1. INTRODUCTION
The Northeastern region of Brazil is home
to a large semiarid expanse referred to
locally as the sertão, or backland.
It encompasses about 900,000 km2 (347,000 mi2) in the
states of Bahia, Sergipe, Alagoas, Pernambuco, Paraíba, Rio Grande
do Norte, Ceará, Piauí, and Minas Gerais (Fig. 1). The region is subject to recurrent droughts, which are often
followed by floods. An individual drought or
flood event may not affect the entire semiarid
Northeastern region (referred to in Brazil as the Polígono das Secas, or Drought Polygon), but
may be of sufficient intensity and magnitude to
warrant emergency measures. Portions of the sertão
have recently (1990-93) experienced a severe 3-yr
drought, affecting close to 11 million people.
A question that recurs in scientific, professional,
and political circles is how to effectively
manage the cycle of droughts and floods of the
Brazilian Northeast. In the aftermath of the
Great Drought of 1877-79, the Brazilian government
initiated a series of policies and strategies
to combat the droughts. These policies
were aimed at providing the means to store
water for use during extended periods of
drought. After more than a century of experience,
the time is now ripe for a reassessment of
these policies. The aim is to seek a more sustainable
solution to the cycle of droughts and floods in the sertão.
"Sustainable" implies a long-term solution, with benefits measured in
decades and even centuries, rather than
months and years. The answer is seen to lie in
the conservation of soil, water, nutrients, and
native vegetation, aimed at arresting the current
spiral of environmental desiccation. This
will eventually make possible the reversion to
nano-, micro-, and regional climates of lesser aridity.
Fig. 1 Limits of drought polygon in the semiarid Brazilian Northeast.
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2. THE DROUGHTS OF THE BRAZILIAN NORTHEAST: HISTORICAL PERSPECTIVE
In the 17th century, at least six droughts
were documented in the Brazilian Northeast:
1603, 1606, 1614, 1645, 1652, and 1692
(Guerra 1981; Rebouças and Marinho 1972).
In his classic work Os Sertões, Cunha (1991)
lists the major droughts of the 18th and 19th
centuries: 1710-11, 1723-27, 1736-37,
1744-45,1777-78, and 1808-09, 1824-25,
1835-37, 1844-45, 1877-79. However, other
studies appear to indicate that the recurrence of
droughts in the sertão is once every eleven years on
the average. In the 20th century, the documented
drought periods are: 1903-04, 1915, 1919,
1930-32, 1942, 1953, 1958, 1970, 1979-83,
and 1990-93, with some spatial variations in
coverage within the drought polygon.
The drought of 1877-79, which killed more
than 500,000 people, prompted the Brazilian
government to formulate, for the first time,
policies and strategies to combat the effects
of the droughts. In 1881, a commission headed by
the French engineer J. J. Revy studied several
potential dam sites in Ceará, and in 1884 received
authorization to start construction of the Cedro dam in Quixadá.
In 1909, the Brazilian government established
the Inspectoria de Obras Contra As Secas
(Inspectorate of Public Works Against
Droughts) or IOCS, and charged it with coordinating
and unifying the government's actions
within the drought polygon, and with developing
a wide-range plan to combat the effects of
the climatic irregularities. The first Director of
IOCS, Miguel Arrojado Lisboa, commissioned
scientific studies aimed at providing baseline information
on the soil, water, and vegetation resources
of the region. The studies of Lofgren
and Luetzelburg on botany, and Crandall, Warring,
Small, Sopper, and Moraes on geology
and hydrogeology are still consulted to this date
(Guerra 1981).
In 1919, a significant drought year, Public
Law No. 3965 announced a renewed effort by
the Brazilian government, with Epitácio Pessoa
as president, to combat droughts by authorizing
the construction of public works needed to irrigate
arable land in the Brazilian Northeast. The
construction of ten major dams was started in
1921. Unfortunately, support for the effort
dwindled soon after the drought subsided.
In 1932, also a drought year, the Brazilian
government authorized the construction of several
public works in the affected region. The
now Inspectoria Federal de Obras Contra As Secas
(IFOCS) had the renewed charge to concentrate
exclusively on the building of dams/reservoirs
and irrigation works. Seventeen dams
were completed within this period. In 1937,
support for the effort dwindled again, as it became
apparent that the drought was no longer
an immediate problem (Guerra 1981).
After the drought of 1958, the Departamento
Nacional de Obras Contra Secas (DNOCS), the
successor to IFOCS, initiated the construction
of the great reservoirs of Orós, Banabuiú, and
Araras. With the importance of surface reservoirs
firmly established, DNOCS embarked on
a program of small dam construction, entitled
Açudagem em Cooperação (Cooperative Dam
Building). The dams were to be built often on
private land and with the cooperation of private
citizens, and with federal support amounting to
50 to 70 percent of the cost of the project.
