Barber, J. (1988). "Mapping of the Groundwater System on Camp Creek Using Geophysical Methods," M.S. Thesis, Department of Rangeland Resources, Oregon State University, Corvallis, Oregon.
Camp Creek, a tributary of the Crooked River, in central Oregon, is located approximately 64 km southeast of Prineville, Oregon. Portions of the main Camp Creek channel were fenced in 1966 to exclude cattle grazing and allow recovery of the riparian vegetation. The objective of this study is to document the extent of the shallow aquifer system in the Camp Creek valley, including water table and aquiclude measurements.
The elevation of the study site ranges from 1130 to 1250 m above mean sea level. The mean annual precipitation varies between 28 and 58 cm. The Camp Creek watershed geology is part of the John Day formation, with fine-grained, thin-bedded, commonly bentonithic, tuffaceous sedimentary claystone, with andesite flows, breccias, and sedimentary rock formations in the upper portions of the watershed. Soil core samples indicate that the Camp Creek valley is composed of alluvium deposits underlain by a sedimentary claystone at a depth of approximately 9 m. The claystone acts as a restrictive layer or aquiclude, with a perched aquifer on top of it.
Measured water table contour lines were shown to be nearly perpendicular to the creek alignment through most of the valley. The majority of subsurface flow in Camp Creek is regional in nature, flowing in a direction parallel to the creek. Near the end of the exclosure, the water table contour lines bend sharply toward the creek, indicating a localized flow pattern, with flow directed into the creek.
The presence of the exclosure, resulting in the reestablishment of riparian vegetation, net bank building, channel aggradation, and subsequent changes in stream gradient above and within the exclosure, may be responsible for raising the groundwater level and slowing the rate of aquifer drainage into the creek. Below the exclosure, the stream level drops, the gradient increases, and consequently, groundwater drains faster into the creek.
The study concluded that the shallow aquifer in the mapped section of Camp Creek is an effluent system feeding the stream even during low flow periods. The hydrologic regime inside the fenced exclosure appears to be unique. Baseflow inside the exclosure is greater than either above or below it, particularly during drought periods. The presence of the aquiclude may be important in producing the flow regimes documented in Camp Creek. Further research is needed to more thoroughly document the role of aquicludes in the hydrologic regime of arid-region streams featuring perched aquifers.
Baurne, G. (1984). "Trap Dams: Artificial Subsurface Storage of Water," Water International, Vol. 9, No. 1, pp. 2-9.
Trap dams are low instream structures designed to fill with coarse sediment over a period of several years following construction. The sediment deposited behind the dam serves as an artificial aquifer for the storage of flood waters and their eventual release as low flow. Surface trap dams are suited to streams that transport large amounts of coarse sediment. In streams that do not transport large amounts of coarse sediment, subsurface trap dams can be built instead. Subsurface trap dams store water in the riverbed.
In arid and semiarid regions, trap dams can provide an effective alternative to conventional dams. Trap dams not only provide water storage but also serve as streamflow moderators. Dams of this type have been built in several countries in Europe and Africa. In southern Austria, a large concrete dam filled with sand and gravel serves to stabilize streamflow, shaving flood peaks and augmenting law flows. Trap dams can be built on a wide range of stream sizes, ranging from small ephemeral streams to more or less perennial streams.
Evaporation is negligible in trap dams, since the water table is usually drained to about 60 cm below the surface. These dams can also serve as agents for the artificial recharge of neighboring aquifers. Since a trap dam will, in practice, leak somewhat, there will be an artificial groundwater recharge through the bottom and sides of the trap dam.
Surface trap dams are designed to fill with coarse sediments during a period of several years following dam construction. To accomplish this, these dams are built in stages, with the height of each stage governed by the need to encourage sedimentation of the coarse sediment sizes. The dam should be designed so that it will withstand a severe storm flow while ensuring the retention of coarse sediments. The high flows must be swift enough to carry away the fine sediments, but not so swift as to discourage the deposition of coarse sediments.
Deposition of fine material (silt) behind the trap dam should be avoided.
To discourage silt deposition, a notch is left on top of the dam at every stage of construction to allow a certain amount of surface flow through the dam. The staged construction of the trap dam cannot be accelerated because it may lead to excessive silt deposition and impair the effectiveness of the deposit as an artificial aquifer.
Economic analysis of individual projects to date has shown the benefits of trap dams over conventional dams. For instance, the gradual loss of storage capacity by sedimentation, which often plagues conventional dams, is not a problem with trap dams.
Cooper, H. H., and M. I. Rorabaugh. "Groundwater Movements and Bank Storage Due to Flood Stages in Surface Streams," U.S. Geological Survey Water Supply Paper 1536-J, U.S. Government Printing Office, Washington, D.C.
Solutions are derived for groundwater flow and bank storage caused by sinusoidal-type stage oscillations in surface streams. Both semi-infinite and finite-width aquifers (i.e., those bounded by a valley wall), are described. Theoretical curves show bank storage response as a function of the following aquifer and flood-wave parameters: (1) transmissivity T, (2) coefficient of storage S, (3) aquifer width L (i.e., distance from streambank to valley wall), (4) maximum rise of stream flood stage H, and (5) period of flood stage oscillation τo.
The dimensionless parameter β characterizes the response of bank storage to the sinusoidal fluctuation in flood stages:
β = (π T τo) / (8 S L2) . . . . . . . . . . . . . . . . . . . . . (1)
In finite-width aquifers, bank storage declines very rapidly when β is large and declines slowly when β is small. For instance, for β > 5, almost all the bank storage will have returned to the stream after one flood-wave period. For β > 0.5, almost all the bank storage would have returned to the stream after two flood-wave periods. On the other hand, in semi-infinite aquifers, the bank storage declines very slowly, with 13.9 percent remaining in storage after 10 flood-wave periods, 4.3 percent after 100 flood-wave periods, and 1.4 percent after 1000 flood-wave periods.
For finite aquifers, the recession of baseflow follows an exponential-decay formula:
Q = Qo e-αt . . . . . . . . . . . . . . . . . . . . . (2)
in which Q is the baseflow at time t after Qo, and α is a recession constant equal to:
α = (π2 T) / (4 S L2) . . . . . . . . . . . . . . . . . . . . . (3)
Therefore, in finite-width aquifers the rate of baseflow recession increases with aquifer diffusivity (T/S) and decreases with aquifer width (L). For semi-infinite aquifers the recession of baseflow does not follow an exponential-decay formula.
Copeland, O. L. (1960). "Watershed Restoration: A Photo-record of Conservation Practices Applied in the Wasatch Mountains of Utah," Journal of Soil and Water Conservation, Vol. 15, pp. 105-120.
A classic example of watershed restoration is documented in this paper, including revealing photographs. The example is the Davis County Experimental Watershed in northern Utah. This area, situated in the Wasatch mountains, between Ogden and Salt Lake City, and ranging from about 5,000 ft to more than 9,000 ft in elevation, typifies the mountain-valley relationships that occur commonly throughout the intermountain west.
In 1847, early pioneers settled around the perennial streams that originate at high elevations and flow into the fertile valley at the base of the mountains. For about 50 years, they utilized the forage, water, and timber from these mountains, unaware of the insidious damage being done on the mountain watersheds by overgrazing and burning.
Some minor summer floods occurred between 1878 and 1906. Soon thereafter, the floods increased in frequency and intensity, culminating in the devastating death-dealing floods of 1923 and 1930. Alarmed residents were spurred to action. Subsequent investigation revealed that the floods had resulted from nonconservative use of the watersheds. Partial destruction to complete obliteration of plant cover by overgrazing bared the soil, excessive trampling by livestock led to soil compaction, and burning destroyed plant cover and litter. As a consequence, infiltration rates were reduced. This led to large amounts of overland flow when the intensity of summer storms exceeded the reduced infiltration rates. The momentum and volume of flow increases rapidly as it moves downslope.
A watershed restoration program was initiated in 1933, supervised by the U.S. Forest Service Intermountain Forest and Range Experiment Station. The program consisted of (1) increased fire protection and restricted grazing to control further deterioration of the watershed, and (2) contour trenching and reseeding to promote restoration of the plant cover. The measures were costly, but brought improved watershed stability and effective flood control to the region. No floods have occurred in this area since its restoration, which attests to the efficacy of land treatments in restoring watershed stability and providing flood control.
DeBano, L. F., J. J. Brejda, and J. H. Brock. (1984). "Enhancement of Riparian Vegetation Following Shrub Control in Arizona Chaparral," Journal of Soil and Water Conservation, Vol. 39, No. 5, September-October, pp. 317-320.
