Case Study of Multiyear Precipitation Variations and the Hydrology of Fort Cobb Reservoir

Impacts of decadal precipitation variations on reservoir inflow, flood releases, and pool elevation were investigated for the Fort Cobb Reservoir, which controls runoff from a 787 km2 agricultural watershed in central Oklahoma. The difference in mean annual precipitation between multiyear dry and wet periods was 33% of the long-term mean and led to a corresponding 100% change in mean reservoir inflow, 170% change in mean annual flood releases from the reservoir, and a maximum drop in conservation pool elevation of 2 m. From a reservoir operations perspective, only the frequency of controlled flood releases was impacted by decadal precipitation variations. These flood releases were sporadic in nature, and the more frequent releases during wet periods were not believed to appreciably enhance stream habitat and riparian vegetation downstream of the reservoir. It was further reasoned that large differences in annual reservoir inflow due to decadal precipitation variations would likely be accompanied by related changes in upstream soil erosion and reservoir sediment loading. With regard to hydrologic and environmental modeling, it was argued that decadal precipitation variations had important implications for model calibration, verification, and subsequent application. Overall, this case study demonstrated watershed and reservoir hydrology were sensitive to decadal precipitation variations and suggested that decadal precipitation variations deserved careful consideration in hydrologic and water quality investigations in central Oklahoma