Author
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Garbrecht, Jurgen |
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Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 11/15/2007 Publication Date: 3/24/2008 Citation: Garbrecht, J.D. 2008. Multi-year precipitation variations and watershed sediment yield in a CEAP benchmark watershed. Journal of Soil and Water Conservation. 63(2):70-76. Interpretive Summary: Five or more consecutive years of predominantly wet or dry conditions are called wet or dry periods. Several such periods occurred in central Oklahoma between 1940 and 2005. Wet or dry periods are believed to significantly effect soil erosion and sediment loading of downstream water bodies, yet the magnitude of the impact is not well documented and remains largely unaccounted for in soil conservation assessment studies. This study investigated the magnitude of the impacts of wet or dry years on watershed runoff and sediment yield for the Fort Cobb Reservoir watershed in central Oklahoma. It was found that runoff and therefore sediment yield was sensitive to wet and dry periods. In the case of the Fort Cobb Reservoir watershed, a 33% difference in mean annual precipitation resulted in a 100% difference in mean annual runoff and 180% difference in mean annual sediment yield. This sensitivity of runoff and sediment yield to wet and dry periods suggest 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 for the evaluation of conservation practices. Furthermore, it was inferred that for computer-based assessments of the effectiveness of soil conservation practices, a thorough testing of the computer-model performance under both wet and dry periods was advisable to ensure accurate simulation results over the full range of prevailing climatic conditions. Technical 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. 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, or 1.5 standard deviations. As a result of non-linear hydrologic linkages between precipitation, runoff and sediment yield, corresponding variations in watershed runoff and sediment yield were comparatively larger. The difference in mean annual runoff between wet and dry periods was 100% of the long-term mean, or 2.1 standard deviations. Sediment yield was estimated using a sediment-discharge relationship. The difference in mean annual sediment yield between wet and dry periods was 183% of the long-term mean, or 1.7 standard deviations. 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. |