The potential water yield increases resulting from brush-to-grass conversions in Arizona chaparral were investigated. Data were collected on the Three Bar experimental watersheds located about 8 km west of Lake Roosevelt in central Arizona. Results for two watersheds were reported, one treated and the other used as a control. Beginning in 1960, the treated watershed received four annual aerial applications of a phenoxy herbicide, which killed 42% of the shrub live oak and 72% of the birchleaf mountain mahogany plants. Surviving plants were hand treated with fenuron in 1964 and with fenuron and karbutylate in 1968. The treatment reduced shrub cover and enabled grass establishment. By 1969, the shrub crown cover in the treated watershed had been reduced to less than 3%. Shrub control increased both the duration and amount of streamflow delivered from the treated watershed. Along with the more favorable hydrologic environment, the density of riparian vegetation increased over time below the treated watershed.
The duration of streamflow during June, July and August was changed dramatically by shrub control. Before the 1959 wildfire, both treated and control watersheds experienced long periods with no streamflow. From 1957 to 1959, the average period with no streamflow was 74 days for the treated watershed, and 76 days for the control watershed. After the fire, streamflow from the treated watershed became perennial and has remained so to date (1984). In contrast, streamflow from the control watershed varied widely. In some years, perennial flow occurred, while in other years there were up to 91 continuous days with no streamflow. The fluctuation in days of zero streamflow for the control watershed was significantly related to the antecedent precipitation that had occurred the previous winter. In contrast, the treated watershed was able to maintain perennial streamflow regardless of the antecedent precipitation, although the total amount was lower in the drier years.
The paper concludes that upslope brush control activities increases water yield and extends the duration of streamflow. The enhancement of riparian vegetation below the treated watershed is attributed to the charge from intermittent to perennial streamflow. The paper addresses the question of whether the additional consumptive use caused by the enhancement of riparian vegetation may serve to offset the increase in water yield sought by the treatment. Since about 85% of the water yield increases occur during the dormant season (November to April), the bulk of the increase is likely to be delivered to downstream uses. For this reason, the enhancement of riparian vegetation is judged to have little impact on the water yield increase resulting from brush-to-grass conversion.
The paper finishes with a warning. The high concentrations of nitratenitrogen in streamflow associated with brush-to-grass conversions in Arizona chaparral may result in a deterioration of downstream water quality, unless this effect is somehow mitigated by the filtering processes that may be attributed to the riparian vegetation. One critical research need is an increased understanding of nutrient cycling processes in riparian areas.
DeBano, L. F., and B. H. Heede. (1987). "Enhancement of Riparian Ecosystems with Channel Structures," Water Resources Bulletin, Vol. 23, No. 3, June, pp. 463-470.
Naturally occurring and manmade structures can be used to enhance the development of healthy riparian areas. Naturally occurring structures are the following: (1) cienagas, (2) log steps, and (3) beaver dams. Manmade structures include large and small channel structures and bank protection devices. These structures affect streamflow hydrology, hydraulics, and sedimentation rates, and can lead to the development of a more favorable environment for the establishment of riparian vegetation. However, when used improperly, they can be destructive to established riparian zones.
Since stream processes are generally slow, a long time may pass before the effects of a certain management action, good or bad, become visible. Also, the effects of large dams may show up a long distance downstream from the dam. Therefore, investigations must be of a wide scope. An interdisciplinary approach is necessary to assure that the complexities of channel hydraulics and riparian ecology are properly understood. The fields of hydrology, fluvial geomorphology, and plant ecology are particularly helpful in the study of riparian ecosystems. Investigations which are not broad in scope are bound to lead to the wrong conclusions.
DeBano, L. F., and L. J. Schmidt. (in press). "Improving Southwestern Riparian Areas Through Watershed Management," General Technical Report RM- , USDA Forest Service Rocky Mountain Forest and Range Experiment Station, Fort Collins, Colorado.
This is a forthcoming comprehensive report on riparian area management.
A summary of the section on water augmentation by riparian area management is given here.
Vegetation cover manipulations, particularly brush-to-grass conversions in chaparral, offer a viable technique for increasing water yield and lengthening the duration of streamflow, thereby enhancing the establishment of riparian vegetation in downstream channels. These conversions, when properly planned, will probably not produce any long-term change in watershed condition (e.g., increase in erosion rates, reduction in plant cover, etc.).
The greatest potential increase in annual streamflow per acre treated can be obtained during timber harvesting in mixed conifer because this vegetation type receives the greatest amount of annual precipitation. Substantial increases also occur during timber harvesting in ponderosa pine. However, in both these commercial forest types, water yield increases, resulting from the effect of timber harvesting on snowmelt, occur mainly during spring when water use by riparian plants is lowest. Duration of streamflow is not changed substantially by timber harvesting in either ponderosa pine or mixed conifer forests. In southwestern pinyon-juniper woodlands, only small increases in water yield can be obtained from tree removal, making it unlikely that treatment of pinyon-juniper woodlands would produce enough additional water to enhance riparian ecosystems. However, in Oregon, selected studies have shown that removal of juniper can change ephemeral streams into perennial streams.
Brush-to-grass conversions in Arizona chaparral appears to be a promising management tool for enhancing riparian areas, because it not only produces the second largest increase in water yield per acre treated, following mixed conifer, but also increases streamflow duration significantly, increases upland habitat diversity, and reduces fire hazard. Brush-to-grass conversions appear to be a viable management alternative for enhancing the hydrology of riparian areas on chaparral watersheds in Arizona and California. However, care must be exercised to ensure that existing riparian communities in untreated chaparral are not endangered by the treatment.
Elmore, W., and R. L. Beschta. (1987). "Riparian Areas: Perceptions in Management," Rangelands, Vol. 9, No. 6, December, pp. 260-265.
Riparian areas are defined as the often narrow strips of land that border creeks, streams, rivers, and other bodies of water. Management strategies designed to restore degraded riparian areas into their original or pristine condition are discussed in this paper. Field observations and photographs from Camp Creek and other eastern Oregon watersheds are used to document riparian area recovery under selected nonstructural management techniques.
A healthy stand of riparian vegetation functions as an effective sediment trap, halting bank erosion and leading to net bank building. Riparian vegetation also plays an important role in the maintenance of summer flows. Bank building in riparian areas can lead to a rise in the water table, slowing the release of subsurface waters to surface waters. In many streams in eastern Oregon, this effect is responsible for the conversion of intermittent streams into perennial streams.
Uncontrolled livestock grazing is judged to be the human activity largely responsible for the degradation of western riparian areas, resulting in gully development, increased bank erosion, lowering of adjacent water tables, and loss of summer flows. Grazing management provides a major opportunity to improve riparian areas without the expenditure of large amounts of money.
For instance, it is known that continuous heavy grazing of riparian areas can have long-lasting detrimental effects. Grazing needs to be closely managed in both riparian and upland areas for recovery of degraded streams to begin. Timing is particularly crucial for riparian areas. Allowing vegetation to grow all summer only to graze it heavily in the fall can eliminate chances for recovery. In some eastern Oregon riparian areas, limiting livestock grazing to the springtime allows for vegetation regrowth throughout the summer, resulting in improved channel and bank stability during periods of high runoff.
Instream structures such as gabions, dikes, check dams, riprap, sills and the like, have been used in the past in an effort to accelerate riparian recovery. Structural solutions, however, are expensive, and will seldom "solve" riparian problems. Building expensive structures without solving the problems associated with management of riparian vegetation allows managers to sidestep difficult decisions. In contrast, nonstructural strategies for riparian restoration are less expensive and allow streams to function in ways that cannot be replicated with artificial structures.
The paper concludes with an exhortation to ranchers, land managers,
biologists, hydrologists, environmentalists, and the general public to engage in a dialogue toward the initiation of riparian management strategies to allow riparian areas to reach their productive potential.
Frickel, D. G. (1972). "Hydrology and Effects of Conservation Structures, Willow Creek Basin, Valley County, Montana, 1954-68," U.S. Geological Survey Water Supply Paper 1532-G, U.S. Government Printing Office, Washington, D.C.
This report presents the results of hydrologic observations in Willow Creek basin in northeastern Montana for the period 1954-68. The effects of conservation structures on both runoff and sediment yields were evaluated on the basis of available data.
During the study period numerous reservoirs and water spreaders were constructed in the basin. This upstream regulation of flow has caused a decrease in both the channel dimensions and the suspended sediment load at the gaging station near the mouth of the basin. The change in channel geometry presumably occurred in response to reduced peak discharges and prolonged periods of low flow brought about by the detention reservoirs.
It is concluded that conservation structures in the basin (i.e., reservoirs and water spreaders) are apparently very effective in reducing peak discharge. Estimates indicate that the peak discharge of the 1962 flood at the gaging station was about 45 percent less than it would have been without these structures. Also, a reduction of 18 percent in annual runoff volume has been documented. It is not known what percentage of this reduction was due directly to the presence of the structures and what percentage can be attributed to normal transmission losses.
