Quantification and simulation of nutrient sources at watershed scale in Mississippi.

Excessive quantity of nutrient produced from various point and non-point source deteriorates the quality of water. For reduction of nutrient from a watershed, it is needed to be quantified followed by implementation of appropriate management practices. In this study, major sources of nutrient in Big Sunflower River Watershed (BSRW) were identified, quantified, and Soil and Water Assessment Tool (SWAT) model was applied for assessment of flow, sediment and nutrient. SWAT was calibrated and validated for streamflow, sediment, total nitrogen (TN), and total phosphorus (TP) for three United States Geological Survey (USGS) gauge stations within BSRW. Moreover, different scenarios, based on agricultural operation and best management practices, were developed. Performance of SWAT was evaluated using Nash-Sutcliffe Efficiency (NSE) and coefficient of determination (R). The performance was good for streamflow during calibration and validation with R and NSE ranging from 0.74 to 0.90 and 0.70 to 0.82 respectively. SWAT performed satisfactorily for sediment, TN and TP except in few extreme conditions, where animal waste was mixed with farm runoff. This study has provided a suitable crop rotation and management practice for efficient management of nutrient in BSRW. Manure applied on field cultivated entirely with soybean was the best practice for reduction of TN with R and NSE ranging from 0.70 to 0.90 and 0.58 to 0.76 respectively. Application of manure only on existing soybean crop-land was the best practice for reduction of TP with R and NSE ranging from 0.51 to 0.69 and 0.41 to 0.60 respectively. Soybean was effective in accumulating both nitrogen and phosphorus from soil. This study will be helpful for efficient planning and management of nutrient through suitable crop rotation and management practice.