Level of watershed subdivision for water quality modeling

Jha, Manoj
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The size and number of subwatersheds can impact a watershed modeling process and subsequent results. The objective of this study was to determine the appropriate level of subwatershed division for simulating flow, sediment, and nutrient. The SWAT model with GIS interface (AVSWAT) was applied to four Iowa watersheds that varied greatly in drainage area. Annual output was analyzed from each simulation, which was executed for 31 years using climatic data representing the period of 1970 to 2000. It was found that the streamflow is not significantly affected by decrease in subwatershed scale, whereas sediment yields were directly related to subwatershed scale. The threshold subwatershed size, i.e. minimum size of a subwatershed at which variation due to different subdivisions tends to stabilize, was found to be around 3 percent of the total drainage area to adequately predict sediment yield. Decreasing the size of subwatersheds beyond this level does not significantly affect the computed sediment yield. Similar analysis on nitrate concentration found 2 percent of the total drainage area as threshold area. This threshold subwatershed size can be used to optimize SWAT input data preparation requirements and simplify the interpretation of results, without compromising simulation accuracy.

Civil and construction engineering, Civil engineering (Environmental engineering), Environmental engineering