Suspended-sediment concentration at the gaging station were reduced about 55 percent during an 11-year period. The capacity of the reservoir system is being depleted by sedimentation at an estimated annual rate of 1.6 percent. Streams in the basin are naturally ephemeral; however, sane may now have long periods of low flow as a result of outflow from the many detention dams in the basin.
Heede, B. H. (1977). "Case Study of a Watershed Rehabilitation Project: Alkali Creek, Colorado," Research Paper RM-189, USDA Forest Service Rocky Mountain Forest and Range Experiment Station, Fort Collins, Colorado, June.
Alkali Creek is located in the White River National Forest, 20 mi south of the town of Silt, Colorado. In 1958, the Alkali Creek Watershed Soil and Water Rehabilitation Project was initiated in response to increased pressure on the land, documented as accelerated gully development throughout the first half of the 20th century. It is surmised that the reason for the gully growth was the combined effect of drought and overgrazing, as well as the overuse of the agricultural lands below the watershed. Overuse of the lowlands led to greater incision of the channels, which resulted in the lowering of the base level for Alkali Creek.
The headwaters of Alkali Creek, about 1 square mile in area, were fenced from 1958 to 1966 to exclude cattle grazing. In 1963, the USDA Forest Service constructed 133 check dams in about half of the gullies located within the project area. Four gullies with a total length of 1,900 ft were converted to vegetation-lined waterways. Seven years later (1970), the previously ephemeral flow became perennial, although there was no noticeable change in normal precipitation for the treatment period. The change from ephemeral to perennial flow is attributed to the establishment of vegetation in the gullies above the check dams.
Sediment deposits above the dams and the luxuriant growth of vegetation enhanced the process of flow stabilization. The perennial watercourses that developed in the structurally-treated gullies exhibited only about one-third of the net erosion of gullies that remained untreated. However, stabilization also proceeded in the untreated gullies. Local base level controls exerted by the structurally-treated gullies, as well as the growth of vegetation, probably contributed to the stabilization. Untreated gullies outside the project area increased their surface area three times more than the untreated gullies inside the project area. Both gully groups had comparable morphological characteristics and belonged to the same stream network.
Check dam treatments should be confined to the mainstem gully and to the large tributaries that control local base levels. Generally, conversion of gullies to vegetation-lined waterways should be restricted to first and second order streams in broad valley bottoms. Periodic field inspections are necessary to assess maintenance needs and prevent failures.
Soil and water rehabilitation projects of the type installed on the Alkali Creek watershed are expensive. However, the on-site benefits demonstrate that the overall project objectives have been achieved. Erosion is still active locally, but soil and water stabilization processes continue to take place. While it is likely that high-intensity storms will cause sane erosion, the enhanced vegetative cover brought upon by the maintenance of perennial flows will continue to mitigate the effects of these natural erosional processes.
Hooper, R., B. P. Van Haveren, and W. L. Jackson. (1987). "The Sheep Creek Resource Conservation Area Project," Proceedings, XVIII Conference of the International Erosion Control Association, Reno, Nevada, February 26-27, pp. 117-126.
The Sheep Creek Resource Conservation Area project was implemented from 1957 to 1966 to stabilize and rehabilitate the upper watershed of Sheep Creek, a tributary of the Paria River in southern Utah. The project was a cooperative effort involving six federal agencies, the Utah Division of Wildlife Resources, and private landowners. Rehabilitation and stabilization measures included construction of detention dams, dike water-spreader systems, gully plugs and check dams. In addition, numerous seedings and brush control projects were undertaken, and livestock grazing was more intensively managed.
As part of the project, the USDI Bureau of Reclamation constructed the Sheep Creek Barrier Dam, a large detention dam on the main stem of Sheep Creek at the lower end of the project area. This dam has been particularly successful in trapping coarse sediments and has contributed to the stabilization and restoration of over 1 mile of deeply incised gully. The dam created a storage pond with an initial capacity of 87.9 ac-ft below the spillway crest elevation. By September 1961, 107.6 ac-ft of sediment had been trapped by the dam. Another 57.7 ac-ft were deposited by November 1964. In 1964, channel aggradation had extended some 2,300 ft upstream of the dam and 21.5 ft above the spillway crest elevation.
There appears to be little free water storage capacity left above the dam. However, vegetation has invaded the sediment deposit and acts to detain water as well as filter out sediment. A small channel, with an average bankfull capacity of 93 cfs, is incised in the sediment wedge that has been created between the spillway and the upstream natural channel. A perennial flow at the dam has resulted from water slowly draining from the reservoir sediments. Approximately 15 acres of riparian habitat have developed on the reservoir sediments.
Flooding occurs on the sediment wedge for all significant runoff events. Flooding serves to spread the flows and reduce runoff peaks, as compared to a gullied condition in which large flows are entirely confined within the banks. In addition, flooding reduces stream energy, encourages sediment deposition, and results in aquifer recharge.
Mechanical or structural watershed treatments should only be applied in situations where the watershed condition is so severely degraded by past management practices as to render the recovery by natural means inefficient. The treatment should permit a watershed to reach a condition of natural stability and function more rapidly than can be achieved without treatment. Whenever possible, the achieved watershed condition should be sustainable with proper land use management, and should not be dependent upon continued structure integrity and maintenance.
Horton, R. E. (1937). "Hydrologic Aspects of the Problem of Stabilizing Streamflow," Journal of Forestry, Vol. 35, No. 11, November, pp. 1015-1027.
Stabilizing streamflow refers to the maintenance of a more nearly constant and uniform flow regime that is provided by nature, without necessarily changing the total runoff volume. It is accomplished by a decrease in surface runoff volume during floods and a corresponding increase in surface runoff whims during law flow periods.
The author discusses three approaches to the problem of stabilizing streamflow. The first, supported mainly by engineers, believes that stabilization can be best accomplished by the construction of surface storage reservoirs. While there are numerous successful examples of this approach, there are still many streams where nature has not provided suitable reservoir sites at desirable locations. Also, the impact of surface storage reservoirs on stream uses other than flow regulation needs to be clearly established.
The second approach, supported by foresters and conservationists, holds that the forests are natural streamflow regulators. It has been claimed that the preservation of upland forests will lead to the elimination of floods and the maintenance of adequate low water flows. However, forest evaporation and evapotranspiration are known to consume large amounts of water, tending to offset the potential benefits of forest flow regulation and stabilization.
A third approach holds that streamflow can be stabilized by increasing soil infiltration. Increased infiltration decreases surface runoff, augments soil moisture, and stabilizes streamflow. Strews are perennial if they are sustained during rainless periods by surface or groundwater storage or both. While simple generalizations are not possible, there is certain to be some increase of minimum streamflow during long dry periods whenever there is an increase in total accretion to the water table.
Possible methods for increasing total infiltration are: (1) increasing the infiltration capacity of the soil by mechanical means, (2) increasing depression storage, (3) decreasing the rate of overland flow, and (4) using grasses as a means to delay surface runoff. It is widely recognized that increasing the infiltration capacity by mechanical means has its pitfalls; therefore, this method is not generally applicable to forests and grasslands. Increasing depression storage has long been practiced to a limited extent in arid and semiarid regions. For instance, strip cropping has been shown to be a simple and effective means of increasing depression storage. The rate of overland flow can be decreased by suitable level terracing, which decreases the slope of the surface and increases ponding time. Grasses delay surface runoff due to the greatly increased resistance to overland flow. The subdivision of the flow by plant stems and leaves leads to decreased surface runoff and a corresponding increase in total infiltration.
The author concludes that stream stabilization can be accomplished in an effective way by increasing total infiltration, primarily by changes in agricultural practices. Because of increased consumptive use by vegetation, stabilization of streamflow is likely to be accompanied by a more or less corresponding decrease in total runoff.
Hough, J. (1986). "Management Alternatives for Increasing Dry Season Baseflow in the Miombo Woodlands of South Africa," Ambio, Vol. 15, No. 6, pp. 341-346.
Water is a scarce and valuable resource in many arid and semiarid regions of the world. In certain forested watersheds, water precipitating in the rainy season can be stored in groundwater reservoirs for later release as baseflow during the dry season. For example, the Miombo woodlands in Southern Africa, if designated as protected areas and properly managed, can be used to improve dry season baseflows in Southern Africa.
The aim of management should be to maximize infiltration and soil moisture storage and to minimize evapotranspiration and surface runoff. Infiltration rates and soil moisture storage capacity are directly related to the organic content of the soil, the avoidance of compaction by excessive livestock trampling or raindrop impact, and the avoidance of fire and exposure of the soil surface to the sun, which produces water repellant colloids in the soil surface. Infiltration is also encouraged by increasing detention storage of water on the soil surface through litter and a good herbaceous ground cover.