In 1967, DNOCS established the program
Engenharia Rural (Rural Engineering), which
entailed the construction of small dams and
reservoirs in upland regions, underground
reservoirs, drilling of water wells, small irrigation
works, and rural construction and electrification.
The program, however, did not prosper,
apparently due to the lack of enthusiasm
shown by the regional offices charged with carrying
out the works. It has been now largely replaced by the
Projeto Sertanejo (Backlands Project).
In addition to dam and reservoir building,
the use of groundwater has been an effective
tool to combat droughts in the sertão.
In the neighboring state of Piauí, the existence of
sedimentary deposits enables a water well production
of 40,000 to 50,000 liters per hour. In
many areas, just one well has made possible the
livelihood of a small town. However, the same
cannot be said for those wells drilled in the
Cristalino, a metamorphic formation consisting
of gneiss, schists, and amphibolites, which underlies
most of the sertão. This water is of poor
quality, rich in salts, and unfit for human consumption.
Moreover, the production of these
wells is small, amounting to only 2,000 to
3,000 liters per hour.
3. CURRENT SCENARIO
For the past 100 years, combating droughts
in the Brazilian Northeast has been accomplished
primarily by providing surface water
storage. Thc reliance on surface reservoirs has
been referred to as the Açudagem policy. There
is now a growing concern that the Açudagem
policy may have focused on combating the effects,
while for the most part brushing aside
some of the underlying causes of the climatic irregularities.
An all-encompassing study of the causes of
droughts and floods in the Brazilian Northeastern
region remains to be carried out. A first level
of analysis is at the atmospheric mesoscale,
which humans seem to be all but powerless to
control. Ocean currents, wind circulation, and
the El Niño effect have been generally ascribed
varying responsibility for the climatic irregularities
which befall the Brazilian Northeast.
A second level of analysis is at the regional
scale, which is largely of geomorphological origin.
In the Brazilian Northeast, the coastal
mountain ranges act as barriers to ocean moisture,
effectively preventing large masses of
moisture-laden air from entering inland regions.
As with the mesoscale, humans appear to
be powerless to control this regional scale component.
A third level of analysis is at the microscale
and its sequel, the nanoscale. The microscale is
typically a second- to third-order catchment,
while the nanoscale is a zero-order catchment,
i.e., a small and distinct area of forest, pasture,
or agricultural land. Historically, it is at these
scales that humans have played a significant
role as "climate changers" [see Denevan's (1966)
classic account of how the Prehispanic Mojos
Indians of the Eastern Bolivian plains managed
to cope with adverse flood and drought conditions].
Through their actions spanning decades
and centuries, humans have been able to alter
the micro- and nanoclimates of the earth, for
better or for worse. Unfortunately, they may
have unwittingly contributed to the desiccation
of the environment. This process of environmental
degradation is now becoming all too
clear, although the basic tenets have been recognized
for quite a while.
In his Personal Narrative, the German scientist
Alexander von Humboldt (Humboldt and
Bonpland 1821) has keenly observed:
"When
forests are destroyed, the springs dry up entirely
or become less abundant. The river beds, remaining
dry during part of the year, arc converted
into torrents whenever great rains fall
onto the adjacent mountains. The sward and
moss disappearing with the brushwood from
the sides of the mountains, the waters falling in
rain are no longer impeded in their course; and
instead of slowly augmenting the levels of the
rivers by progressive filtration, they furrow during
heavy showers the sides of the hills, bear
down the loosened soil, and form those sudden
inundations that devastate the country. Hence
it results that the destruction of forests, the
want of permanent springs, and the occurrence
of floods, are three phenomena closely related together."
The currency of Humboldt's words and their
impact in a global context need no elaboration.
Their relevance to the drought and flood problem
in the Brazilian Northeast can be readily ascertained.
As long as the removal of the sertão's
native vegetation remains uncontrolled, the
micro- and nanoclimates of the region will
continue to experience a gradual process of desiccation,
eventually leading to more intense droughts and floods.
4. CLIMATE, VEGETATION, AND THE IMPACT OF HUMAN ACTIVITIES
Paradoxically, the Brazilian Northeast, with
its marked moisture gradients and diversity of
microclimates, is a region particularly well suited
to the study of the relationship between climate,
vegetation, and human activities. Duque
(1973) has suggested a classification of the various
ecological regions of the drought polygon,
in order of increasing moisture, into: (1) seridó,
(2) sertão, (3) caatinga, (4) cariri velho, (5) curimatau,
(6) carrasco, (7) cerrado, (8) serra, (9)
agreste, and (10) mata. This classification embodies
a spectrum of environmental moisture,
indeed a "field laboratory" where judicious observation
may reveal the interaction between
climate and vegetation, and the extent of
human influence.
Table 1 shows mean annual
rainfall and evaporation/rainfall ratios for several
ecological regions of the Brazilian Northeast.
