Assessing the impacts of flow diversion terraces on stream water and sediment yields at a watershed level using SWAT model

Around the world, soil erosion is a major concern for the sustainability of agricultural systems and a threat to the integrity of aquatic ecosystems. Soil conservation Beneficial Management Practices (BMPs) have been widely used to reduce soil loss from cultivated lands and minimize degradation in water quality. As an effective BMP, flow diversion terraces (FDT) have been widely implemented in Canada and their impacts at field levels have been well documented. The objective of this study was to use the Soil and Water Assessment Tool (SWAT) to assess the efficacy of FDT systems on maintaining surface water quality at the watershed level in the Black Brook Watershed (BBW) in northwestern New Brunswick. The SWAT model was calibrated with three years of data (1992–1994) when limited amount of FDT had been implemented in the watershed. For this period, we found that SWAT performed well in predicting the seasonal variation of water yield (R2 = 0.91) and moderately well for sediment yield (R2 = 0.5). For key water variables, the SWAT model captured the variation of soluble phosphorous very well (R2 = 0.81). However, the SWAT model over-predicted the amount of nitrate loading (R2 = 0.02), and poorly represented the general trend of nitrate. The calibrated model was used to assess seasonal and annual effects of FDT on water quality for the entire watershed during the period of 1995–2005. The results indicated that FDT implemented in BBW contributed to the reduction of sediment yield by 4 tonne ha−1 yr−1 on average, which represented a reduction of 56%. We also found that the FDT system as a BMP not only reduced the sediment loading at a watershed level, but also reduced water yield during the summer growing seasons. FDT systems contributed to the reduction of water yield by 158 mm yr−1, which represented a reduction of 20%.