Evaluating the Transport and Fate of Nutrients in Large Scale River Basins Using an Integrated Modeling System - Landscape Ecology for Sustainable Environment and Culture - page 186

Fig. 10.4 SWAT hydrologic processes and interactions

percolation. Groundwater flow contribution to total streamflow is simulated by
routing a shallow aquifer storage component to the stream (Arnold and Allen
1996 ). Channel routing is simulated using either the variable-storage method or the
Muskingum method; both methods being variations of the kinematic wave model.
Erosion and sediment yield are estimated for each HRU with the Modified Uni-
versal Soil Loss Equation (Williams and Berndt 1977 ). The channel sediment routing
uses a modification of Bagnold's sediment transport equation (Bagnold 1977 ) that
estimates the transport concentration capacity as a function of velocity. The model
either deposits excess sediment or re-entrains sediment through channel erosion
depending on the sediment load entering the channel. The delivery ratio is estimated
for each particle size as a linear function of fall velocity, travel time, and flow depth.
SWAT simulates the complete soil nutrient cycle for nitrogen and phosphorus.
The soil nitrogen cycle is simulated using five different pools; two are inorganic
forms (ammonium and nitrate) while the other three are organic forms (fresh,
stable, and active). Similarly, SWAT simulates six different pools of phosphorus in
soil; three are inorganic forms and the rest are organic forms. Primary biochemical
transformations of nitrogen and phosphorus are simulated. Nitrate export with
runoff, lateral flow, and percolation are estimated as products of the volume of
water and the average concentration of nitrate in the soil layer. Organic nitrogen
and organic phosphorus transport with sediment is calculated with a loading
function developed by McElroy et al. ( 1976 ) and modified by Williams and Hann
( 1978 ) for application to individual runoff events. The loading function estimates
daily organic nitrogen and phosphorus runoff loss based on the concentrations of
constituents in the top soil layer, the sediment yield, and an enrichment ratio. The
amount of soluble phosphorus removed in runoff is predicted using labile phos-
phorus concentration in the top soil layer, the runoff volume and a phosphorus soil