Losses by evapotranspiration are usually responsible for a large percentage of the total water loss from the system. Therefore, reduction in the amount of evapotranspiration, through manipulation of vegetation structure and species composition, offers the most potential for increasing dry season baseflow. Conversion of woodlands to grasslands should increase dry season baseflow in the short term by reducing evapotranspiration from deep in the soil. However, increased grass biomass leads to higher fire intensity, causing increased soil erosion and decreased infiltration. Therefore, woodland clearance appears undesirable except on gentle slopes when accompanied by early burning.
The use of shallow rooting tree species may have significant potential for increasing dry season baseflows. More research is needed in this area. The higher evapotranspiration rates of evergreens also have significant implications. Conversion of phreatophytic vegetation from evergreen to deciduous should increase dry season baseflows in most Miombo areas of Southern Africa. Where eroded watersheds exist, revegetation should be encouraged. Evidence is unclear as to the effect of dambo (meadow) drainage. The latter might be beneficial in reducing dry season evapotranspiration, provided erosion and woodland encroachment are prevented. However, meadow drainage depletes groundwater storage, which may lead to a reduction in dry season baseflows.
A viable model for managing Miombo catchments to maximize dry season baseflows might be the following: An open woodland of tall mature trees with a continuous herbaceous sward on steep slopes, open grassy strips on gentle slopes, open grasslands on level areas, and a thinned or deciduous riparian forest. Fire may provide a convenient and inexpensive management tool.
Ingebo, P. A. (1971). "Suppression of Channel-side Chaparral Cover Increases Streamflow," Journal of Soil and Water Conservation, Vol. 26, No. 2, March-April, pp. 79-81.
Two small watersheds in central Arizona were instrumented to determine the effect of manipulating chaparral cover on streamflow. The watersheds are located about 6 miles southwest of Prescott, Arizona, in the headwaters of the Hassayampa river, at an elevation of 6,000 to 7,000 ft. Watershed A (303 ac) was designated as the control watershed and watershed B (246 ac) as the treated watershed.
Pelleted fenuron was placed by hand under channel-side shrubs and trees, predominantly oak-mountainmahogany chaparral. Treatment was confined to areas along well defined channels. The intent was to treat all shrubs and trees that use water from the moist channel environment. By eliminating the deep-rooted plants within this zone, water concentrating downslope along the stream channel would be subject to reduced evapotranspiration losses before becoming streamflow. Bounds of the treated zone were set at 30 ft vertical but no more than 75 ft horizontal distance from the channel.
Following treatment, streamflow in watershed B charged from intermittent to perennial. In contrast, streamflow in watershed A has remained intermittent. Both volume and duration of streamflow responded to the channel-side treatment in watershed B. Streams were flowing in both watersheds at the time of treatment in March 1967. Streamflow in the control watershed became intermittent in late June 1967. In contrast, flow in the treated watershed did not stop, and its rate of recession was less than in previous years. Following winter and spring runoff, watershed A again entered a period of intermittent flow, while watershed B continued to maintain perennial flow.
Prior to treatment, steamflow in the two watersheds was quite well synchronized. Since treatment, however, streamflow in the two watersheds is no longer synchronized. Watershed B has flowed continuously, while watershed A has flowed for 201 days in the first post-treatment water year and for 227 days in the second.
The fenuron treatment will probably eventually kill 80% or more of the chaparral cover, even the highly resistant shrub live oak. Native grasses and forbs in the intershrub areas have exhibited renewed vigor since the elimination of competition. By the second year after treatment, flannel mullein (Verbascum thapsus) had established itself in the moist channels of the treated area. It is not known what effect this rank growth might have on water yield. Streamflow is being monitored for fenuron contamination.
Kennon, F. W. (1966). "Hydrologic Effects of Small Reservoirs in Sandstone Creek Watershed, Beckham and Roger Mills Counties, Western Oklahoma," U.S. Geological Survey Water Supply Paper 1839-C, U.S. Government Printing Office, Washington, D.C.
The hydrologic effects of a group of 22 flood-retarding reservoirs located in Sandstone Creek watershed in western Oklahoma were reported in this study. The capacity of the reservoirs ranged from 102 to 4,192 ac-ft. Complete monthly water budgets for each reservoir were prepared for the period starting October 1, 1958, and ending September 30, 1960. Reservoir seepage was found to be almost equal to spillage. The seepage reappears as surface flow in the principal stream channels downstream of the reservoirs, and thus Sandstone Creek has been effectively converted from an ephemeral stream into a perennial stream. The perennial flow has accelerated the growth of riparian vegetation, which has triggered a change in channel geometry and a reduction in channel conveyance. Water losses attributable to the reservoir were found to be about 20% of the natural runoff.
Most, if not all, reservoirs are built on top of beds of relatively impervious shale. Therefore, there is negligible loss of water by percolation to deep groundwater reservoirs. Most of the seepage water must flow under or around the dams and reappear as surface flow in the main channels below the reservoirs. Comparisons of estimated reservoir seepage and downstream channel flows support the thesis that substantially all reservoir seepage reappears downstream as surface flow.
Prior to the construction of the reservoirs, Sandstone Creek was an ephemeral stream. After the completion of the reservoirs in early 1952, the creek remained ephemeral for almost 6 years. However, in November 1957, perennial flow was established. It is conjectured that almost 6 years were required for groundwater levels to build up in the valley alluvium in order to sustain a continuous flow downstream.
There was a marked change in the amount and duration of streamflow before and after the establishment of perennial flow. For instance, the streamflow in 1953 was 57 ac-ft, while in 1959 it was 887 ac-ft, even though total rainfall in 1959 was somewhat lower than in 1953. The establishment of perennial flow encouraged the growth of riparian vegetation. In turn, the vegetation changed the geometry of the channels from rectangular shape, with erodible vertical banks, to V-shaped, with more stable banks. The increased presence of vegetation has resulted in a loss in channel conveyance for medium and high flows.
Kirkby, M. (1988). "Hillslope Runoff Processes and Models," Journal of Hydrology, Vol. 100, pp. 315-339.
Hillslope hydrology is concerned with the separation of precipitation into overland flow and subsurface flow as it passes through the vegetation and soil, and the effect that this separation may have on the downstream runoff hydrograph. Three distinct flow processes are identified for modeling purposes: (1) Hortonian overland flow, (2) saturation overland flow, and (3) return flow.
Hortonian overland flow is produced when the rainfall rate exceeds the current infiltration capacity of the soil. Saturation overland flow is produced when the storage capacity of the soil is completely filled, so that all subsequent applications of water at the surface, regardless of their rate of application, are forced to flow over the surface. Return flow is produced when subsurface flow is constrained to flow out of the soil, in areas of profile concavity and/or flow convergence in plan, or where soil thickness and/or permeability are decreasing in the downslope direction. These flow types follow different paths, and are, therefore, subject to different attenuation rates.
Subsurface flow may move either in a general vertical or lateral (i.e., downslope) direction. Movement in the downslope direction may occur in the following cases: (1) where saturated vertical discharge decreases with depth, forcing lateral flow, (2) where percolating water reaches a perched water table, and (3) where lateral hydraulic conductivity is much greater than vertical hydraulic conductivity.
Within the soil, vertical hydraulic conductivity generally decreases downwards, although there are many exceptions to this rule. Where vertical hydraulic conductivity decreases downwards, moisture content tends to rise until, if percolation is rapid enough, saturation is reached. A saturated layer then builds up as a perched water table. The decline of hydraulic conductivity with depth tends to encourage movement of flow in the lateral direction. Comparable mechanisms of flow diversion occur at the surface, giving rise to Hortonian flow and/or saturation overland flow.
This paper reviews the state-of-the-art in the modeling of hillslope hydrologic processes, including the simple cases such as infiltration, saturated lateral subsurface flow, macropore and pipe flow, and overland flow. An exception to the simple cases is provided by subsurface flow toward streambanks, where hydraulic potential surfaces commonly intersect the land surface with a steep gradient. The paper concludes that much research remains to be done in the field of hillslope hydrologic processes and models.
Kondolf, G. M., L. M. Maloney, and J. G. Williams. (1987). "Effects of Bank Storage and Well Pumping on Baseflow, Camel River, Monterey County, California," Journal of Hydrology, Vol. 91, pp. 351-369.
This study uses surface and groundwater data from the Camel River, in Monterey County, California, to document the relationship between bank storage, well pumping, and baseflow in a typical alluvial setting in the west. Baseflow is defined as the natural groundwater discharge to a stream, or alternatively, as the portion of the streamflow that originates in groundwater storage. Sources of baseflow include: (1) geologic configurations that lead to springs, (2) soil water draining from upland slopes, and (3) groundwater stored temporarily in bank alluvium. Soil water from upland slopes is the main source of baseflow in most lower order streams. Under favorable conditions, water can be stored in bank alluvium during floods, to be released back to the stream later, as flood stages recede. The water temporarily stored in bank alluvium is referred to as bank storage.
