Submitted to: Journal Hydrologic Engineering
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/10/2011
Publication Date: 5/1/2011
Citation: Garbrecht, J.D. 2011. Effects of climate variations and soil conservation on sedimentation of a west-central Oklahoma reservoir. Journal Hydrologic Engineering. 16(11):899-913. Interpretive Summary: Soil conservation practices on agricultural land are effective at reducing soil erosion and sediment delivery to receiving channels. However, it is unclear how much upstream conservation practices contribute to reducing sediment yield and alleviating sedimentation problems at an off-site location further downstream the channel system. In this study, the integrated effects of land use conversion, soil conservation practices, and a climate shift on watershed sediment yield and sedimentation of the Fort Cobb Reservoir in west central Oklahoma were investigated. Findings revealed that reductions in downstream sediment yield as a result of upstream soil conservation practices may go unrecognized amid large but common variations in climate and runoff. Specifically, modest, yet persistent, variations in average annual precipitation can lead to changes in sediment yield that are as large as, or even larger than, those associated with soil conservation practices. In the Fort Cobb Reservoir watershed, a wetter climate in recent decades, led to increased runoff, erosion and sediment transport that offset to a large extent the reduction in sediment yield reduction achieved by soil conservation practices. Thus, projections of watershed sediment yield, anticipation of downstream sedimentation problems, and preparations for remediation are highly uncertain due to unpredictable variations in climate. Contingency plans for additional soil conservations efforts will be helpful for maintaining critical watershed drainage and reservoir storage capacity given potential future climate intensification, such as anticipated in Oklahoma due to global warming.
Technical Abstract: During the second half of the 20th century, extensive soil conservation practices were implemented on the Fort Cobb Reservoir watershed in West-Central Oklahoma. Sediment and flow observations were made on major tributaries in 1943-1950 (pre-conservation time period), and again in 2004-2008 (post-conservation time period). These data were used to compare watershed sediment yield and reservoir sedimentation during pre- and post-conservation periods. Suspended sediment - discharge rating curves were developed for each time period and used to estimate average annual watershed sediment yield. Initial sediment yield estimates for pre- and post conservation conditions were 254'900 [Mg/yr] (3.306 [Mg/yr/ha]) and 247'700 [Mg/yr] (3.213 [Mg/yr/ha]), respectively. The apparent lack of responsiveness of sediment yield to conservation efforts was explained by a shift in the mid-1980s towards wetter climatic conditions. The wetter climate led to increased soil erosion, sediment transport and sediment yield that offset the reduction in sediment yield due to conservation efforts. About a 60% to 65% reduction in sediment yield would have been achieved by conservation efforts if the climate had been constant. The mutually offsetting effects of a wetter climate and conservation efforts on sediment yield also resulted in unabated sedimentation of the Fort Cobb Reservoir. The interaction between wetter climate and effectiveness of conservation practices was the basis for projecting future sedimentation rates and reservoir lifespan, which are shown to vary over a wide range depending on climate assumptions through the next decades.