Modeling stream water nutrient concentrations and loadings in response to weather condition and forest harvesting

A process-based biogeochemistry model, ForNBM, was used to simulate stream water nutrient concentrations and monthly and cumulative stream nutrient loadings according to the monthly air temperature and precipitation records at the Nashwaak Experimental Watershed in central New Brunswick, Canada. Compared with the field measurement data from 1973 to 1985, the simulation results show that the best simulation of stream water nutrient concentrations is for Mg 2+ with r 2 = 0.59, and the poorest is for NH 4 +-N with r 2 = 0.48. The best simulation of the monthly stream nutrient loadings is for Ca 2+ with r 2 = 0.66, and the poorest is for NO 3 −-N with r 2 = 0.53. All simulations of the cumulative stream nutrient loadings have r 2 values more than 0.90. Seasonal precipitation and air temperature played the most important role in determining the patterns of the stream nutrient concentrations and loadings. The stream nutrient concentrations were high during the winter and low during the spring. The peak values of the stream nutrient loadings appeared in the springs and the low values appeared in the winters. Although snowmelt diluted the stream water nutrient concentrations during the springs, it resulted in high stream nutrient loadings due to stream flooding. According to the assessments of the model simulations and the field measurements, the increased watershed soil nutrient losses through the stream exports after forest harvesting may not cause the decline of the watershed soil productivity because (1) the annual increments of the stream nutrient exports were small, with 2.34 kg ha −1 year −1 for NO 3 −-N, 0.93 for NH 4 +-N, 3.34 for Ca 2+, 0.30 for Mg 2+, and 5.21 for K + and (2) the increased watershed soil nutrient losses occurred during the stage of forest regeneration when vegetation growth requires low soil nutrient supply.