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Supervisor Diane Jacobs
County of San Diego
Dear Supervisor Jacobs:
This letter provides comments in reference to the DRAFT document entitled "County of San Diego--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The document defines "sustainable yield" as "the amount of groundwater that can be withdrawn [from a basin] annually without producing an undesirable result" (page 6).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 strongly implies that current practice is to equate sustainable yield with annual recharge. It is now widely recognized that this may not the case. Most of the annual recharge feeds the surface water; therefore, the undesirable result (a reduction in the quantity of surface water) cannot be avoided when basing a policy of groundwater use on annual recharge.
The proposed 50% storage-volume policy stated in item 4.1 on page 22 is ill-conceived, patently out-of-date, and contrary to the stated definition of "sustainable yield" of page 6. 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. The terms "volume" and "recharge" should not be confused. If pumping recharge amounts can be shown to negatively affect local ecosystems and established surface-water rights, basing policy limits on storage-volume amounts (effectively, mining groundwater) will be even worse.
The proposed 50% storage-volume policy should be replaced with a more sensible policy that considers the experience of the past decade. There is a growing body of evidence that shows that sustainable yield should be taken as a fraction of the "recharge" amount, to be determined after detailed surface-water and ground-water studies. Caution is needed when dealing with this issue, since the negative effects of groundwater mining can be long ranging and long lasting.
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. (1999) (U.S. Geological Survey Circular 1186). This report describes "safe yield" as the maximum pumpage for which the consequences are considered acceptable. The burden of proof regarding groundwater sustainability is on the government to determine, based on basin-specific studies, if the consequences of pumping are acceptable.
Sincerely yours,
Victor M. Ponce
Professor of Civil and Environmental Engineering
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