Hydrology and water quality across gradients of agricultural intensity in the Little River watershed area, New Brunswick, Canada

Abstract

Impacts of suspended sediment and agrochemicals on water quality comprise one of the dominant environmental concerns affecting the agricultural sector today. Three sub- watersheds within the Little River watershed (New Brunswick, Canada) were instrumented to continuously monitor discharge and sample water for suspended sediment and agrochemi- cal analysis with the objective of determining the impacts of different agriculture and forestry intensities on water yield and quality on a long-term basis. The subwatersheds included (1) Black Brook watershed (BBW), a predominantly agricultural (65%) watershed with potato as the major crop; (2) Little River watershed (LRW), a combination of forest (77%) and agricultural (16%) land uses; and (3) Upper Little River watershed (ULRW), a forest- and wetland-dominated watershed (94%). Data from 2003 to 2007 showed that average water yields were 0.588, 0.849, and 0.901 million m 3 km -2 y -1 (53.78, 77.65, and 82.41 million ft 3 mi -2 yr -1 ) for the BBW, LRW, and ULRW, respectively, indicating that water yield decreased with increasing agricultural intensity. Average suspended sediment yield in the receiving water decreased from high agricultural intensity to low agricultural intensity, with values of 181.60, 121.60, and 57.00 Mg km -2 y -1 (470.34, 314.94, and 147.63 tn mi -2 yr -1 ) for the BBW, LRW, and ULRW, respectively, as did flow-weighted sediment concentra- tion (0.33, 0.13, and 0.06 mg L -1 for the BBW, LRW, and ULRW, respectively). Average flow-weighted nitrate-nitrogen concentration in the water of the BBW was 4.39 mg L -1 compared to 1.24 mg L -1 in the LRW and 0.45 mg L -1 in the ULRW; ortho-phosphorus in the BBW was 56.43 μg L -1 compared to 24.29 μg L -1 in the LRW and 19.56 μg L -1 in the ULRW. Major ions such as potassium, magnesium, and calcium followed similar trends to those observed for suspended sediments and dissolved nutrients across the watersheds. Water pH showed little variation among watersheds, but water conductivity and water temperature were highly related to the intensity of agriculture. While the differences across the watersheds were generally consistent, differences among years were variable-dependent. Except for the ortho-phosphorus load, chemical loads of nitrogen, potassium, calcium, and magnesium were closely related to annual precipitation, but chemical concentrations did not show much correlation with either rainfall amount or erosivity. The robust time-series data available for this study enabled meaningful assessment of agricultural impacts at a landscape scale.