This table shows that the evaporation/rainfall
ratio, a key climatic parameter, is a
function of type of vegetation, tending to decrease
with increasing environmental moisture,
as the vegetation changes from xerophytic to
mesophytic. In Duque's study (1973), evaporation
is interpreted as measured pan evaporation.
TABLE 1. Mean annual rainfall and evaporation/rainfall ratios in several ecological regions of the Brazilian Northeast (Duque 1973).
|
Region |
Location |
Data years |
Rainfall (mm) |
Evaporation/ rainfall ratio |
Seridó |
Cruzeta, RN |
1940-46 |
497 |
5.8 |
Seridó |
Quixeramobim, CE |
1912-58 |
750 |
2.5 |
Caatinga |
Francisco Floresta, PE |
1939-58 |
395 |
4.8 |
Caatinga |
Monteiro, PA |
1942-54 |
489 |
3.6 |
Caatinga |
Paratinga, BA |
1947-55 |
659 |
3.2 |
Caatinga |
Barra, BA |
1946-54 |
692 |
2.5 |
Caatinga |
Juazeiro, CE |
1940-54 |
800 |
2.5 |
Caatinga |
Ibipetuba,BA |
1939-58 |
844 |
2.2 |
Sertão |
Souza, PA |
1939-58 |
750 |
2.5 |
Sertão |
Iguatu, CE |
1912-56 |
838 |
2.2 |
Agreste |
Natal, RN |
1940-57 |
1038 |
2.0 |
Agreste |
Conquista, BA |
1931-54 |
680 |
1.8 |
Agreste |
Pesqueira, PE |
1912-43 |
713 |
1.7 |
Agreste |
Jacobina, BA |
1945-55 |
893 |
1.5 |
Agreste |
Itaberaba, BA |
1954 |
942 |
1.3 |
Mata |
Itabaianinha, SE |
1945-55 |
997 |
1.1 |
Mata |
Ibura, PE |
1945-57 |
1500 |
0.9 |
Mata |
Aracajú, SE |
1945-55 |
1274 |
0.9 |
Mata |
Cruz das Almas, BA |
1950-55 |
935 |
0.8 |
Mata |
Maceió, AL |
1923-54 |
1300 |
0.7 |
Mata |
Teresina, PI |
1911-54 |
1390 |
0.7 |
Mata |
Ondina, BA |
1945-55 |
1831 |
0.5 |
The German botanist Luetzelburg (1923)
was able to document the relationship between
climate, vegetation, and human activities in the
sertão. He observed:
"The decimating of forests
depleted the soil of organic salts, which were
washed away by the torrential rains. Extraneous
species invaded abandoned agricultural
lands, gradually leading to a more xerophytic
landscape, in tune with the increasing aridity
of the soil. In this way the vegetation of today
was gradually formed: a caatinga or carrasco
where before there was mata with complete soil
cover by vegetation."
A close relationship exists between climate
and vegetation. The question is the extent to
which human activities can influence this relationship
at the various spatial and temporal
scales. The complexity of the natural environment,
with its myriad of physical, chemical,
and biological interactions, and the practical
impossibility of gathering adequate data on a
long-term basis does not permit a clear answer
to this question.
5. EVAPORATION VS. EVAPOTRANSPIRATION
To understand the relationship between climate
and vegetation, we examine the roles of
evaporation and evapotranspiration in the context
of the mesoscale and regional hydrologic
balances. To provide a common ground, these
and related terms are defined here:
Evaporation is a physico-chemical process by
which water from water bodies and the surface
and near-surface of the earth is vaporized
and returned to the atmosphere.
Transpiration is a physiological process by
which plants pump moisture from the soil
and transport it upwards through their roots
and stems towards the leaf surface, where it
becomes available for evaporation.
Evapotranspiration refers to the combined effect
of transpiration and evaporation.
Potential evapotranspiration is the evapotranspiration
that would take place under conditions
such that there is an ample supply of
moisture at all times.
Actual evapotranspiration is the evapotranspiration
that would take place under conditions where moisture is limiting.
In a terrestrial ecosystem, evaporation is produced
from three sources:
From vegetated surfaces, through transpiration,
From nonvegetated surfaces, i.e., bare soil and cultural features
of the landscape (pavements, roof tops, etc.), and
From water bodies, such as natural lakes,
rivers, and surface reservoirs and ponds.
In hydrologic practice, however, usage of the term
"evaporation" is restricted to that of an aquatic
ecosystem, i.e., a water body such as a lake, river,
reservoir, or ocean. On the other hand, the term
"evapotranspiration" refers to the total
evaporation from a terrestrial ecosystem, which comprises:
Evapotranspiration proper, i.e., the evaporation
from vegetated surfaces; and
Evaporation, i.e., the evaporation from nonvegetated
surfaces and the evaporation from water bodies.
6. GLOBAL AND REGIONAL HYDROLOGIC BALANCES
The Earth's land areas are divided into:
(1) endosed areas, which have no runoff; and
(2) peripheral areas, which have measurable runoff.