Most water in bank storage is discharged to the stream soon after the flood starts receding. However, as long as the stage continues to fall, a hydraulic gradient from bank to stream is maintained, and a longer-tern, seasonal bank storage effect can be expected. Three conditions are necessary for significant bank storage effects: (1) the stage must increase markedly during high flows, forcing the stream to become influent, (2) there must be a sufficient volume of bank storage relative to the amount of streamflow, and (3) the bank material must have a high hydraulic conductivity.
The first condition is readily satisfied by downstream reaches, because these reaches are subject to large stage variations during floods. The second condition also favors downstream reaches, because they are more likely to be flanked by alluvium. The third condition is satisfied by high-gradient, straight-to-braided streams, where sand and gravel are likely to predominate over silts and clays. In contrast, low-gradient, meandering streams are not likely to satisfy the third condition. In addition, rivers formed by infrequent events are also likely to produce coarse deposits because of the greater competence of flood flows. Watershed lithology is another important factor to be considered. For example, rivers that drain granitic terrain are more likely to have coarse-grained alluvium than rivers that drain shales.
In 1982, a moderately wet year, substantial bank storage effects were detected in the Cannel valley during the two months following the last flood peak of the season. However, in 1983, an extremely wet year, large amounts of moisture draining from the upland slopes overwhelmed the bank storage contributions. Thus, the importance of bank storage will depend on its relation (in timing and magnitude) to baseflow originating in upland slopes. On regulated rivers, where baseflow from upland slopes is impounded, bank storage may represent an important source of baseflow to downstream users.
While irrigation usually results in net moisture accrual to bank storage, well pumping is shown to deplete it. In the Carmel valley, well pumping can lower the local water table and change the character of the stream from effluent to influent. The persistence of this effect was noted, and its effect on baseflow and the negative impact on fisheries habitat, loss of riparian vegetation, and channel stability were documented.
Lewis, G. L. (1984). "Nebraska's Shrinking Platte River Channel: Hydrologic Aspects and Implications," Proceedings, American Society of Civil Engineers Hydraulics Division Specialty Conference, Couer d'Alene, Idaho, August, pp. 639-643.
Substantial reductions in channel width (to about 30% of the initial value) along a reach of the Platte river, Nebraska, downstream of Kingsley dam, are documented. The channel narrowing is linked to the uncontrolled growth of vegetation and associated streambank building following the construction of Kingsley dam (1941).
Mean annual flows, annual no-flow days, and historical channel width data (as early as 1860) are used to show that, contrary to popular opinion, mean channel width in the study reach decreased markedly during periods of increasing peak annual flow. This evidence defies the conclusion that channel width is directly related to the "dominant" or peak annual flow rate.
Channel widths in the study reach actually increased during dry periods when no-flow days (causing vegetation to die out) were prevalent. Significant reductions in channel width were documented following the elimination of no-flow days by the upstream flow regulation provided by Kingsley dam. The paper concludes that the change from intermittent to perennial flow was the principal cause of vegetative channel encroachment and subsequent channel narrowing.
Meinzer, O. E. (1927). "Plants as Indicators of Ground Water," U.S. Geological Survey Water Supply Paper 577, U.S. Government Printing Office, Washington, D.C.
There are two types of flora in the desert: (1) xerophytes, which adapt to prolonged absence of rainfall by maintaining a nearly dormant condition, and (2) phreatophytes, which are able to obtain a perennial and secure supply of water by sending their roots down to the water table or to the capillary fringe. The phreatophytes form a fairly definite group in desert regions and a less definite group in humid regions. The more arid the region, the sharper the contrast between phreatophytes and xerophytes.
Observations of the relation between the depth to the water table and the occurrence of certain plant species in arid regions give convincing evidence of the groundwater adaptation in some species and its absence in others. In the arid regions of the West, tracts of shallow groundwater occur in three principal situations: (1) in the canyons and other localities in the mountains where the water is held up by impermeable bedrock near the surface; (2) in the lowest parts of the principal basins or intermountain valleys; and (3) at certain intermediate locations featuring natural barriers to groundwater flow. The largest areas of plants that feed on groundwater are on the valley lowlands, but distinctive plants of this group also grow in upland areas, where they can become reliable indicators of the presence of shallow groundwater.
The following are principal species of plants that habitually feed on groundwater: (1) rushes (juncus), sedges (Scirpus), and cat-tails (Typha), (2) reeds (Phragmites communis) and cane, (3) wild rye (Elymus condensatus), (4) salt grass (Distichlis spicata), (5) sacaton (sporobolus airoides), (6) pickleweed (Allenrolfea occidentalis), (7) rabbit brush (Chrysothamnus graveolens), (8) arrow weed (Pluchea sericea), batamote bush (Baccharis glutinosa) and jacate (Hymenoclea monogyna), (9) the saltbrush species, (10) greasewood (Sarcobatus vermiculatus), (11) mesquite (Prosopis juliflora, Prosopis vetulina, etc.), (12) alfalfa, (13) willow (Salix gooddingii, Salix fluviatilis), (14) desert willow (Chilopsis linearis), (15) cottonwood and other poplars, (16) buffalo berry, elderberry, blackberry, gooseberry and hackberry bushes, shrubby cinquefoil, and wild roses, and (17) Washington palm and other palms.
These plants are of great practical value as indicators of the occurrence of groundwater in arid regions. They give evidence to supplement that furnished by the topography or geology and which is more specific as to the precise locations where the water occurs near the surface. They cannot be ignored in any groundwater survey in a desert region. The phreatophytes indicate not only the occurrence of groundwater but also to some extent its depth. Some species will grow where the water table is virtually at the surface; others have minimum and maximum depths to groundwater to guarantee luxuriant growth. With a few exceptions, the greatest depth to which groundwater is known to be lifted by plants is about 15 m.
Motts, W. S., and A. L. O'Brien. (1981). "Geology and Hydrology of Wetlands in Massachusetts," Publication No. 123, Water Resources Research Center, University of Massachusetts at Amherst, Mass.
This report provides detailed data on three Massachusetts wetlands of varying character in diverse settings. The findings reveal a wide range of differences in the extent to which wetlands (1) modify the character of basin runoff; (2) influence the recharge and discharge relationships of underlying aquifers; and (3) affect potential groundwater development.
The report proposes a classification to define the hydrologic response of wetlands using geologic, hydrologic, and topographic factors. Important geologic and hydrologic factors include the character and thickness of surficial materials, bedrock type, and aquifer connectivity and properties. Important topographic positions are the wetland's position in the drainage basin and its absolute and relative size.
The artificial recharge of aquifers is one of the groundwater management alternatives considered in this report. Artificial recharge involves the management of surface water to convert increasing amounts of it into subsurface and groundwater. With effective artificial aquifer recharge, the amount of water available for pumping could be increased.
The total yield of municipal wells can be increased with artificial recharge methods such as check dams and disruption of channel bottoms over primary recharge areas. The simplest method to divert water underground is to mechanically increase the permeability of a stream bottom by upturning and disturbing the streambed with bulldozers or similar earth-moving equipment.
In an artificial recharge study in Nonewaug Basin, near Watertown, Connecticut, the well yield was doubled by means of a check dam and infiltration galleries. This dam raised the water level of Nonewaug Creek a few feet, which was enough to supply the gradient needed to feed surface water into the infiltration galleries. The amount of infiltration probably can be substantially increased by merely retaining water in small reservoirs lying over the primary recharge area.
Mull, R. (1986). "Low Flow Sustained by Ground Water," Chapter 4 in River Flow Modelling and Forecasting, D. A. Kraijenhoff and J. R. Moll, eds., D. Reidel Publishing Company, Dordrecht, Holland.
The term low flow in natural rivers is qualitatively indicated by a low water level. Law flow results from the interaction between groundwater and surface water. Under effluent conditions groundwater exfiltrates into surface-water courses. Under influent conditions surface water infiltrates into the subsurface. Subsurface water is comprised of an unsaturated zone and a saturated zone. In the unsaturated zone the preferred path of movement of moisture is vertical toward the saturated zone. In the saturated zone the preferred path of movement of moisture is horizontal toward discharge areas.
Aquifers have three distinct zones: (1) a recharge zone, (2) a transition zone, and (3) a discharge zone. Within the recharge zone, water originating from precipitation or surface water systems recharges the groundwater. In the transition zone impermeable layers near the surface or cultural land uses (buildings, etc.) can prevent recharge of the aquifer. In the discharge zone groundwater exfiltrates into surface drainage systems. In mountainous areas effluent and influent conditions can occur alternately, leading to intermittent streams.
