Simulation of stream nitrate-nitrogen export using the Soil and Water Assessment Tool model in a dairy farming watershed with an external water source

Abstract

Quantitative assessment of stream nitrate-nitrogen (NO₃-N) export in a watershed with an external underground water source is complex because the underground flux for both the water discharge and NO₃-N cannot be directly measured. A modeling approach, using the Soil and Water Assessment Tool (SWAT), was tested to simulate the NO₃-N export in the 672 km² (417 mi²) Shibetsu Watershed with external water source (EXT) in eastern Hokkaido, Japan. The EXT and its NO₃-N loading contents were added as the assumed point source discharge and loading in the SWAT model. The results yielded high Nash-Suttcliffe coefficient (Ens) values for daily streamflow (over 0.5) and monthly NO₃-N loading (over 0.7) for the simulation during the calibration and validation periods. The simulated seasonal patterns of discharge and NO₃-N loading were also well matched with the measured data during 2004 to 2008. However, the spatial patterns of NO₃-N concentrations showed a poor <i>r</i>² correlation (<i>r</i>² = 0.34) with the measured data in 2004. These results showed that, although the model had limitations for the simulation of spatial patterns, the method of adding EXT and NO₃-N loading as assumed points in SWAT was reasonable for the assessment of the stream NO₃-N export. Therefore, the calibrated model was repeated without adding EXT and NO₃-N loading as assumed points. Approximately 66% of the NO₃-N export was from the watershed itself, and the NO₃-N export was high in the grasslands. These results indicated that most of the NO₃-N export was related to dairy farming in the Shibetsu Watershed, and the best management practices for controlling nonpoint source pollution should focus on manure applications on grasslands in the future. Meanwhile, the EXT (47% of streamflow) contributed to 34% of the annual NO₃-N loading, which may cause the overestimation of the stream NO₃-N export when using only the measured data. Therefore, the EXT should not be ignored when estimating the annual NO₃-N loading.