Analysis of the In-stream Water Quality Component of SWAT (Soil Water Assessment Tool)

Abstract

SWAT (Soil Water Assessment Tool) is a watershed mass-balance loading model. Recentlyequations from the water quality model QUAL2E have been incorporated into the SWAT model.QUAL2E is a steady-state model used to predict the concentration of in-stream parameters basedon constant streamflow and inputs. The validity of the incorporation of the QUAL2E equationsinto SWAT has never been adequately confirmed. In order to test the validity of the water qualitycomponent of SWAT, two approaches were used. The first approached extracted the waterquality subroutine from SWAT and emulated a steady-state simulation for comparison withQUAL2E output. The second approach utilized the entire SWAT program emulating a steady-statesimulation for comparison with a comparable simulation in QUAL2E. The extractedsubroutine steady-state simulations had a low standard error of prediction (0.071) compared tothe QUAL2E output, with all of the calculated variables very similar at low concentrations.However, as initial concentrations increased the standard error also increased. The full SWATmodel exhibited differences from the QUAL2E model in 2, 6 and 24 hour element retention timesimulations. SWAT gave the same results regardless of retention time, with the differencesbetween the output of the two models decreasing as the duration of retention time increased. Thegreatest discrepancies between the two models occurred in the DO and chlorophyll-asimulations. A minor change in the SWAT program, removing a default which made allretention times equal to one day, caused SWAT output in the 2 and 6 hour retention timesimulations to better emulate the QUAL2E output. A limited test using modified algorithms fororganic and dissolved phosphorus in the SWAT model was able to produce identical results to theQUAL2E simulation, leading to the conclusion that the QUAL2E equations may not have beencorrectly incorporated into the SWAT model. More analysis needs to be done to evaluate SWATwater quality algorithms.