DOOMED RESERVOIRS ON THE HIGH PLAINS? CLIMATE CHANGE AND GROUNDWATER MINING LEAD TO PROFOUNDLY INEFFICIENT SURFACE WATER STORAGE

A number of major reservoirs on the High Plains have become notably inefficient over time, losing to evaporation 50% or more of the water flowing into them. This has occurred chiefly because of steady streamflow declines since the 1960's, while evaporation rates have remained steady or increased. Streamflow changes through the mid-1980's can be correlated with water level declines in the High Plains Aquifer (HPA). Subsequent changes appear to be related to short or long-term climate change.

Water balance for four federal reservoirs in western Kansas is indicative of this phenomenon: Cedar Bluff, Keith Sebelius, Webster and Kirwin. Reservoir inflow has declined by 90%, 89%, 96% and 57% respectively. The ratio of annual evaporation to inflow for these reservoirs shows increases of 114%, 92%, 68% and 38% respectively since impoundment. Since 1990 total evaporated volumes relative to total inflows amounted to 68%, 83%, 24% and 44% respectively. Several of these reservoirs lie in intensive water management districts, and this magnitude of evaporative loss may prove to be intolerable. Conversion to subsurface storage may be the best alternative in such cases.

Continued global warming will likely exacerbate these trends. Recent global climate model results (IPCC4) can be used to predict future changes in streamflow. Since 1980 stream discharge above each of these four reservoirs exhibits a log-linear correlation (r=0.54) with Palmer Drought Severity Index (PDSI). Pre-1975 discharge shows little correlation with PDSI, and appears most consistent with a predominantly baseflow source (the HPA). Monthly mean temperature and precipitation for the 21st Century was obtained from the mean of 14 separate IPCC4 models of the SRES-A1B scenario. The models predict little change in precipitation, with much larger increase in temperature, leading to more negative annual average PDSI. Extrapolation using the observed PDSI-stream discharge correlation indicates that inflows to the four reservoirs may decrease by up to an additional 90%. High Plains streamflow declines initially related to groundwater withdrawals prior to 1975 will continue, with future declines primarily attributable to climate change. The combination of these declines threatens the sustainability of a number of storage reservoirs on the High Plains.