Watershed sediment yield reduction through soil conservation in a west-central Oklahoma watershed

Authors: Garbrecht, Jurgen; Starks, Patrick

Submitted to: Journal of Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/28/2009
Publication Date: 5/8/2009
Citation: Garbrecht, J.D., Starks, P.J. 2009. Watershed sediment yield reduction through soil conservation in a west-central Oklahoma watershed. Journal of Ecohydrology. 2(3):313-320.

Interpretive Summary: Runoff and sediment yield from agricultural watersheds often lead to downstream water quality problems. A variety of soil and water conservation programs have been developed and implemented to improve downstream water quality. However, soil conservation measures generally do not translate into an immediate reduction in measurable sediment yield at the outlet of large watersheds. In this study, impacts of extensive soil conservation measures on watershed sediment yield are investigated for the Fort Cobb Reservoir watershed, a 300 sq mi mixed agricultural watershed in west-central Oklahoma. Sediment and discharge measurements were taken in 1943-1948 prior the implementation of conservation practices, and again in 2004-2007 after implementation of extensive conservation practices. A comparison of sediment yield of these two measurement periods showed a substantial reduction in watershed sediment yield. This reduction was attributed to land use and management changes and conservation practices implemented in the second half of the 20th century. Even though it generally is difficult to identify impacts of upstream conservation practices on sediment yield at the watershed outlet during the short time span of a particular conservation project, targeted and widespread conservation efforts in the Fort Cobb Reservoir watershed have led, over a 60 year time period, to a sizable reduction in sediment yield at the watershed outlet.

Technical Abstract: Soil conservation practices on the Fort Cobb Reservoir watershed in West-Central Oklahoma were few before the 1950s. In the second half of the 20th century, extensive soil conservation measures were implemented to protect agriculturally fertile but erosion-prone soils. Fortuitously, the U.S. Geological Survey (USGS) collected instantaneous suspended-sediment and discharge measurements on major tributaries within the watershed in 1943-1948 and again in 2004-2007, called pre- and post-conservation periods. These measurements offered the opportunity to compare channel suspended-sediment yield before and after implementation of conservation practices. With these measurements, a separate suspended sediment-discharge rating curve was developed for each the pre- and post-conservation period. Average annual sediment yield at the watershed outlet was estimated by evaluating the respective sediment-discharge rating curve with an 18-year daily discharge record near the outlet of the watershed. Average annual sediment yield was estimated to be 760 [Mg/yr/km2] and 108 [Mg/yr/km2] for the pre- and post-conservation periods, respectively. The substantial reduction in suspended-sediment yield was related to land use and management changes and the wide range of conservation practices implemented in the second half of the 20th century. Even though it generally is difficult to identify impacts of upstream conservation practices on sediment yield at the watershed outlet during the short time span of a particular conservation project, targeted and widespread conservation efforts in the Fort Cobb Reservoir watershed have led, in time, to a sizable and measurable reduction in watershed sediment yield.