For effluent conditions water flows from the groundwater into the surface waters. During rainless periods the outflow is greater than the recharge and, consequently, the stream discharge and low flow level decrease continuously. The rate of recession is a function of the hydraulic and geometric properties of the system. The hydraulic parameters of the aquifer are the transmissivity and storage coefficient. Aquifers can be of two types: (1) confined, and (2) unconfined. In confined aquifers, water is stored between two impermeable layers. The storage of a confined aquifer is related to its depth and to the compressibility of water. Due to the low compressibility of water, the water volume that can be released from a confined aquifer is relatively small and, therefore, the aquifer can be depleted relatively fast. Conversely, the volume of water that can be released from an unconfined aquifer is directly related to the pore volume. Thus, storage coefficients of unconfined aquifers are much larger than those of confined aquifers.
The rate of groundwater recharge is usually smaller than the net rate of infiltration into the soil. The difference is accounted for in soil evaporation and evapotranspiration by vegetation. The greater the distance between land surface and water table, the smoother the historic records of water table depth. For water tables at great depths the accretion is more or less constant in time. For shallow aquifers, large fluctuations in the rate of groundwater recharge can be experienced, particularly during droughts.
Law flow modeling can be accomplished by assuming that the depth of the aquifer is much smaller than its horizontal area. Therefore, the vertical velocity can be assumed constant, reducing the system to one of transient two-dimensional flow over porous media, to be modeled with the unsteady two-dimensional diffusion equation. Initial and boundary conditions and aquifer hydraulic parameters are necessary to simulate the transient response of the subsurface-surface flow system. The solution can be accomplished numerically, using either finite difference or finite element methods.
Oosterbaan, R. J. (1982). "Modern Interferences in Traditional Water Resources in Baluchistan," in Annual Report, International Institute for Land Reclamation and Improvement, Wageningen, the Netherlands. pp. 23-34.
This paper describes the interference by modern water resources development on traditional water conservation methods such as the Kushkaba, Sailaba and Karez systems used in Baluchistan, Pakistan. The Kushkaba system consists of a series of small earth embankments constructed across upland slopes for water harvesting purposes. Rainfall that runs down the slope is detained by the embankment and forced to infiltrate into the soil, augmenting the soil moisture. The main crop produced under the Khuskaba system is wheat, grown exclusively in the winter season. The Sailaba system is similar in principle to the Kushkaba, but it is built on the flood plain, for the purpose of retaining flood waters and encouraging infiltration.
The Karez is an underground tunnel, dug in a gentle upslope direction until it reaches the watertable. The groundwater then flows into the Karez and through it to the outlet at the beginning of the tunnel, where the water is captured and used for irrigation. The most appropriate site for a Karez is an alluvial fan at the foot of a mountain range. There, large amounts of water infiltrate into the soil, thus replenishing the groundwater and feeding the Karez. Karez systems usually provide a relatively stable discharge throughout the year.
Modern water resources developments such as infiltration dams, subsurface interception walls, and tubewells, interfere with the traditional ways of conserving water resources in Baluchistan. For instance, Islamic water laws attach no property rights to water resources. The law only attaches rights to the use of water to those who have constructed works to produce the water. Thus, the builders of a Karez have exclusive rights to the water extracted from it. However, if a dam is built upstream of the Karez, the same law applies to the builders of the dam. The fact that this dam reduces the downstream quantities of water constitutes no legal problem.
Careful hydrologic investigations are needed to evaluate the impacts of modern water resource developments on traditional water conservation systems in Baluchistan.
Oregon State University Water Resources Research Institute. (1986). "Estimating and Measuring Impacts of Nonstructural Methods for Increasing Basin Water Yield," Report of the Third Interuniversity Water Workshop, Portland State University, Portland, Oregon, May 9, 1986, July, 47 pages.
The workshop was convened to discuss approaches and methodologies for estimating the changes in timing, duration, quantity and quality of water that can be linked to riparian area management and associated watershed improvement practices. The workshop record was used as the basis for the report.
The participants' feelings regarding the potential for riparian area enhancement were positive. They were in agreement that many benefits could be realized by the development of riparian management policies. There was a concern that the focus on water yield was misguided, and that a more appropriate focus would be on the timing of flow, particularly the amount and duration of flow during August. Several participants felt that although there was an abundance of qualitative data regarding summer flow augmentation, little quantitative data was available. They called for increased data collection efforts to fill this gap. A number of areas in need of further research were identified. Among them are the following: How does the rising water table affect flow? What is the best place to measure water yield and storage? What is the current state of knowledge? What is the cost, availability and reliability of more data?
The participants acknowledged that sane estimates of flow changes already exist, and that additional estimates could be developed. However, a contrast was noted between the seeming clarity of the field evidence and the apparent difficulty to perform precise quantitative assessments. The participants discussed sane ideas for models to develop estimates and monitor changes in surface and subsurface flow conditions. The importance of a program of public involvement in order to ease the cost of data gathering and watershed restoration was addressed. It was also felt that it was important to involve all concerned agencies in monitoring changes in hydrologic and riparian systems.
The participants reckoned that important questions are in need of answers if the experience and observations of Camp Creek are to be fully understood and used as the basis for management practices in other areas. There is a need to learn if Camp Creek is unique and if the methodology used at Camp Creek is valid. There is a need to know how many other potential Camp Creeks there are, to set up demonstration projects in suitable locations, and to develop the knowledge base to warrant extrapolations.
The participants suggested that the following variables be used in evaluations of riparian hydrology: (1) inflow-outflow, (2) aquifer storage, (3) soil moisture, (4) vegetation type, (5) relative humidity and temperature, (6) stream morphology, (7) changes in channel geometry, and (8) plant communities (quantities, types, growth stages and water use). Actions suggested were the following: (1) establish coordinated resource management groups, (2) continue efforts to estimate water benefits due to riparian area management, and (3) focus on vegetation changes rather than on structural control.
Pilgrim, D. H., D. D. Huff, and T. D. Steele. (1978). "A Field Evaluation of Surface and Subsurface Runoff. II. Runoff Processes," Journal of Hydrology, Vol. 38, pp. 319-341.
Field measurements using radioisotope tracers on a 18.3 x 48.8 m plot near Stanford, California provided detailed information on simultaneous surface and subsurface runoff processes. Runoff processes were shown to be markedly nonuniform, both spatially over the plot, and laterally and vertically within the soil. Hortonian overland flow, saturation overland flow, and subsurface throughflow were all observed on the plot. Subsurface throughflow refers to moisture that infiltrates into the surface soil horizon, but moves rapidly through macropores in a predominantly lateral direction, and quickly reaches the surface drainage network.
Subsurface stormflow is commonly referred to as interflow. Some researchers believe that unsaturated subsurface flow can produce long-term baseflow, but not interflow contributions to storm runoff. Other studies have suggested that temporarily saturated layers or pathways must occur above the water table during and immediately following a storm to allow appreciable lateral movement of subsurface water.
In addition to subsurface flow contributions, sources of intermediate-term storm runoff may include return flow from streambank storage, delayed outflow from storage in flood plains, and true surface runoff from areas with long delay times, such as ponds and marshes. Intermediate-term storm runoff is, then, very complex, encompassing a variety of processes. Several of these processes may be operating at the same time in a given watershed, and it is probable that different processes predominate in different watersheds, particularly under different climatological regimes or geological settings. It is also probable that one of these processes may be predominant in a given watershed at a certain time. Therefore, results from field studies tend to be site-specific.
An outstanding result from this field study was the observation of great spatial variability in the runoff processes, even though the plot was selected after an exhaustive search to find a site that appeared to be of uniform characteristics. Marked irregularities in infiltration and in surface and subsurface runoff were documented. The three types of runoff processes, Hortonian overland flow, saturation overland flow, and subsurface throughflow, all made significant contributions to total storm runoff.
Subsurface flow had a characteristically short time of travel, probably by finding its way through root holes, animal holes, and other macropores in the soil structure. During storm periods, lateral (i.e., downslope) flow occurred in a saturated layer located within the surface soil horizon. After an initial flushing effect, the subsurface flow consisted almost entirely of recently infiltrated water. The subsurface flow contained moderate concentrations of suspended sediment, not unlike those prevalent in typical surface streams under average runoff conditions. There is evidence to indicate that most of this suspended sediment originated in the ground surface, by entrainment following rainfall drop detachment.
Rorabaugh, M. I. (1963). "Estimating Changes in Bank Storage and Ground-Water Contribution to Streamflow," Publication No. 63, International Association for Scientific Hydrology, pp. 432-441.
A method for estimating groundwater outflow and forecasting the streamflow recession curve for a finite aquifer with parallel boundaries is described. Total groundwater outflow is treated in two parts. The component of outflow related to bank storage is computed from river fluctuations. The component of outflow related to recharge by irrigation and precipitation is computed from water levels in a well.
