In any drought the most immediate concern is storage sufficiency. Can existing storage meet projected water demand? For long term drought, climate change, and increasing water demand, a key issue becomes storage efficiency. Are losses from surface reservoir storage so large that subsurface storage is superior?

One place where a transition is underway from surface to subsurface storage is the Smoky Hill River near Hays, Kansas. Hays' Smoky Hill River Wellfield (SHRW) produces from a narrow alluvial valley stream-aquifer system, which is highly dependent on streamflow. Declining streamflow above SHRW has forced Hays to restrict pumping to less than half its water right of 2250 ac-ft/yr. Geophysical determination of SHRW groundwater storage volume was made to allow detailed prediction of drought performance. This analysis demonstrated that streamflow declines since 1979 have doubled the probability of aquifer depletion to 30% had Hays extracted 2250 ac-ft/yr. Streamflow above SHRW has declined by up to 90%, attributable to operations at Cedar Bluff Reservoir 20 miles upstream and changing farming practices. Inflow to the reservoir has declined by 50% since its construction in 1949. As a result, evaporation losses now equal or exceed inflows most years, and reservoir releases were curtailed in 1979. This effectively reduced the drainage area supplying streamflow to SHRW by 90%, leading to the steep decline in average annual streamflow and impacting Hays' senior water right. Based on this, a plan to trigger recharge releases from the reservoir when wellfield water levels decrease awaits state approval. Wellfield reconfiguration and this change will reduce the chances of aquifer depletion to near zero at full production, using water that would have evaporated from the reservoir to recharge the alluvial aquifer instead.

Other locations that might benefit from conversion to subsurface storage include the near-empty Optima Lake in the Oklahoma Panhandle, and Las Vegas, New Mexico, where decreasing inflows and extreme evaporation rates are problematic. Failure of storage as snowpack at Las Vegas has precipitated several water crises there, and alluvial aquifers represent the only immediate hope for a solution. In these and many other cases increasing storage efficiency offers the best hope for avoiding future crises.