Impacts of Decadal Precipitation Variations on Watershed Sediment Yield and Implications for the COnservation Effects Assessment Project

Authors: Garbrecht, Jurgen; Starks, Patrick; Steiner, Jean; Schneider, Jeanne

Submitted to: Soil and Water Conservation Society Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 9/15/2006
Publication Date: 10/11/2006
Citation: Garbrecht, J.D., Starks, P.J., Steiner, J.L., Schneider, J.M. 2006. Impacts of decadal precipitation variations on watershed sediment yield and implications for the conservation effects assessment project[abstract]. Soil and Water Conservation Society Proceedings. p. 40.

Interpretive Summary: Abstract Only.

Technical Abstract: A case study was conducted on the Fort Cobb Reservoir watershed in central Oklahoma, to investigate the impacts of persistent, multi-year precipitation variations on watershed runoff and sediment yield. The significance of the findings was discussed with regard to the Conservation Effects Assessment Project (CEAP) conducted by agencies within the USDA. The study demonstrated that several persistent, multi-year precipitation variations occurred in the central Oklahoma region between 1940 and 2005. These persistent precipitation variations were called wet and dry periods. The difference in mean annual precipitation between wet and dry periods was about 30 percent of the long term mean. To investigate the impact of wet and dry periods on watershed runoff, the 1940-1959 stream flow record at Cobb Creek near Fort Cobb was augmented with the 1959-2005 reservoir inflow estimates by the Bureau of Reclamation. Mean annual stream flow showed a difference of about 100 percent of the long term mean between wet and dry precipitation periods. Associated changes in soil erosion and sediment transport likely impacted watershed sediment yield and reservoir sediment loading. Sediment yield was estimated based on suspended sediment measurements at three Cobb Creek stream flow gaging stations for 2004 and 2005. A “suspended sediment – discharge” rating curve was developed and the 1940-2005 suspended sediment record was reconstructed with the available stream flow data. This reconstructed sediment record reflected suspended sediment for 1940-2005 flow conditions, and for land use and agronomic practices prevailing during the 2004-2005 sediment sampling period. The mean annual sediment yield for wet and dry precipitation periods showed a difference of over 150 percent of the long term mean. Thus, stream flow and watershed sediment yield, and by extension soil erosion and sediment transport, were sensitive to persistent, multi-year precipitation variations. Similar findings are likely to apply for the movement of agrichemicals, since many such substances are adsorbed to soil particles and transported with sediment. With regard to CEAP, any assessment of effectiveness of conservation practices can be expected to depend, in addition to conservation measures, on the particular precipitation period used in the evaluation. This raises the question as to which precipitation period should be selected for base-line conditions to define effectiveness. Without an agreed upon comparison standard, various values of effectiveness could be obtained for the same conservation practice depending on the precipitation period used. Persistent precipitation variations can also affect calibration and validation of simulation models used in the assessment of conservation practices. The selection of an “appropriate” base-line precipitation period for the assessment of effectiveness of conservation practices should be debated among policy makers, practitioners, conservation specialists, land owners and climatologists. Until then the determined effectiveness of a conservation measure or practice should be reported with reference to the particular precipitation period and base-line condition used in the model calibration and subsequent assessment.