Impact of tile drainage on water budget and spatial distribution of sediment generating areas in an agricultural watershed

A recently developed model, SWATDRAIN, was used to assess the alterations in water balance components, discharge, and sediment loads due to tile drainage practices in a heavily tile drained watershed in Ontario, Canada. Furthermore, the model was implemented to determine the spatial variability of sediment loads which can be explained by a combination of spatially distributed variables within a watershed, including those controlling the hydrology, geology, soil and land use. Three scenarios were examined across the watershed, including conventional drainage (existing condition), controlled drainage, and no tile drainage. The model predicted that streamflow was not significantly impacted due to tile drainage, while the total runoff and sediment loads from the basin due to controlled drainage were increased by 27.1% and 22.2%, respectively, while removing tile drain infrastructures resulted in 37.1% increase in surface runoff and 55% increase in sediment load from the watershed The areas with high sediment load generation were identified by the model and the impact of tile drainage in producing sediment in those areas was assessed. The results showed that the sediment load generation rate in the areas with the highest load (class V) increased by 8% only due to controlled drainage, while in the second ranked sediment generating areas (class IV), the sediment load generation rate was increased by 32%.