Abstract

Agriculturally-driven land use change and hydrologic modifications have influenced solute transport in midwestern U.S. streams. A clear understanding of the mechanisms driving nutrient export from agricultural watersheds will be critical in mitigating diffuse nutrient pollution, given anticipated shifts in hydrology associated with a changing climate. Specifically, more frequent, intense precipitation and altered snow patterns are predicted for the upper Midwest. We used four years of high-frequency nitrate (NO3--N) sensor data from two tile-drained, agricultural watersheds in Indiana to explore NO3--N export for 200 storms. We used concentration-discharge (C-Q) relationships and two indices, the hysteresis index (HI) and flushing index (FI), to understand physicochemical controls of NO3--N export across time scales. On both annual and seasonal time scales, we found NO3--N concentrations were largely chemostatic; however, patterns in FI suggested C-Q relationships for individual storms were highly variable, which may influence estimates of watershed-scale NO3--N export. We also found storm NO3--N export was strongly driven by mobilization of distal sources, given the predominance of counterclockwise hysteresis. In both watersheds, HI and FI values varied seasonally and with storm size, and patterns were linked to changes in hydrologic connectivity related to variation in seasonal tile drain flow. Variation in storm-specific NO3--N yields was driven by event runoff, storm duration, and antecedent basin moisture, rather than antecedent precipitation. Overall, we found that high-frequency NO3--N data accurately documented the magnitude of the ecological challenge presented by storm-driven nutrient export in agricultural watersheds.

Keywords

Nitrate; Concentration-discharge relationship; Agricultural watershed; Extreme events; Hysteresis index; Flushing index

Published in

Biogeochemistry
2021, volume: 156, number: 3, pages: 319-334
Publisher: SPRINGER

Authors' information