The Case for Subsurface Storage of Water in Agricultural Basins
University of California, Davis
For most of human history, water supplies have come predominantly from existing surface water bodies, and in more recent decades, from engineered reservoirs. As population and hence demand for water grew, more and bigger surface storage projects were constructed, including conveyance structures for wheeling water long distances. As the Green Revolution and the associated large increase in food production ramped up during the middle part of the 20th century, the irrigation water needed for the revolution came initially from surface storage systems, but in the second half of the century groundwater development was increasingly used to satisfy demand. This system, with managed water storage being done mainly with surface reservoirs, and groundwater being used to supplement increasingly larger fractions of the water supply, is becoming increasingly unreliable because of groundwater overdraft and groundwater quality degradation. Furthermore, in many agricultural basins, the irrigation and urban water demands are no longer being met by the existing water stores, with simultaneous, severe depletion of both the surface and subsurface stores of water during droughts. Even moderate amounts of groundwater production have caused significant reductions in lakes, rivers and wetlands that were previously sustained by groundwater discharge, while also in effect converting many basins into closed hydrologic basins in which most of the water exits by evapotranspiration of applied irrigation water. In these newly closed hydrologic basins, just as in other closed basins such as Death Valley and the Great Salt Lake, groundwater salinization is inevitable. Resolving the storage problem and water security in the face of drought can be accomplished by long-term planning and alternative land management that produces much greater groundwater recharge during wet years and a greater emphasis on subsurface storage instead of the traditional emphasis on surface storage. In turn, such a reimagining of our water storage systems is the only path toward (1) reversing the ongoing declines in regional groundwater quality caused by non-point source contamination from irrigation water and hydrologic basin closure, and (2) recovering some the ecosystem functions that were formerly supported by the underlying groundwater systems being sufficiently ‘full’ of water to discharge to the surface. In short, neither the quantity nor the quality of groundwater in many agricultural basins is sustainable unless those basins are used much more proactively for subsurface storage, which would in turn have significant benefits for the surface ecology and environmental quality.