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L-THIA model, water quality, sediment, Snake River Basin, Idaho


Watershed management requires the determination of both point and non-point sources of pollution within a watershed. The primary non-point source pollutants in a typical watershed are nutrients (mainly nitrogen and phosphorus), sediment, and pesticides. In the Snake River Basin, in Idaho, nutrients from non-point sources (primarily agricultural) are delivered to streams via storm water and irrigation runoff. However, the objectives of this study were to estimate the phosphorus loading from different sources in the Snake River Basin due to storm water events, to calculate the total nitrogen/phosphorus (TN/TP) ratios for land uses, and to compare and a precipitation runoff model and statistical regression with the measurements. The study used the Long-Term Hydrologic Impact Assessment (L-THIA) model to perform the analysis and to estimate the loss and gain in phosphorus loading. The gain was due to ground water discharge and point sources from industrial and commercial trout farms. The loss was attributed to phosphorus absorbance, existence of riparian vegetation, ground water discharge, or dilution from spring inflows. The results showed that phosphorus is the limiting nutrient. The L-THIA model gave more accurate results than the simple statistical regression.