Careful How You Define “Sustainable Yield” and “Safe Yield”

Tom Maddock, department head, University of Arizona’s Department of Hydrology and Water Resources, contributed this Guest View.

The U.S. Fish and Wildlife opinion requires that the Fort Huachuca area reduce water use to “sustainable yield” by 2011. Some may interpret this to mean if the annual groundwater pumping equals the annual amount of recharge, the system will be operating in “sustainable yield” mode. Such an operating procedure will not sustain the San Pedro River, its associated riparian ecosystem, or even its groundwater supply.
The San Pedro River Basin hydrology is simple in concept but complex in detail. Basically, precipitation falls in the mountains, flows down the mountain streams to the alluvial fans in the pediment, and infiltrates into the groundwater system where it slowly flows within an aquifer. Ultimately, it discharges to the riparian system, either as base flows to the stream or evapotranspiration from the plants, trees and bare ground.

Prior to development, the natural recharge processes of the San Pedro River Basin were in equilibrium with the natural discharge processes. There were wet years when the recharge exceeded the discharge, but these were balanced by the dry years when the discharge exceeded the recharge. The point is that the river and its ecosystem were sustained by the aquifer discharge which, on the average, was equal to the aquifer recharge. The system was in balance. But consider this: If one pumps all the natural recharge, where is the water for the discharge to the stream and its riparian ecosystem? There isn’t any! Rather than saving the riparian system, pumping all the groundwater recharge every year ultimately guarantees its death.

One should not confuse sustainable yield with safe yield, although both concepts lead to the detriment of riparian systems. To define safe yield, we need to define capture. The groundwater pumped by a well from an aquifer is either derived from a decrease in storage in the aquifer, a reduction in previous discharge from the aquifer, an increase in the recharge, or a combination of these changes. Capture is defined as the increase in recharge plus the decrease in discharge. Figure 1 illustrates the sources of capture: 1) an increase in groundwater recharge from losing streams (or increased infiltration), (2) a decrease in groundwater discharge to gaining streams (or interception of baseflow) and (3) the reduction in the component of evapotranspiration that is derived from the saturated zone.

If we restrict the groundwater pumping to the capture, there will no longer be a decrease in storage. Because the loss of storage associated with groundwater pumping manifests itself as cones of depression, no further loss of storage means no further increase in the well’s cones of depression. Restricting the groundwater withdrawals to what may be captured is a definition of safe yield for the aquifer. However, since the capture sources are from the stream and the riparian ecosystem, such a restriction of pumping is not a safe yield for the stream or riparian ecosystem! Not only applicable to issues on the San Pedro, sustained yield and safe yield concepts apply to other basins within the Southwest.

Obviously, there are additional complexities to the hydrologic system and its water balance. Some have suggested that waste waters be treated and either recharged to the aquifer or discharged to the river. However, the waste waters are always going to be less than the waters extracted because of consumptive use, so there will always be a loss in groundwater storage even if all the treated effluent is injected back into the aquifer. The storage loss will manifest itself as a cone of depression that will capture surface water and riparian evapotranspiration. If all the treated effluent is discharged to the river, essentially what we are left with is a great Ponzi game with huge storage losses producing cones of depression that ultimately capture water from the river and its riparian ecosystem. Even after all the wells stop pumping, this capture will continue in order to fill the cones of depression. Regardless of where the treated effluent is discharged, over the long term there will be substantial loss of water in the stream, loss of riparian ecosystem, and loss of groundwater storage. This is neither sustainable yield nor a safe yield for the stream or riparian ecosystem!

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