DOWNSTREAM RESPONSE OF RUNOFF, SEDIMENT, AND PHOSPHOROUS TO SPATIAL VARIATIONS IN CONSERVATION PRACTICE IMPLEMENTAION

Authors: Van Liew, Michael; Daniel, John; Arnold, Jeffrey

Submitted to: Proceedings of the World Water and Environmental Resources Congress
Publication Type: Proceedings
Publication Acceptance Date: 2/9/2005
Publication Date: 5/16/2005
Citation: Van Liew, M.W., Daniel, J.A., Arnold, J.G. 2005. Downstream response of runoff, sediment, and phosphorous to spatial variations in conservation practice implementaion. In: Proceedings of the World Water and Environmental Resources Congress. May 15-19, 2005, Anchorage, Alaska. 2005 CDROM.

Interpretive Summary: For a number of years many kinds of conservation programs have been put in place to conserve water use, protect agricultural lands from soil erosion, and to reduce nutrient levels in rivers and lakes downstream of cropland areas that have been fertilized with nitrogen and phosphorous compounds. Although studies have shown the benefit of these programs on conserving soil and water resources and improving water quality 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. In this study an investigation was carried out to determine how a particular conservation practice would affect soil and phosphorous losses from an agricultural 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 and phosphorous losses for the 52.5 mi2 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. The changes from one type of crop to Bermuda grass were put into practice in the upper, middle, and lower portions of the Lake Creek watershed on cultivated lands that were the most erosive 2.5%, 5.0%, and 7.5% of the total watershed area. SWAT was then used to simulate soil and phosphorous losses downstream of the proposed conversions at the watershed outlet. Of the three types of cropping system conversions simulated by the model, test results show that the largest percent reductions in soil loss occur for a change in winter wheat to Bermuda, followed by changes in sorghum-wheat to Bermuda and then peanut-wheat to Bermuda. Conversion of 2.5% of the most erosive cropland to Bermuda grass resulted in a 15.3%, 9.6%, and 7.1% reduction in soil loss for winter wheat, sorghum-wheat, and peanut-wheat, respectively. Model simulations show that soil loss reductions resulting from the land conversion were most pronounced in the upper portion of the Lake Creek watershed where slope steepness and soil conditions more strongly influence erosion rates than in the middle and lower reaches of the watershed. Of the three types of cropping system conversions, results of this study indicate that the largest percent reductions in phosphorus loss occur for a change in sorghum-wheat to Bermuda, followed by changes in peanut-wheat to Bermuda and winter wheat to Bermuda. For example, conversion of 2.5% of the most erosive cropland to Bermuda grass resulted in a 6.8%, 5.1%, and 2.0% reduction in phosphorous loss for sorghum-wheat, peanut-wheat, and winter wheat, respectively. Whether the location of the conservation practice was in the upper, middle, or lower reaches of the Lake Creek watershed made little difference on reductions in phosphorous loss. This investigation provides valuable information that shows the relative changes in soil and phosphorous losses that might be expected to occur at the watershed outlet if conservation practices were put into practice in the most erosive upper, middle, or lower portions of the 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 is used to simulate the impact of BMP implementation on runoff, sediment yields, and total phosphorous yields for the 136 km2 Lake Creek watershed in Southwestern Oklahoma. Changes in cultivated crops from winter wheat, sorghum-wheat, and peanut-wheat to Bermuda grass at upper, middle, and lower locations in the watershed are implemented at three BMP levels, representing 2.5%, 5.0%, and 7.5% of the total watershed area. Changes in runoff, sediment, and phosphorous are simulated by the model for each of the three BMP levels. Of the three types of cropping system conversions, test results show that the largest percent reductions in sediment occur for a change in wheat to Bermuda, followed by sorghum-wheat and then peanut-wheat. A 2.5% level BMP implementation in the upper portion of the Lake Creek watershed results in a 15.3%, 9.6%, and 7.1% reduction in sediment for wheat, sorghum-wheat, and peanut-wheat, respectively. Model simulations show that sediment reductions resulting from BMP implementation are most pronounced in the upper reaches of the watershed where topographic and soil conditions more strongly influence erosion rates than in the middle and lower reaches of the watershed. Of the three types of cropping system conversions, results of this study indicate that the largest percent reductions in phosphorus occur for a change in sorghum-wheat to Bermuda, followed by changes in peanut-wheat and winter wheat. With BMP implementation at the 2.5% level in the upper portion of the Lake Creek watershed, percent reductions are 6.8%, 5.1%, and 2.0% for sorghum-wheat, peanut-wheat, and winter wheat systems, respectively. Unlike changes in sediment, the effect of BMP location on reductions in phosphorous is nearly negligible. This investigation provides preliminary information that quantifies the relative changes in runoff, sediment yield, and phosphorous yield that might be expected to occur if conservation practices were implemented in the upper, middle, or lower portions of the watershed.