Identifying the roles of overland flow characteristics and vegetated buffer systems for nonpoint source pollution control

Nonpoint source pollution (NPS) is a serious problem causing the degradation of soil and water quality. Concentrated overland flow is the primary transport mechanism for a large amount of NPS pollutants from hillslope areas to downslope areas in a watershed. Vegetated buffer systems (VBS) are one of the most sustainable best management systems for NPS pollution control. VBS mitigate the water's erosive energy using vegetation cover and enhanced infiltration and increase pollutant removal via sorption and vegetation uptake.; In this study, a soil erosion model to identify transitional overland flow regions was developed using the kinematic wave overland flow theory. Spatial data, including digital elevation models (DEMs), soil, and landcover, were used in the GIS-based model algorithm. The model was calibrated and validated using gully head locations in a large agricultural watershed, which were identified using 1-m aerial photography.; The model performance was better than two previous approaches; the overall accuracy of the nLS model was 72 to 87 % in one calibration subwatershed and the mean overall accuracy was 75 to 89 % in four validation subwatersheds, showing that the model well predicted potential transitional erosion areas at different-size watersheds. However, the user accuracy in calibration and validation was still low. To improve the user accuracy and study the effects of DEM resolution, finer resolution DEMs were generated from a differential GPS field survey and compared to the USGS 30-m DEMs. Two interpolation methods (IDW and TIN) were used to convert the point data to raster files. The results showed that 3-m DEMs produced the best prediction for gully head locations in a grassland hillslope. Two out of three gully head locations were identified using 3-m DEMs interpolated by both IDW and TIN. Only one gully head location was determined using 10-m DEMs interpolated by IDW. No locations were delineated using 30-m DEMs. Overall model accuracy of 3-m DEMs (IDW) was improved to 12 % in comparison to the previous study.; Results from a field-scale study determined the trapping efficiency of VBS was 99 % of TSS, 97 % of T-N, 88% of T-P, and 87 % of PO4 --P on average. A major factor affecting performance was infiltration. Infiltration differences between measured and modeled data significantly decreased in October as the vegetation senesced. Vegetation clipping did not influence the runoff ratio or water quality, indicating interception was not a primary factor in this experiment. A 60-cm DEMs, developed by ground survey, showed more detailed slope variation and flow direction, particularly for smoother buffer topography. Manning's kinematic method yielded more accurate times of concentration than Darcy-Weisbach method in the bias error analysis. Finally, information gained from this study can improve assessing soil erosion process due to concentrated overland flow as well as enhance soil and water quality from better VBS design.