Conservation and sediment yield on the Fort Cobb reservoir watershed

Authors: Garbrecht, Jurgen; Starks, Patrick; Moriasi, Daniel

Submitted to: National Sedimentaton Laboratory (NSL)- 50 Years of Soil & Water Research in a Changing Agricultural Environment
Publication Type: Proceedings
Publication Acceptance Date: 7/15/2008
Publication Date: 9/30/2009
Citation: Garbrecht, J.D., Starks, P.J., Moriasi, D.N. 2009. Conservation and sediment yield on the Fort Cobb reservoir watershed. National Sedimentaton Laboratory (NSL)- 50 Years of Soil & Water Research in a Changing Agricultural Environment, Sept. 3-5, 2008, Oxford, MS. 2009 CD-ROM.

Interpretive Summary: Past soil and water conservation research has demonstrated the effectiveness of conservation practices on cropland to reduce overland soil erosion and sediment delivery to channels. However, soil conservation and channel stabilization may not always translate into an immediate sediment yield reduction at the outlet of the watershed. Hence, the apparent disconnect between upstream conservation practices and reduction of watershed sediment yield, at least within customary project durations of a few years. In this study, impacts of conservation activities on reducing sediment yield was demonstrated by contrasting sediment yield and discharge measurements taken by the USGS on the Fort Cobb Reservoir watershed during 2004-2007 with similar measurements taken more than half a century earlier. A substantial reduction in annual sediment yield between 1943-1948 and 2004-2007 was uncovered and related to conversion of cropland to rangeland and to the targeted implementation of conservation practices in the second half of the 20th century. Thus, while it may be difficult to identify immediate impacts of upstream conservation practices on watershed sediment yield, this study demonstrated that targeted and sustained conservation efforts in Oklahoma can, in time and with some delay, lead to a sizable reduction in sediment yield at the watershed outlet.

Technical Abstract: Prior to about 1950, conservation practices on the Fort Cobb Reservoir watershed in West-Central Oklahoma were few and mostly demonstration type projects. Extensive soil conservation measures were implemented in the second half of the 20th century. Fortuitously, the U.S. Geological Survey collected instantaneous suspended-sediment and discharge measurements on major channels during 1943-1948 and again in 2004-2007. These data offered the opportunity to estimate the reduction of watershed-scale suspended-sediment yield as a result of upstream conservation practices. Average annual suspended-sediment yield at the watershed outlet for 1943-1948 was estimated to be 760 [Mg/yr/km2], and for 2004-2007 it was 108 [Mg/yr/km2]. The substantial reduction in suspended-sediment yield was related to conversion of cropland to grassland, and to a wide range of conservation practices implemented in the second half of the 20th century. While it is generally difficult to identify conservation impacts on sediment yield at the watershed outlet during the short time span of a conservation project, this study shows that targeted and widespread conservation efforts in Central Oklahoma can lead, over time, to a sizable reduction in watershed sediment yield, as was the case for the Fort Cobb Reservoir watershed.