Of the precipitation falling on the Earth's land
area on an annual basis, on the average, 64 percent
is returned to the atmosphere by evaporation
and evapotranspiration, and 34 percent is
delivered to the oceans as runoff (i.e., river discharge).
The remaining 2 percent constitutes
direct groundwater flow into the oceans
(L'vovich 1979). Since the latter is relatively
small compared to the other three (evaporation,
evapotranspiration, and runoff), it is customary
to neglect it on practical grounds.
In regional context, the hydrologic balance
is driven by the prevailing climate. In semiarid
regions, the sum of evaporation and evapotranspiration
is high relative to precipitation, on the
order of 90 percent. Conversely, in humid regions,
the sum of evaporation and evapotranspiration
is likely to be about 50 percent
(USDA 1940; L'vovich 1979). The comparison
is more dramatic when it is recognized that a
10 percent runoff in a semiarid region may
amount to only 50 mm yr1 (2 in yr1), while a 50
percent runoff in a humid region may reach
1,000 mm yr1 (40 in yr1).
An exception to the aforementioned pattern
is provided by the swamps and marshes, usually located
in subhumid and humid regions, where the sum
of evaporation and evapotranspiration is typically
more than 90 percent of precipitation, with
runoff being reduced to less than 10 percent.
This is due to the small or negligible surface
gradient, which acts to discourage runoff while
encouraging evaporation and evapotranspiration.
A routine hydrologic budget does not
distinguish between evaporation and evapotranspiration
proper, and often uses either term to refer
to the combined process. Furthermore, the
combined amount is considered a loss, at least
as far as the terrestrial ecosystem is concerned.
However, it is noted that the answer to the
relationship between climate and vegetation lies in
the fundamental distinction between evaporation
and evapotranspiration. In semiarid regions,
evaporation is the dominant process,
with evapotranspiration being the actual
evapotranspiration, which falls well below its potential
value. Conversely, in humid regions, evapotranspiration
is the dominant process, readily
reaching its potential value, while evaporation
is restricted to that of water bodies only.
A hydrologic balance in a semiarid region,
with 500 mm (20 in) of annual precipitation,
may be as follows:
70 percent of precipitation going to evaporation;
20 percent to actual evapotranspiration, primarily of xerophytes; and
10 percent to runoff.
Conversely, the hydrologic balance in a humid
region, with 2,000 mm (80 in) of annual precipitation,
may be as follows:
40 percent of precipitation going to the potential
evapotranspiration of mesophytes and hydrophres;
10 percent to evaporation, mostly from water bodies; and
50 percent to runoff.
The special case of a wetland maybe as follows:
40 percent of precipitation going to evaporation,
mostly from water bodies,
50 percent going to potential evapotranspiration, primarily of hydrophytes, and
10 percent to runoff.
As shown in Table 1, the seridó, an arid
ecosystem, has a high evaporation potential
(evaporation/rainfall ratio in the range 2.5
to 5.8); therefore, little runoff. In this case, evaporation
is high, while actual evapotranspiration
from the sparse xerophytic vegetation is low.
Conversely, the mata, a humid ecosystem, has a
low evaporation potential (evaporation/rainfall
ratio typically less than 1.0); therefore, substantial
runoff. In this case, evaporation, mostly
from water bodies, is low, while evapotranspiration
from the dense mesophytic vegetation is
high, readily reaching its potential value.
Figure 2 shows a graphical interpretation of
the components of the water balance for the
range of climates experienced in the sertão,
showing the unmet potential for evaporation
or evapotranspiration. This unmet potential is
substantial for arid and semiarid ecosystems,
and close to zero for humid ecosystems. Thus,
evapotranspiration and evaporation are seen to
be quite different processes. Evapotranspiration
is intrinsic to the development of the phytomass,
while evaporation is not. Evapotranspiration
is a result of life on this planet, while
evaporation proceeds in the absence of life.
Evapotranspiration enhances soil formation
and preservation, while evaporation does not.
Evapotranspiration results in the production of
food and other biomass useful to animals,
while evaporation does not. Evapotranspiration
directly sustains life, while evaporation does not.
Fig. 2 Graphical model of the water balance for a wide range of climatic conditions.
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The albedo of vegetated surfaces, as well as
that of water bodies, is lower than the albedo
of nonvegetated surfaces. Thus, vegetated surfaces
and water bodies are able to store more
incoming short-wave energy, and to release it
gradually as outgoing long-wave heat, particularly
at night, where the long-wave heat dominates
the radiation balance. This warming of
the lower atmosphere causes air lifting, favoring
the condensation of water vapor and the
formation of rainfall (Balek 1983). Thus, the
atmospheric moisture resulting from evapotranspiration
and evaporation from water bodies
has a tendency to condense and precipitate
back, while that resulting from evaporation
from nonvegetated surfaces does not. The
denser and greener the vegetative surface, the
more marked is the recirculation effect (Salati
et al. 1978; Salati et al. 1979; Salati and Vose 1984).