Assume a drainage basin having uniform, homogeneous, and isotropic characteristics (permeability, coefficient of storage, and aquifer thickness are constant in space and time); distances from stream to groundwater divides or geologic boundaries of no flow are equal at all places in the basin; and groundwater level is everywhere at stream level. The aquifer response to an instantaneous water table increase (recharge) of ho at time to is the following:
For Tt/(L2S) > 0.2:
q = 2T (ho /L) e -π2Tt / (4L2S) . . . . . . . . . . . . . . . . . . . . . (1)
For Tt/(L2S) < 0.2:
q = ho [ST/ (tπ)l/2 . . . . . . . . . . . . . . . . . . . . . (2)
in which q = groundwater discharge per unit stream length (one side) at any time t after to ; T = transmissivity; S = coefficient of storage; and L= distance from stream to groundwater divide.
Since the equations are linear, the principle of superposition is valid. Therefore, for a series of recharge impulses, the type curve (Eqs. 1 or 2) can be applied successively and the discharge at any time computed as the sum of the incremental discharges.
Equations 1 and 2 were used to forecast low flows for the Bitterroot Valley in western Montana. In 1961, the snowmelt season was of short duration, and bank storage effects represented about 7 percent of the total outflow. However, in 1962, the river remained at moderately high stages for four months, and bank storage effects were approximately 25 percent of the total outflow. Verification of the forecasts in the valley is not possible due to complex conditions. Forecasts have been 30 to 60 percent below the observed flows. The observed discharge includes groundwater flow plus random effects of rain or snowmelt occurring during the forecast period.
Stabler, F. (1985). "Increasing Summer Flow in Small Streams Through Management of Riparian Areas and Adjacent Vegetation: A Synthesis," in Riparian Ecosystems and Their Management: Reconciling Conflicting Uses, Proceedings, First North American Riparian Conference, April 16-18, Tucson, Arizona; also as General Technical Report RM-120, USDA Forest Service Rocky Mountain Forest and Range Experiment Station, Fort Collins, Colorado, pp. 206-210.
The potential for increased summer streamflows through construction of small instream structures, removal of streamside vegetation, and management of livestock grazing is investigated in this paper through a literature review and analysis.
Small dams (beaver dams, check dams, or gabions) may increase the size of the zone of saturation, or the saturated wedge, contained within the valley bottom. Small dams may be specially effective in increasing summer flows in streams in which channel downcutting has resulted in a greatly reduced saturated wedge. The possibility that channel downcutting may lead to loss of summer flows should be seriously considered.
Subsurface flow can be intercepted and transpired by vegetation on valley slopes to a much greater degree than commonly assumed. Therefore, removal of deep-rooted valley slope vegetation can reduce transmission losses as water moves toward the saturated zone in valley bottoms.
Studies of removal of woody riparian vegetation have led to increases in summer streamflow, but water savings have been at most moderate. On the other hand, it is now widely recognized that riparian vegetation may serve other useful purposes. Besides furnishing materials for beaver dams and debris jams, riparian vegetation provides shading and maintenance of a moist microclimate. Other water-conserving functions of riparian vegetation include the building of organic soils and the stabilization of stream channels to arrest channel degradation and stop further lowering of the water table. During overbank flows, riparian vegetation helps slow and spread flows and may also encourage a more efficient streambottom aquifer recharge.
The mechanisms by which summer flow increase might result from removal of livestock may be hypothesized. Removal of livestock often leads to reestablishment of beaver and woody riparian vegetation. In heavily gullied streams, recovery of riparian vegetation may lead to channel aggradation, improved channel morphology, and improved storage capacity. Removal of livestock may also result in increased groundcover and reduced soil surface compaction in areas contributing to streamflow, resulting in increased infiltration and soil moisture storage.
The information reviewed in this paper suggests that the complexities of streamflow generation and maintenance of summer flows in small upland watersheds in arid and semiarid regions are not fully understood. Where dry-season water is at a premium, an improved knowledge of summer streamflow generation could be of benefit in many ways.
Stallman, R. W., and I. S. Papadopulos. (1966). "Measurement of Hydraulic Diffusivity of Wedge-Shaped Aquifers Drained by Streams," U.S. Geological Survey Professional Paper 514, U.S. Government Printing Office, Washington, D.C.
Many unconfined aquifers are drained by perennial streams. Unconfined aquifers are usually recharged by infiltrated precipitation. A fraction of the infiltrated precipitation is returned to the atmosphere through evapotranspiration by vegetation. The remaining portion goes on to recharge the aquifers, eventually draining laterally into adjacent streams.
An important aquifer property is its hydraulic diffusivity, defined as the ratio of transmissivity T and coefficient of storage S. When geometric and site parameters are kept constant, the hydraulic diffusivity determines to a large extent the rate of aquifer drainage. The greater the hydraulic diffusivity, the faster the rate of aquifer drainage. Aquifer drainage results in a lowering of the water table and the associated depletion of groundwater resources.
Accurate measurements of hydraulic diffusivity are possible in aquifers which are: (1) hydraulically bounded by streams; (2) recharged by relatively infrequent additions of moisture; and (3) isolated by a thick unsaturated zone from diurnal and/or seasonal fluctuations in evapotranspiration.
This paper presents 120 aquifer response type curves to calculate hydraulic diffusivity as a function of the following data: (1) total observation well rise due to spring-thaw recharge; (2) difference between observed well level (at time t after end of recharge) and prerecharge well level recession trend; (3) distance from the observation well to the apex of an aquifer wedge drained by adjacent streams; (4) radius from the apex of the wedge to a circumference along which the water table levels are presumed to be unaffected by the aquifer drainage; and (5) distance and angular position between observation well, wedge apex, and general alignment of adjacent draining streams.
Given the distance from observation well to wedge apex and its angular position, and well level recession data (after spring-thaw recharge), the hydraulic diffusivity (T/S) of the aquifer can be calculated by using one of the type curves presented in this paper. Then, an estimate of coefficient of storage S allows the calculation of transmissivity T.
Stephens, D. B., and R. Knowlton, Jr. (1986). "Soil Water Movement and Recharge Through Sand in a Semiarid Site in New Mexico," Water Resources Research, Vol. 22, No. 6, June, pp. 881-889.
Recharge is the rate at which water is replenished in an aquifer. Groundwater recharge is generally one of the most difficult components of the hydrologic budget to quantify. In the southwestern United States, natural recharge is widely believed to be concentrated in mountain terrain where infiltrating snowmelt and runoff penetrate deep into the bedrock, along mountain-front alluvial fans where runoff percolates into permeable sediments, and along major ephemeral stream channels in valleys.
A type of natural recharge which is prevalent in humid and subhumid climates is diffuse recharge, caused by precipitation and snowmelt infiltrating over extensive areas and eventually reaching the water table. In semiarid climates, diffuse recharge is often assumed to be negligible. This is attributed to the following: (1) the extensive occurrence of near-surface caliche deposits, which are frequently considered to mark the downward limit of soil moisture movement; (2) vegetative landscapes featuring only the hardiest of drought-resistant species; (3) the dry appearance of surficial soils; and (4) the absence of significant amounts of groundwater discharge to support perennial streams. There have been only a relatively few field studies documenting diffuse aquifer recharge in arid and semiarid regions.
This paper presents results of monitoring of soil moisture to calculate diffuse recharge in the semiarid environment of the Sevilleta National Forest, near Socorro, New Mexico. Results show that net diffuse recharge does occur beneath a sandy, sparsely vegetated area in a semiarid climate where annual evapotranspiration greatly exceeds annual precipitation. Average annual diffuse recharge calculated over a 19-month period ranged from 0.7 cm/y to 3.7 cm/y. During the period of measurement, monthly average values of diffuse recharge varied by more than three orders of magnitude. Both winter frontal storms and intense summer thunderstorms were shown to lead to deep infiltration, and presumably, groundwater recharge.
While qualifying their study as preliminary, the authors conclude that sparsely vegetated, gently sloping, unconsolidated, permeable, sandy soils may be areas where significant amounts of diffuse recharge can occur. In regional groundwater basin studies, these possible recharge areas should be identified for the purpose of obtaining more reliable predictions of aquifer productivity and groundwater replenishment or depletion
U.S. Department of Agriculture. (1940). "Influences of Vegetation and Watershed Treatment on Runoff, Silting, and Stream Flow: A Progress Report on Research," Miscellaneous Publication No. 397, Washington, D.C., July, 80 pages.
This early document contains a thorough study of the relationship between surface and subsurface runoff, including their causes and effects, and reports on USDA research (up to 1940). First, it deals with the factors affecting infiltration, which increases subsurface runoff and augments baseflow while decreasing surface runoff and reducing peak flows. It discusses the effect of soil porosity, content of organic matter, plant roots, plant and animal life, terrain slope, and saturated subsurface flow on infiltration rates.
Tests have revealed a direct correlation between the soil's organic content and its water-holding capacity. Channels left by decayed roots encourage soil infiltration and increase subsurface water storage. While the roots are alive, they force their way into the soil. After the roots die, as happens more or less annually with the herbaceous vegetation and at longer intervals with the shrubs and trees, they decay, leaving channels through which water can penetrate into the soil.
