AGNPS Model Assessment for a Mixed Forested Watershed in Thailand

Abstract

Watershed modeling, as a tool to identify environmental problems, is becoming more popular. The modeling approach will allow evaluating present scenarios and possible remedial measures and strategies. The present study attempts to verify the suitability of the Agricultural Non-Point Source (AGNPS) pollution model developed for an agricultural watershed, for a mixed forested watershed. The study watershed, Huai Nong Prong in Southeastern Thailand, is a mixed forested watershed with 26% forest, 25% mangrove forests, and 36% agro-forests. Extensive fieldwork was completed to collect data and information needed for the model preparation and application. The study revealed that the AGNPS model produces satisfactory results regarding runoff volume and soluble nitrogen yields for the watershed. The sediment yield prediction is marginal for the selected watershed, partially attributed to the mangroves and the pools in the streams, which act as sediment traps. This suggests that mangroves in the downstream end of the watershed could act as natural wetlands capable of neutralizing or reducing environmental problems created by a watershed. The model, however, could not accurately simulate the peak flows, suggesting the peak flow simulating approach in AGNPS does not suitably predict peak flows from mixed forested watersheds.