On the basis of the foregoing analysis, it is
concluded that arid and semiarid ecosystems
should be managed with the objective of maintaining
or enhancing the ratio of evapotranspiration
to evaporation. To the extent possible,
this entails preserving native vegetation to
achieve a balance between natural (forest
and/or range) and human-induced ecosystems
(agricultural and/or urban). An increase in this
ratio, through conservation management, increases
the biotic potential of the ecosystem,
from which all forms of life ultimately benefit.
Conversely, a decrease in this ratio, through deforestation,
overgrazing, and other forms of
land degradation, decreases the biotic potential
of the ecosystem, eventually compromising all forms of life.
It is seen that evapotranspiration and evaporation
are quite different processes; thus, the
justification for accounting for them separately,
particularly in the management of water
resources in arid and semiarid regions.
It follows that human misuse of natural vegetation
leads to the eventual replacement of
evapotranspiration by evaporation, to an alteration
of the nano-, micro-, and eventually even
regional climates on a long-term basis, and to
the gradual desiccation of the environment,
typically from subhumid to semiarid, and from
semiarid to arid.
Arid and semiarid regions have irregular
rainfall patterns that lead to recurrent
droughts, often followed by destructive floods.
Subhumid regions are less affected by
droughts, while humid regions are almost free
from them. In the Brazilian Northeast, the
seridó is characterized by a rainfall regime
having 8 dry months per year, and the caatinga 7
to 9 dry months per year. On the other hand,
the mata has only 4 to 5 dry months per year
(Duque 1973). This seasonal variability does
not portray the whole picture, because the
daily variability may be even more marked.
Duque (1973) has concluded that a drought
year in the sertão may be that year where it
rains about half of the annual rainfall in just 1
month, and about half of that month's rainfall
in just 1 day.
7. THE CASE FOR CONSERVATION MANAGEMENT
In his studies of the sertão hydrology, Silva
(1937) observed:
"Forests conserve the ambient
freshness, and evaporation within the forests is
much less than evaporation on the plains.
Forests retain moisture, only to release it gradually
to ambient air and soil. It is evident, then,
that forests exert a beneficial action upon the
hydrologic regime, regularizing it."
Thus, the objective of conservation management
of an arid/semiarid ecosystem should be
to preserve the ratio of evapotranspiration to
evaporation. This entails the establishment of
conservation policies, and the development of
strategies and technologies to conserve water,
soil, and nutrients. In regions already degraded
by human activities, conservation management
may take a more active role and be aimed at
stopping and reverting the degradation process,
eventually to return the degraded ecosystem to
its former state of productive stability.
The conservation of water, soil, and vegetation
is a complex undertaking. Its Achilles heel
is its distributed nature, i.e., the fact that it
works better when it is extensively executed at
the micro- and nanoscales. Historically, it is at
these scales that humans have played a role as
climate changers. Therefore, it follows that if
humans, through their actions, can contribute
to the desiccation of the climate, they can also,
with renewed will, contribute to the humidification
of the same. Thus, native vegetation
must no longer be looked upon as merely a resource
to be exploited, but also as nature's
instrument for the regularization of the climate.
In a societal context, the conservation of
water, soil, and vegetation is based on federal,
state, and local policies and laws and aimed at
preserving a healthy balance between natural
ecosystems and their human-induced counterparts,
and to manage all with the overall
objective of conservation.
In a technological context, the conservation
of water, soil, and vegetation is based on the
following principles:
The avoidance of splash erosion. This is
accomplished by covering the earth's surface
with vegetative canopy and/or litter, in order to
minimize splash erosion by raindrop impact.
Splash erosion is not only soil loss in itself, but
may also be a cause of additional soil loss, by
contributing to the sealing of soil surfaces,
which greatly increases runoff (USDA 1940; Le Bissonnais and Singer 1993).
The decrease in the speed of surface
runoff. This limits the potential of surface
runoff to concentrate, that is, to reach a high
discharge lasting a relatively short period of
time, and minimizes floods. The decrease in the
speed of surface runoff is accomplished by:
Covering the earth's surface, preferably with
live short vegetative canopy, to provide a
myriad of obstacles in the path of runoff, thereby
increasing overall roughness and flow depths, and
thus, decreasing the flow velocity; and
Modifying the micro- and nanorelief of the
earth's surface, to reduce surface runoff slopes
to manageable levels by means of terracing,
small detention and retention ponds, plowing
on the contour, tree basins, land imprinting
(Anderson 1987), and other similar measures.
The decrease in the volume of surface
runoff. This is accomplished by covering the
earth's surface with vegetation. The vegetation
not only protects the earth's surface against
raindrop impact and reduces the speed of surface
runoff, but also increases infiltration rates
and amounts, through macropores left by live
and dead roots, and through the enhancement
of soil structure by the preservation of soil flora,
fauna, and related humic processes. Generally,
the greater the size and density of vegetation
and the greater the biological activity in the
underlying soil, the greater is the "sponge" effect
of a vegetative ecosystem.
