Modeling the hydrochemistry of the Choptank River Basin using GWLF and Arc/Info: 1. Model calibration and validation

We modeled nutrient export in the Choptank River Basin on the coastal plain of the Chesapeake drainage, using a modified version of a lumped-parameter, hydrochemical model (GWLF). Calibration was performed using long-term (WY1980–WY1990) hydrochemistry data from a gauged site. The calibrated model reproduced water yields, TN, and TP export with cumulative errors of <1% over the 11-year calibration period and with annual RMS errors of 10–50%. Model validation was done with independent measurements at the same gauged site (WY1991 to WY1996) and at another nearby independently gauged site (WY1991 to WY1995). Local adjustment of the groundwater recession coefficient and the dissolved N concentration in agricultural stormflow was essential for successful application at the second site. GWLF appears to be a useful model for estimation of fluxes of water, N and P from ungauged areas with accuracies of 10–50% at annual time scales.