Simulation of nutrients from an agricultural watershed in Japan using the SWAT model

The Soil and Water Assessment Tool (SWAT) model was applied to the Yamada River watershed to investigate the potential for using the model for watersheds characterized by circular irrigated paddy and livestock production systems in Japan. The Yamada River watershed, a typical mosaic land use in an agricultural watershed in the Ibaraki Prefecture, includes small valleys; where the paddy fields with a circular irrigation system are located along the river on lowlands and a forest, vegetable fields, and livestock barns are located in the surrounding uplands. The runoff characteristics were significantly different between the irrigation and non-irrigation periods in the study site, and the observed runoff was well simulated using the SWAT model by setting different parameters for the irrigation and non-irrigation periods. The Nash-Sutcliffe coefficient (E _Ns), coefficient of determination (r ²), and relative error (RE) statistics were 0.86, 0.87, and 2.0 for the irrigation period and were 0.67, 0.68, and 2.6 for the non-irrigation period. However, the observed nutrient concentrations were not simulated well. The E _NS and r ² values were very weak and the RE values were not satisfactory. Improved estimates of storage nutrients in the vegetable fields and livestock wastes should be considered for further SWAT simulation, as well as improved algorithms for simulating rice paddy systems.