To summarize, an arid/semiarid ecosystem
managed for conservation should preserve the
ratio of evapotranspiration to evaporation. This
entails putting in place the strategies and
technologies to conserve water, soil, and nutrients.
Can a terrestrial ecosystem be managed for
conservation while at the same time increasing
the availability of water resources for human
use? The answer to this question remains elusive
(Balek 1983). In the long term, the availability
of more water for evapotranspiration results
in the availability of more water for
runoff, in a sort of multiplying effect
resembling a cybernetic machine. This is due to the
increased rate of soil creation, resulting in
increased catchment wetting and increased
replenishment of groundwater (L'vovich 1979;
Ponce and Shetty 1995). In other words, the
more water going to evapotranspiration, the
more water there will be available for runoff in
the long run. The converse bears repeating: The
less water going to evapotranspiration, the less
water there will be available for runoff.
To elaborate on this point, we compare typical
runoff coefficients for the arid/semiarid and
subhumid/humid regions of the globe. Basins
in arid/semiarid regions have runoff coefficients
of 0.05 to 0.2, while basins in subhumid/humid
regions have runoff coefficients of 0.3
to 0.5, and may be even highs in certain cases
(L'vovich 1979). To give an example, the runoff
coefficient of Riacho Feiticeiro, a tributary of
the Jaguaribe river, in the heart of the sertão of
Ceará, is 0.09. On the other hand, the runoff
coefficient of the humid Amazon basin is 0.45
(Salati et al. 1979). Yet the evapotranspiration
of humid regions greatly exceeds the actual
evapotranspiration of arid regions. The culprit
loss is seen to be not evapotranspiration, but
rather the high evaporation from the nonvegetared
surfaces which prevail in arid/semiarid regions.
Thus, the case is made for the conservation
of arid/semiarid terrestrial ecosystems as
the preeminent tool of long-term climatic management.
8. CASE STUDY 1: THE AMAZON BASIN'S RAINFALL AND EVAPOTRANSPIRATION REGIME
The Amazon river basin comprises an area of
approximately 7,000,000 km2 (2.7 million mi2)
in the central and eastern portions of South
America, encompassing parts of Venezuela,
Colombia, Ecuador, Peru, Bolivia, and Brazil.
The region is s humid ecosystem comprising a
tropical rainforest with an abundance of rainfall.
Rainfall, however, is not evenly distributed
in the basin. At Belém, Pará, close to the mouth
of the Amazon river, mean annual rainfall is
2,600 mm (102 in). In the northwest, close to
the Brazil-Colombia border, it attains a maximum
of 3,600 mm (142 in). Yet toward the
south, in the region transitional to the Brazilian
shield, the mean annual isohyets are around
2,000 mm (79 in) (Sioli 1985).
A characteristic of this huge humid terrestrial
ecosystem is its rapid recirculation of moisture,
from rainfall to evapotranspiration, and again
to rainfall. Salati et al. (1978; 1979) and Salati
and Vose (1984) have revealed that approximately
50 percent of the rainfall in the lower
Amazon basin returns directly to the atmosphere,
only to condense and fall again. In this
way, differences in precipitation between wet
and dry seasons are substantially attenuated,
and the intensity of droughts and floods is diminished.
It is clear, then, that the Amazon rainforest
ecosystem influences its own climatic and rainfall
regimes, and that at least in this case, more
vegetation meths more evapotranspiration, and
more evapotranspiration means more rainfall and
certainly more runoff. It is noted that the
Amazon river basin has a mean annual runoff
estimated at 220,000 m3s-1 at its mouth (UNESCO 1978),
amounting to roughly one-sixth
of the total runoff contributed to the oceans by
the Earth's landmasses.
9. CASE STUDY 2: THE UPPER PARAGUAY RIVER BASIN EVAPORATION/RAINFALL RATIOS
The Upper Paraguay River basin comprises
an area of 496,000 km2 (192,000 mi2) in southern
central South America, 80 percent of
which lies in Central Western Brazil
(Mato Grosso and Mato Grosso do Sul) and the
remaining 20 percent in eastern Bolivia. The
basin is home to the Pantanal of Mato Grosso,
the largest wetland in the world, encompassing
136,700km2 (52,800 mi2). The word "pantanal"
means great swamp, implying that the region is
subject to extensive and recurrent flooding.
However, large portions of the Pantanal are
flooded only during the annual crest of the
Upper Paraguay River and its tributaries, and
much nonflooded terra firma is interspersed
throughout the region. The mixture of permanent
swamp, seasonal swamp, and terra firma,
as well as the contiguity of the Pantanal to four
major South American ecosystems (the tropical
Amazon rainforest to the north and northwest,
the subhumid savanna woodlands of Central
Brazil to the northeast, east, and southe, the
humid Atlantic forest to the south, and the
semiarid scrub forest of Eastern Bolivia and
Western Paraguay to the west and southwest)
has contributed to the richness and variety of
its vegetation and fauna.
