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 (WY 1980-WY1990) hydrochemistry data from a gauged site. The calibrated model r eproduced water yields, TN, and TP export with cumulative errors of < 1% ov er the 11-year calibration period and with annual RMS errors of 10-50%. Mod el validation was done with independent measurements at the same gauged sit e (WY1991 to WY1996) and at another nearby independently gauged site (WY199 1 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 mode l for estimation of fluxes of water, N and P from ungauged areas with accur acies of 10-50% at annual time scales.