Water Quality Modeling of Two Agricultural Fields in Southern Quebec Using SWAT

To study the dynamics of nutrient transport at the field scale, we collected data from two tile-drained agricultural fields in the Pike River watershed of southern Quebec. A two-year data set was used to calibrate and validate the Soil and Water Assessment Tool (SWAT) for sediment, nitrate, and phosphorus loads exiting the field through surface runoff and tile drainage. We found that SWAT output on water quality required an accurate estimation of the timing and form of field management practices employed. After calibration, the monthly coefficients of performance (Cp) over four site-years varied from 0.23 to 0.89 for sediment loads, from 0.48 to 1.35 for nitrate loads, and from 0.38 to 0.67 for total phosphorus loads. Subsurface nitrate loads accounted for 97.7% and 86.7% of the total nitrate yield, while particulate phosphorus accounted for 61.2% and 87.7% of total phosphorus load on sites 1 and 2, respectively. SWAT underestimated nitrate loads in subsurface drainage during spring snowmelt and large storms. Sediments and particulate phosphorus predictions were most accurate of all simulated parameters, whereas dissolved phosphorus was marginally overestimated year-round. Overall, SWAT satisfactorily reproduced field observations for sediment and nutrient transport and could be used to compare the impacts of implementing different best management practices (BMP) on individual fields for the study site.