The Upper Paraguay River basin features
marked climatic gradients, from humid and
subhumid in the uplands surrounding the Pantanal
to the north, east, and south, to semiarid
in the basin center lowlands. Mean annual
precipitation varies from 1,800 mm (71 in) in the
Chapada dos Parecis surrounding the Pantanal
to the north, to 850 mm (33 in) at the mouth of
the Taquari River in the basin center. Annual
potential evapotranspiration varies from
1,300mm (51in) in Chapada dos Parecis to
1,400mm (55in) at the mouth of the Taquari
River. Therefore, the evapotranspiration/ rainfall
ratios are 0.72 for the Chapada dos Parecis, and
1.65 at the mouth of the Taquari River (Alfonsi
and Camargo 1986; Alvarenga et al. 1984).
It is seen that the humid upland regions
surrounding the Pantanal have consistently low
evapotranspiration/rainfall ratios (0.72), while
the semiarid lowland region located in the basin
center has a high evapotranspiration/rainfall
ratio (1.65). The more mesophytic the vegetation,
the less the potential (and actual)
evapotranspiration, with evaporation reduced to that
of water bodies only. Conversely, the more xerophytic
the vegetation, the greater the potential
evaporation, with evapotranspiration reduced
to the actual evapotranspiration of the
xerophytic plants.
10. CASE STUDY 3: SOIL AND WATER CONSERVATION WITH BRAZIL'S NATIONAL PROGRAM OF MICROBASINS
In 1975, the government of the state of
Paraná, Brazil, tied the awarding of agricultural
credits to the execution of soil conservation
works. The program, however, had limited success
because the conservation works were performed
using the farmer's property, not the
encompassing watershed, as a unit of management.
In this way, an individual farmer's conservation
solution became his neighbor's or somebody
else's problem. Gradually, is became
apparent that this straregy was doomed to failure.
Learning from this mistake, the focus of
conservation management eventually shifted from
each individual property to the microbasin as a
whole, the latter encompassing a watershed of
about 2.500 ha (6.175 ac). This led to the establishment,
in 1987, of the Programa Nacional de Microbacias Hidrográficas (PNMH)
(National Program of Microbasins). Among the specific
objectives of PNMH are:
An adequate management of renewable
natural resources, primarily soil and water;
A decrease in the risks of droughts and
floods; and
A reduction of the processes of soil
degradation, principally erosion.
In Cruz Alta, Rio Grande do Sul, Brazil,
the techniques of conservation management have
been applied since 1985. One of the first
conservation measures was the construction of
terraces across property lines, with the fences
eliminated. In this way, water retention was
improved, with the land now able to soak up
100mm (4 in) of rainfall without appreciable
runoff. Other applicable soil and water conservation
measures were also implemented, and
the success of the program did not wait to be
announced. To date, the Programa Estadual de
Microbacias Hidrográficas (State Program of
Microbasins) of his Grande do Sul reaches 155
municipalities, comprising 315 microbasins,
12,000 families, and 294,000 ha (726,500 ac) of
land. In addition to soil conservation, water
was apparently also conserved, because the
productivity of soy beans increased 54 percent, corn 85
percent, wheat 40 percent, and beans 74
percent (Globo Rural 1993).
In the state of Paraná, the Sáo Roque microbasins,
in the municipality of Realeza, close
to the Argentine and Paraguayan borders, has
been applying conservation measures for the
past few years. The basin is located in steep
and rocky terrain, not suited for agriculture
in the absence of management. The renewed emphasis
on soil and water conservation has now
turned viable all the small rural properties of
the region. The construction of terraces to
hold on to the water and soil and the application
of techniques such as soil decompaction,
crop rotation, and green manure has doubled
the productivity of the soil in just 2 to 3 years.
Other examples, such as that of Tupãssi in
western Paraná, show that conservation management
works by improving water retention,
while at the same time reducing erosion.
Agricultural credits were tied to the construction of
tcrraces, and farmer support quickly ensued.
The land is now able to sustain 150 mm (6 in)
of rainfall in a few hours, without surface
runoff (Globo Rural 1993).
The success of PNMH in the Brazilian South
shows conclusively that conservation management
can lead to the replacement of evaporation
by evapotranspiration, even in the case of
the human-induced agricultural ecosystems.
11. A BLUEPRINT FOR THE SERTÃO'S SURVIVAL
The case for conservation management having
been made, we now outline a comprehensive
strategy to manage the cycle of droughts
and floods in the semiarid Brazilian Northeast.
The strategy consists of the following five measures,
in order of perceived importance:
Conservation management,
Xerophytic silviculture and agriculture,
Surface water storage,
Subsurface water use, and
Interbasin water transfer.
