Phosphorus and cadmium transport via surface runoff in Songkhla Lake catchment using AnnAGNPS and TREX models

Abstract

To model transport of phosphorus and cadmium via surface runoff from its catchments to Songhkla Lake. The lake is located in southern Thailand covering an area of 1,042 sq.km with a drainage area of 7,687 sq.km. As economy in the area based on agriculture, non-point source pollution loading from the surrounding drainage area to the lake is of great concern. Surface runoff is considered as a major mechanism to transport non-point source pollutants. To model non-point source pollution transport via surface runoff, spatially distributed parameter models are commonly used. In this study, Substance Flux Analysis (SFA) was used as a tool to obtain better estimates of potential phosphorus and cadmium loadings from the catchment. AnnAGNPS and TREX models were used to simulate the transport of phosphorus and cadmium from the drainage area to the lake. The catchment was divided into 8 sub-watershed based on delineation results and individually modeled, calibrated and validated. The selected parameters for calibration include hydraulic conductivity, soil erodibility, flow resistance, crop management factor and chemical distribution coefficient. The results obtained from validation process reveal the relative loadings of phosphorus and cadmium from different parts of the watershed. Almost one third of the phosphorous and cadmium were contributed from U-Tapao and Eastern Coast #4 sub-watersheds (34%). Klong Pa Payom & Thanae and Phru Poh & Rattaphum sub-watersheds contributed at approximately 15% each. Three scenarios, comprises of change of fertilizer application rate, change of fertilizer type, and change of crop types, were conducted in the most contributed sub-watershed, U-Tapao and Eastern Coast #4. The scenarios demonstrated the changing of fertilizer type from high cadmium type (3 mg-Cd/kg) to low cadmium type (1 mg-Cd/kg) leads to 5 times declining of cadmium load to the lake. This information suggests an effective approach to minimize cadmium loadings by changing of phosphate fertilizer type. Decision makers and/or planners should take this information into account when making a consideration for crops planning or agricultural extension.