Streamflow declines on the Great Plains are causing many Federal reservoirs to become profoundly inefficient, and potentially unsustainable. For example the near-complete cessation of streamflow above Optima Lake, OK, has caused that reservoir to be effectively empty for several decades. Great Plains streamflow declines are historically related to groundwater mining, but will continue in the future as a consequence of climate change. A number of these reservoirs now lose >50% of inflow volume to evaporation. Streamflow changes through the mid-1980's can be correlated with water level declines in the High Plains Aquifer and land use changes. Subsequent streamflow changes appear to be related to short or long-term climate change, and particularly threaten Cedar Bluff and Keith Sebelius reservoirs in Kansas. Water balance for the four westernmost Federal reservoirs in Kansas (Cedar Bluff, Keith Sebelius, Webster and Kirwin) is indicative of this phenomenon. At those reservoirs inflow has declined by 90%, 89%, 96% and 57% respectively since the 1950's. Since 1990 total evaporated volumes relative to total inflows amounted to 68%, 83%, 24% and 44% respectively.

Continued global warming will likely exacerbate these trends. Inflows show increasing dependence on climate (Palmer Drought Severity Index or PDSI) vs. time, as groundwater levels and baseflow contributions decline. Post-1980 log-linear fits of historical discharge vs. PDSI (r=0.54) can be used with future PDSI predictions based on IPCC climate models to assess future streamflow trends. The climate models predict little change in precipitation, but large increase in temperature, leading to more negative annual average PDSI. Extrapolation using the observed annual PDSI-stream discharge correlation indicates that inflows to the four reservoirs is likely decrease by an additional 90% from current levels, and annual ratios of evaporation/inflow will consistently exceed 1.0 (i.e. net loss) by 2050. Anthropogenic threats to Great Plains streamflows began with groundwater mining, but will continue with climate change. Ultimately, surface storage of water may prove unsustainable in this region, leaving groundwater storage as the remaining option.