Controls on spatial variation of hyporheic zone denitrification in the Columbia River Basin

Abstract:

Denitrification in the hyporheic zone of river corridors is a crucial pathway to remove excess nitrogen (N) in the rivers from anthropogenic activities, such as fossil fuel combustion, and inorganic fertilizer application. However, previous studies of the effectiveness of river corridor in excess nitrogen removal via denitrification were often limited to the reach scale and low-order stream watersheds.

We introduce a basin-scale river corridor model accounting for denitrification in the hyporheic zone (HZ) at the level of the National Hydrography Dataset Plus (NHDPlus) stream reaches for the Columbia River Basin (CRB). Our study aims to answer two questions: 1) what is the relative importance of hydrologic variability and substrate variability in controlling spatial HZ denitrification in the CRB? 2) which watershed characteristics can better explain the spatial variability of HZ denitrification in the CRB?

Our model results showed that the hydrologic exchange flux explains most spatial variability in the cumulative denitrification. The relationship between the hydrologic variability/substrate availability and HZ denitrification does not vary with the stream orders, but it changes with different land uses. For example, the cumulative HZ denitrification in the urban and agriculture streams is higher than that in the forest streams by the available dissolved organic carbon. Random forest model built with the watershed properties and the modeled cumulative denitrification amount at the NHDPlus reaches identified that the stream morphology parameters (D50, stream slope) and land use (fraction of forest) are important variables in explaining the spatial variability of HZ denitrification. These results suggest that the combined effects of hydrologic variability in streams and land use influenced substrate availability control the spatial variability of HZ denitrification at the basin scale. The developed model will serve as a useful tool to identify the limiting factors in removing excess N pollution in large river basins.