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The keystone strategy is conservation management,
without which the rest are eventually
doomed to failure. Conservation management
entails the conservation of the water, soil, and
nutrient resources of the semiarid Brazilian
Northeast, with the objective of preserving the
ratio of evapotranspiration to evaporation. In
practice, this means the formulation of policies
and laws, and the implementation of strategies
and technologies to conserve natural vegetation,
and to retain water, soil, and nutrients. Agricultural
practices should focus on the techniques
of sustainable agriculture, without which soil,
land, and environmental degradation soon set in.
Duque (1949; 1964; 1973; 1982) pioneered
the Lavouras Xerófilas, i.e., the practice of silviculture
and agriculture using native vegetation,
well adapted to the harsh climatic conditions in
the sertão. Some of that xerophytic plants are
able to survive drought conditions by accumulating
reserves within their organisms, others by
consuming relatively small amounts of water,
and yet others by absorbing moisture from the
atmosphere at night. The lavouras xerófilas are
indeed "cashcrops" for export, as well as
valuable raw materials suitable for diverse uses.
Duque (1982) has observed:
"It was the cotton
of good quality, the oiticica of the alluvial
plains, the mamona of the arenaceous shales,
and the caroá of the caatingas that allowed industrious
northcasterners to create the industrial
park of textiles, vegetable oils, dyes, etc., that
provide an honest occupation to the working
classes, adding value to the finished products."
Duque (1964; 1982) points out the many
advantages of xerophytic crops; they are:
Drought resistant,
Perennial,
Generate export products,
Encourage the development of local industry,
Provide canopy coverage for erosion control, and
They are well known to local farmers.
The benefits of conservation management
are a function of the scale and intensity of
management. While the benefits of small-scale
conservation projects are often observed immediately,
the benefits of large-scale projects may
take years, or even decades, to accrue. In the
meantime, a policy of surface water storage to
ameliorate the effects of droughts and floods
should be in place. This policy provides for immediate
drought and flood relief; however, as
the sole policy for drought and flood control it
is counterproductive because it encourages
evaporation from water bodies to the detriment
of evapotranspiration. It is noted that a drop of
water that evaporates directly is a drop that
does not evapotranspire; therefore, it does not
concribute to biomass production. In addition,
human populations have a tendency to concentrate
in the vicinity of reservoirs, increasing the
pressure on the fragile semiarid ecosystems.
Close to 3 trillion cubic meters of water lie at
various depths in the Brazilian northeastern
sertão. (Globo Ciencia 1993). These deposits are
located primarily along fracture zones of the
Cristalino and in sedimentary formations. It
has been conservatively estimated that about
two billion cubic meters of this water can be
exploited annually for irrigation, without
appreciable groundwater depletion. Yet some scientists
readily point out that the groundwater of the
Cristalino is saline, making it unfit for agricultural
or human use. With appropriate resource-depletion
regulations in place, and with
improved desalinization technologies, the
groundwater of the region may eventually
contribute its share to the sustainable development of the sertão.
Interbasin water transfer is yet another strategy
to cope with the sertão's drought problems.
After Silva (1937), who outlined the transfer of
water from the São Francisco River basin, other
equally ambitious schemes have been proposed,
including transfering water from the Parnaiba
and Tocantins Rivers, located to the northwest,
in the states of Piauí and Maranhão. Yet, to this
date, water remains to be imported to the sertão.
The idea of importing water to the thirsty
sertão merits further analysis. Basic ecological
reasoning helps throw light on this issue. The
sertão is semiarid, and the lack of water is endemic
to it. The Parnaiba and Tocantins river
basins are humid, and they have an ample supply
of water. Experience tells us that tropical
semiarid environments, when managed properly,
tend to be healthier than their humid counterparts.
In conditions of water surplus, the
Amazon rainforest being a good example, many
species thrive, including insects and others that
prove harmful to humans. More importantly,
however, tropical humid ecosystems such as the
Amazon basin tend to be poor in exportable
nutrients, relying instead on the immediate
recirculation of a relatively small quantity, to be
used over and over (Salati and Vose 1984).
Thus, the human species has a natural tendency
to strive toward the middle of the climatic spectrum,
readily populating semiarid and subhumid
regions, where the chances of livelihood
and ultimate survival are enhanced (Lugo and Morris 1982).
12. CONCLUDING REMARKS
The Brazilian Northeastern region and its
growing population will continue, for the foreseeable
future, to be saddled with the prospect
of droughts and floods. The laissez-faire
approach to the problem is to continue to build
surface storage features to store ever more
water, while the water itself remains a scarce
and vanishing commodity.
An integral approach to the problem leads to
a fivefold strategy, with conservation management
as the keystone. An increased reliance on
xerophytic agriculture, the judicious use of surface
and groundwater, and the eventual
importation of water from neighboring humid basins
complete the overall strategy.
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