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Supervisor Dianne Jacob
County of San Diego
Dear Supervisor Jacob:
This letter provides comments in reference to the DRAFT document entitled "Guidelines for Determining Significance and Report Format and Content Requirements--Groundwater Resources," prepared by the Land Use and Environment Group, and circulated for public review January 11-February 9, 2007.
The document defines sustainable yield as "the amount of groundwater that can be withdrawn [from a basin] annually without producing an undesirable result" (page 6). The reference for this statement is the authoritative book by Todd (1959). However, later in the same paragraph, it states that "the sustainable yield of a basin is often considered to be the annual average recharge for the basin." This statement implies that current practice is to equate sustainable yield with annual recharge, a concept which is flawed and has been widely discredited (see, for instance, M. Sophocleous: "From safe yield to sustainable development of water resources--The Kansas experience," Journal of Hydrology, 235, 2000). Significantly, Sophocleous argues that the sustainable yield of an aquifer must be considerably less than recharge if adequate amounts of water are to be available to sustain both the quantity and quality of streams, springs, wetlands, and ground-water dependent ecosystems.
The proposed 50% storage-volume policy stated in Section 4.1 of the Guidelines (page 22) is ill-conceived, patently out-of-date, and contrary to the stated definition of sustainable yield (page 6). We note that using 50% of aquifer storage volume as a basis for measuring groundwater exploitation defies every concept of sustainability painstakingly developed in the past two decades on a global reach. The terms "volume" and "recharge" should not be confused. If pumping annual recharge amounts can negatively affect local ecosystems and established surface-water rights, the pumping to a 50% aquifer storage-volume limit (that is, the mining of groundwater) will be even more negative.
The proposed 50% storage-volume policy should be repealed outright, and replaced with a more sensible policy that is in accordance with the times. There is a growing body of evidence that shows that sustainable yield should be taken as a fraction of the "recharge" amount, the fraction to be established after detailed conjunctive surface-water and ground-water studies (see, for instance, Hahn et al., in Environmental Geology, 33(1), December 1997). Caution is recommended when dealing with this issue, since the negative effects of groundwater mining can be long ranging and long lasting. At risk are the surface water itself, and the ecosystems and people who are relying solely on the local groundwater resources.
For further clarification on this issue, please refer to the authoritative U.S. Government report entitled "Sustainability of Ground-Water Resources" by Alley et al., U.S. Geological Survey Circular 1186 (1999). This report defines "safe yield" as the maximum pumpage for which the consequences are considered acceptable. The burden of proof regarding groundwater sustainability is on the government, which should determine, based on basin-specific studies, if the consequences of pumping are acceptable.
Sincerely yours,
Victor M. Ponce
Professor of Civil and Environmental Engineering
cc: Mario Covic, DPLU
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