PNNL

Abstract

Denitrification in the hyporheic zone of river corridors is a crucial pathway to remove excess nitrogen (N) in rivers from anthropogenic activities. However, previous modeling studies of the effectiveness of river corridors in removing excess nitrogen via denitrification were often limited to the reach-scale and low-order stream watersheds. We developed a basin-scale river corridor model based on SWAT-MRMT-R to account for denitrification in the hyporheic zone (HZ) for the Columbia River Basin (CRB) with random forest models to identify what factors control the spatial variation of HZ denitrification in streams with different sizes and land uses. Our modeling results suggest that the combined effects of hydrologic variability in streams and substrate availability influenced by land-use control the spatial variability of HZ denitrification at the basin scale. Hyporheic exchange flux can explain the denitrification extent in different-sized streams, while among the streams affected by different land uses, the combination of hyporheic exchange flux and stream concentration of dissolved organic carbon (DOC) can explain the denitrification differences. Also, we can generalize that the most influential watershed and stream variables controlling denitrification variation are stream morphology parameters (D50, stream slope), climate (annual precipitation), and stream DOC-related parameters (percent of forest area). The modeling framework in our study can serve as a useful tool to identify the limiting factors in removing excess N pollution in large river basins where direct measurement is often infeasible.