DECADAL PRECIPITATION AND INFLOWS INTO FORT COBB RESERVOIR, OKLAHOMA

Authors: Garbrecht, Jurgen; Schneider, Jeanne; Van Liew, Michael

Submitted to: Environmental and Water Resources Institute World Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 4/1/2006
Publication Date: 5/22/2006
Citation: Garbrecht, J.D., Schneider, J.M., Van Liew, M.W. 2006. Decadal precipitation and inflows into Fort Cobb Reservoir, Oklahoma. In: Graham, R., editor. Proceedings of the World Environmental and Water Resources Congress, May 21-25, 2006, Omaha, Nebraska. 2006 CDROM.

Interpretive Summary: Decade-long precipitation variations exist in the Great Plains of the United States and have strong impact on average annual runoff from agricultural watersheds. Changes in annual reservoir inflow and flood release volumes due to persistent multi-year dry or wet periods were investigated for the Fort Cobb Reservoir. An analysis of the 1940-2004 annual precipitation revealed three dry and one wet period, called decadal variations. Corresponding strong variations were found in the reservoir inflow and flood release records. The sensitivity of watershed response, reservoir inflow and flood release volume to decadal precipitation variations suggested that water resources, soil erosion, sediment transport, and water quality assessments in central Oklahoma should account for decadal precipitation variations. Along the same lines, water resources and water quality investigations based on numerical model simulations need to include decadal precipitation variations in the calibration, validation and application phase of the numerical model simulations.

Technical Abstract: Decade-long precipitation variations exist in the Great Plains of the United States and have strong impact on average annual runoff from agricultural watersheds. Here the impact of decade-long precipitation variations between 1940 and 2004 on annual reservoir inflow and flood-release volumes were investigated for the Fort Cobb Reservoir in central Oklahoma. A time-series analysis revealed three below average precipitation periods (dry periods) and one above average precipitation period (wet period). The difference in mean annual precipitation between dry and wet periods was 33% of the 1940-2004 mean annual precipitation. The impact of a shift in mean annual precipitation from dry to wet period was a doubling in mean annual reservoir inflow and a 170% increase in mean annual flood releases. From these findings it was inferred that during wet periods the higher volume of flood releases is likely to lead to more frequent bank full discharge downstream of the dam. However, due to the sporadic nature of flood releases and the generally dry streambed downstream of the dam, the increased stream flow during wet periods will likely not enhance the aquatic ecosystem or riparian habitat. Also, the strong and non-linear relationship between runoff and soil erosion/transport suggests that sediment loading of the reservoir is equally if not more sensitive to decadal precipitation variations than watershed runoff. This inference may equally be extended to the movement, transport and reservoir loading of agro-chemicals. Finally, the sensitivity of the watershed response to decadal precipitation variations suggests that model simulations of watershed runoff and water quality need to account for decadal precipitation variations in their calibration and application.