In deep soils, particularly those covered by vegetation and a mat of litter, deep infiltration continues after the surface soil becomes saturated, until contact is made between soil water and groundwater. Water thus added to groundwater will rarely reappear in rivers in time to add to the crest of flood resulting from large amounts of surface runoff. In general, extremely dry soils are less capable of absorbing water than moist soils. Where soils are unable to absorb rainfall in appreciable quantities, the phenomenon is generally due to the disturbance of the normal infiltration processes in the surface layers. During intense rains large amounts of surface runoff frequently originate from sloping, cultivated fields in which dry soil is found 2 or 3 in. below the surface.
A certain degree of control can be exercised over rainfall after it reaches the land surface. Rainfall's usefulness to mankind is largely determined by its fate after it comes in contact with the land. If allowed to run-off over the surface it may cause erosion. If it is absorbed by the soil, its runoff is retarded, it becomes available for the growth of plants useful to man, and it may go on to replenish groundwater supplies. In time, a significant portion may reappear in streams, augmenting the low water flow to the benefit of downstream users.
This paper also discusses the role of vegetation in increasing subsurface runoff and decreasing surface runoff, including the effect of vegetation on infiltration, interception, reduction of soil evaporation, consumptive use, adsorption by plant litter, runoff and snowmelt retardation, and protection from freezing. For instance, experimental evidence shows that spring floods increase and summer flows decrease following extensive forest fires. Another section of the paper deals with consequences of change in vegetal cover, including clearing and cultivation, fire logging, overgrazing, and improper pasture management. The paper finishes with a section on runoff and erosion control by cropping practices and mechanical measures, for example, crop rotation, strip cropping, terracing, contour furrows, and structural controls.
U.S. General Accounting Office. (1988). "Public Rangelands: Some Riparian Areas Restored but Widespread Improvement Will Be Slow," Report to Congressional Requesters, GAO/RCED-88-105, Washington, D.C. June.
This General Accounting Office (GAO) report examines federal efforts to restore degraded riparian areas on public rangelands. The report discusses the progress achieved to date (1988) and the extent of the problem that remains. It also assesses constraints that will impede more widespread progress in the future.
Over the last 20 years, the USDI Bureau of Land Management (BLM) and the USDA Forest Service have restored a number of degraded riparian areas on public rangelands in the West. The successes, achieved primarily by improved livestock grazing management, demonstrate dramatically the extent of improvement that is possible. They also demonstrate that there are no technical barriers to improving riparian areas and that the basic restoration approaches used on successful projects can essentially be applied to all riparian areas on federal lands.
GAO reviewed 17 BLM and 5 Forest Service projects in which riparian areas have been restored. These projects were spread throughout the 10 western states to include a wide range of climatic and geographic conditions and to illustrate different techniques of riparian management. Although specific approaches to restoring riparian areas varied with the characteristics of the land, GAO noted that the overriding factor in achieving success was the improved management of livestock grazing to give the native vegetation more opportunity to grow. In sane cases, fences were built to keep the livestock out of the area, either permanently or until vegetation had recovered and streambanks were stabilized. In others, livestock continued to graze in the area, but their use was restricted by herding, fences, or a combination of both, or to a shorter period of time, a specific season, or only part of the area.
The report identified specific examples of successful efforts to restore riparian areas on public rangelands in the West. It also sought to determine why these efforts were successful, and whether the management techniques used on these successful efforts can be applied to the restoration of riparian areas throughout the West.
The report concludes that BEM and the Forest Service know how to restore riparian areas, with no major technical or scientific obstacles to overcome. The projects reviewed dramatically demonstrate the level of improvement that can be made in riparian areas to provide more forage for livestock, better habitat for wildlife, and other watershed and recreational values. In its introduction, the report reviews the significance of riparian areas and describes hydrologic and other related processes. The report specifically mentions the adverse effects of overgrazing on riparian areas, including increased bank erosion, lowering of the surrounding water tables, and changing "the whole character of the streams from perennially flowing to intermittent water courses that dry up in the summer months."
Van Haveren, B. P. (1986). "Management of Instream Flows Through Runoff Detention and Retention," Water Resources Bulletin, Vol. 22, No. 3, June, pp. 399-404.
Retention and detention structures and land treatments, implemented for soil and water conservation purposes, often have favorable effects on streamflow hydrographs. Decreases in peak flows and increases in low flows have been documented. This paper discusses runoff control as a means to restore or enhance instream flow values. Runoff control can be achieved with water retention and detention structures, land surface modifications, and/or vegetation management.
The chosen treatment will depend on watershed condition, desired hydrograph modifications, watershed size, and physical characteristics. Three general approaches are available to the land manager: (1) in-channel detention or retention of flow, (2) increase in depression storage, and (3) increase in infiltration capacity.
Detention or retention reservoirs, particularly those of large storage capacity in relation to the contributing drainage area, may be very effective in modifying the runoff hydrograph. Flood peaks are usually reduced, and the recession of the storm hydrograph is prolonged. Both detention and retention structures eventually fill with sediment, reducing or eliminating water storage capacity. However, a large sediment deposit behind a stable barrier dam may function as a shallow aquifer, yielding a dependable supply of water to downstream users, after an initial period required for law flow maintenance (aquifer saturation). As a byproduct of runoff detention and retention, the conversion of ephemeral and intermittent streams into perennial streams has been observed in selected soil and water conservation projects in several western states.
Increases in depression storage are commonly achieved with contour furrows and trenches. These have the effect of increasing ponding time and, therefore, total infiltration. Contour furrowing is probably the most common mechanical land treatment applied to rangelands in the United States. Contour furrows store less water than contour trenches, disturb less ground, and are less expensive to construct.
Increases in infiltration capacity can be achieved by vegetation management. However, the proper treatments tend to be site-specific, and, therefore, must be chosen with care. Infiltration is strongly related to vegetation cover, but the relationship is specific to each plant community. The relationship may also vary with season and storm characteristics. Cover appears to be very important in the early stages of a storm event, but not as important as soil factors in the later stages of the event. Before initiating a vegetation management project for runoff control and watershed improvement, it is necessary to perform a thorough analysis of soil, vegetation type, and infiltration relationships.
Wilcox, B. P., M. K. Wood, and J. M. Tromble. (1988). "Factors Influencing Infiltrability of Semiarid Mountain Slopes," Journal of Range Management, Vol. 41, No. 3, May, pp. 197-206.
Factors influencing the infiltrability of semiarid mountain slopes were studied in this paper. Infiltrability is defined as the infiltration flux resulting when water at atmospheric pressure is made freely available at the soil surface. The objective of the study was to determine the effects of selected site variables on the infiltrability of semiarid slopes with gradients varying in the range 0-70 percent.
The study was conducted in the northern Guadalupe Mountains of southeastern New Mexico. Field work was conducted on and adjacent to a great fault scarp known as the Guadalupe Rim. Elevation of the study area ranges from 1,200 to 2,000 m. The climate is semiarid and is characterized by relatively mild winters and warm temperatures throughout the year. Average annual precipitation is about 50 cm per year. Approximately 80 percent of the precipitation occurs between May and October. Most soils in the study area are shallow and are developed from dolomite or dolomite residuum. Textures are gravelly loams and gravelly clay loams. Soils are well drained with moderate permeability. Rock outcrops are common on steep slopes. Succulent desert and evergreen woodland formations are present in the study area, which is seasonally grazed by sheep and cattle at moderate stocking levels (up to 2 ha per animal). Mule deer are also abundant in the area.
A portable rainfall simulator was used to apply rainfall to plots of about 1 square meter in size. Infiltrability was calculated as the difference between application rate and runoff rate. Other components of the water budget (interception, surface detention, evaporation) were judged to be minor and, therefore, not explicitly accounted for in this study. Two years of data were used to draw conclusions regarding infiltrability of the study site.
Infiltrability was shown to be strongly correlated with total vegetal cover. The relative importance of grasses, shrubs, and litter was dependent on their respective abundance, especially grass. Basal vegetal cover was a poorer indication of soil infiltrability than aerial vegetal cover. Correlations between infiltrability and basal cover were nonsignificant. In general, vegetation influences surface hydrologic properties by decreasing velocity of overland flow, increasing surface roughness, and enhancing soil infiltrability by root activity and addition of organic matter. Vegetal cover also reduces the impact energy of raindrops, substantially reducing splash erosion and the formation of less permeable soil crusts.
Of all the soil variables measured, infiltrability was better correlated with soil depth (depth to bedrock). Soil depth limits soil water storage capacity, and, as storage capacity is reached, infiltrability slows. Infiltrability was shown to be negatively correlated with bulk density and rock cover, and poorly or negatively correlated with bare ground percentage. In addition, it was found to be positively correlated with slope gradient, possibly because interflow increases with slope.