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 2004-2005 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 non- linear 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. Sediment yield from agricultural watersheds is traditionally ascribed to land use, land management and agronomic practices. However, soil erosion and sediment transport and yield are in fact caused by rainfall and runoff, whereas land use, land management and agronomic practices affect the location's erosion potential. Thus, rainfall-runoff is a primary determinant in watershed sediment yield, and any lasting change in rainfall and runoff should be expected to lead to a corresponding change in sediment yield, assuming all other factors remain the same. Lasting changes in rainfall and runoff can be caused by persistent, multi-year precipitation variations which are defined as sequences of consecutive years that have annual precipitation values predominantly above or below the long-term average. Precipitation variations that last 5 or more years are termed wet or dry periods, or pluvial and drought periods. Wet or dry periods in a precipitation record are not uncommon (Gray et al., 2004; Garbrecht and Rossel, 2002; NRC, 1998). Notable examples include the Dust Bowl years