DOWNSTREAM SEDIMENT RESPONSE TO CONSERVATION PRACTICE IMPLEMENTATION

Authors: Van Liew, Michael; Arnold, Jeffrey

Submitted to: Annual International SWAT Conference
Publication Type: Proceedings
Publication Acceptance Date: 1/27/2005
Publication Date: 7/13/2005
Citation: Van Liew, M.W., Arnold, J.G. 2005. Downstream sediment response to conservation practice implementation. In: Proceedings of Annual International SWAT Conference, July 13-15, 2005, Zurich, Switzerland. 2005 CDROM.

Interpretive Summary: For decades many kinds of conservation programs have been put in place to conserve water, protect agricultural lands from soil erosion, and to reduce pollution levels in rivers and lakes downstream of cropland areas. Although a number of studies have shown the benefit of these programs on conserving soil and water resources immediately downstream of the location where they were put into practice, the environmental benefits of these programs on much larger, watershed sized areas is not well known. Evaluating the impacts of conservation practices within a watershed provides valuable information to better understand how these practices can be used to reduce soil losses and pollution levels in streams and water bodies that are well downstream of the conservation practices. An investigation was carried out to determine how a particular conservation practice would affect soil losses from an experimental watershed. A computer model referred to as the Soil and Water Assessment Tool (SWAT) was used to simulate the impact of a proposed conservation practice on downstream soil losses for the 136 km2 Lake Creek watershed in Southwestern Oklahoma. The proposed conservation practice consisted of converting an area of land where a cultivated crop is grown to a permanent grass cover. Three types of conversions included changes from winter wheat, sorghum-wheat, and peanut-wheat to Bermuda grass. Model changes from one type of crop to Bermuda grass were implemented in the upper most portion of the Lake Creek watershed on cultivated lands that were the most erodible 2.5%, 5.0%, and 7.5% of the total watershed area. SWAT was then used to simulate soil losses at three locations downstream of the proposed conversions. Of the three types of cropping system conversions simulated by the model, results show that the largest percent reductions in soil loss occurred for a change from winter wheat to Bermuda, followed by changes from sorghum-wheat to Bermuda and then peanut-wheat to Bermuda. Conversion of 2.5% of the most erodible cropland to Bermuda grass resulted in a 15.1%, 9.2%, and 6.7% reduction in soil loss for winter wheat, sorghum-wheat, and peanut-wheat, respectively. Reductions in soil loss were most pronounced in the upper reaches of the watershed and became increasingly less pronounced further downstream near the watershed outlet. For example, model simulations show that for the most erodible 5.0% of cultivated land that was converted from winter wheat to Bermuda grass, the reduction in soil loss was 49.3%, 36.5%, and 23.2% for contributing watershed areas of 36.5%, 62.6%, and 100%, respectively. This study shows the relative changes in soil loss that would be expected to occur downstream of a proposed conservation practice that is put into place in the erodible, upper areas of a watershed.

Technical Abstract: In recent years watershed scale hydrologic models have been developed to assess the environmental conditions of a watershed and evaluate the impact of Best Management Practice (BMP) implementation. In this study the Soil and Water Assessment Tool was used to simulate the impact of BMP implementation on downstream sediment yields for the 136 km2 Lake Creek watershed in Southwestern Oklahoma, USA under dry, average, and wet climatic conditions. Changes in cultivated crops from winter wheat, sorghum-wheat, and peanut-wheat to Bermuda grass located in the upper most portion of the watershed are implemented at BMP levels, representing the most erodible 2.5%, 5.0%, and 7.5% of the total watershed area. Resultant changes in sediment yield were then simulated at three locations downstream of the proposed conversions. Of the three types of cropping system conversions simulated by the model, test results show that the largest percent reductions in sediment occured for a change from winter wheat to Bermuda, followed by changes from sorghum-wheat and then peanut-wheat to Bermuda. Under average climatic conditions for the Lake Creek watershed, BMP implementation on the most 2.5% of land area resulted in a 15.1%, 9.2%, and 6.7% reduction in sediment yield for wheat, sorghum-wheat, and peanut-wheat, respectively. Sediment reductions were most pronounced in the upper reaches of the watershed and became increasingly less pronounced further downstream, due to the dampening effect of averaging sediment yields from larger, contributing watershed areas. At the 5.0% BMP implementation level, sediment reduction for the conversion of wheat to Bermuda under average climatic conditions was 49.3%, 36.5%, and 23.2% for contributing watershed areas of 36.5%, 62.6%, and 100%, respectively. Simulation results suggested that the impact of decadal scale variations in precipitation was minimal on percent sediment reductions for the three cropping systems. This investigation provides preliminary information that quantifies the relative changes in sediment yield that would be expected to occur downstream if conservation practices were implemented in the erodible, upper areas of the watershed.