CASE STUDY OF MULTI-YEAR PRECIPITATION VARIATIONS AND THE HYDROLOGY OF FORT COBB RESERVOIR

Authors: Garbrecht, Jurgen; Schneider, Jeanne

Submitted to: Journal Hydrologic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/5/2007
Publication Date: 2/1/2008
Citation: Garbrecht, J.D., Schneider, J.M. 2008. Case study of multi-year precipitation variations and the hydrology of Fort Cobb reservoir. Journal Hydrologic Engineering. 13(2):64-70.

Interpretive Summary: Persistent variations in annual precipitation and corresponding watershed runoff can affect downstream reservoir operations, recreational activities, water quality, and stream and shoreline ecosystems. The impacts of decade-long precipitation variations on the hydrology of the Fort Cobb Reservoir in central Oklahoma are addressed in this case study. One wet and three dry periods were identified in the 1940-2004 precipitation record. The difference in annual precipitation between dry and wet periods was 33% of the mean, and led to a corresponding 100% change in reservoir inflow, 170% change in annual flood releases from the reservoir, and a maximum pool elevation drop of 2 m from the top of the conservation pool. It was further reasoned that large differences in annual reservoir inflow due to multi-year 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 was sensitive to decadal precipitation variations and suggested that decadal precipitation variations deserved careful consideration in hydrologic and water quality investigations in central Oklahoma. The findings and implications of this study should be of particular interest for investigations that attempt to quantify downstream impacts of upstream watershed management actions in the presence of decade-long precipitation variations.

Technical Abstract: Impacts of decadal precipitation variations on reservoir inflow, flood releases, and pool elevation were investigated for the Fort Cobb Reservoir, a reservoir that controls runoff from an 800 km2 agricultural watershed in central Oklahoma. Difference in mean annual precipitation between multi-year dry and wet periods was 33% of 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 pool elevation of 2 m. From a reservoir operational aspect, only the frequency of controlled flood releases was impacted. However, flood releases were sporadic in nature and 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 was sensitive to decadal precipitation variations and suggested that decadal precipitation variations deserved careful consideration in hydrologic and water quality investigations in central Oklahoma.