Calibration and validation of a non-point source pollution model

Surface runoff and sediment transport influence soil quality and the qualit y of receiving waters. Simulation models are useful tools to describe drain age behavior and sediment yield to develop management strategies for agricu ltural waters. The objective of this study was to investigate the performan ce of the Agricultural Non-Point Source Pollution Model (AGNPS) and modifie d versions comparing measured and predicted surface runoff and sediment yie ld at the drainage outlet. The study was carried out using 52 rainfall-runo ff events from two small watersheds in Bavaria, 22 for calibration and 30 f or validation. Evaluation of model outputs was based on graphical displays contrasting measured and predicted values for each rainfall-runoff event, a nd standard statistics such as coefficient of efficiency. A comparison betw een three different surface runoff methods el (uncalibrated curve number (C N) method), Q(2) (calibrated CN method), and Q(3) (Lutz method) showed that the uncalibrated CN method underestimated measured surface runoff consider able, while the Lutz method outperformed the calibrated CN method and was b etter than the uncalibrated CN method. The modifications made to sediment y ield calculations encompassed: (i) replacement of the Universal Soil Loss E quation LS factor algorithm by one based on stream power theory (variant S- 2), and (ii) linkage of channel erosion by individual categories of particl e-size to runoff velocity (variant S-3) The sediment yield predictions by S -2 and S-3 outperformed the predictions of S-1 (USLE). Calculations based o n S-3 gave the best agreement when compared to measured sediment yield valu es at the drainage outlet. (C) 2000 Elsevier Science B.V. All rights reserv ed.