Multi-year precipitation variations and watershed sediment yield in a CEAP benchmark watershed

Abstract

A case study was conducted on the Fort Cobb Reservoir watershed in central Oklahoma to investigate impacts and implications of persistent multi-year precipitation variations on watershed runoff and sediment yield. Sediment yield was calculated from a sediment-discharge relationship representing 2004 to 2005 land use, agronomic practices, and conservation measures. Several persistent multi-year precipitation variations, called wet and dry periods, occurred in central Oklahoma between 1940 and 2005. The difference in mean annual precipitation between wet and dry periods was 33% of the long-term mean. As a result of nonlinear hydrologic linkages between precipitation, runoff and sediment yield, corresponding variations in watershed runoff and calculated sediment yield were comparatively larger. The difference in mean annual runoff between wet and dry periods was 100% of the long-term mean, and for mean annual calculated sediment yield it was 183% of the long-term mean. With regard to the Conservation Effects Assessment Project (CEAP), the sensitivity of runoff and therefore of sediment yield to wet and dry periods suggests that measures of conservation program effectiveness depend on climatic conditions used in their evaluation and that great care should be taken to select a climate record representative of prevailing climate conditions. Furthermore, it was inferred that the calibration of simulation models used in the conservation effects assessment may be biased if performed with climatic data representing either just a wet or a dry period. In the presence of multi-year precipitation variations, a thorough model validation for both wet and dry periods is recommended to ensure accurate simulation results over the full range of prevailing climatic conditions.