Modeling Riverine Nitrogen Transport through the Coupled Noah-MP-CN and RAPID River Routing Model

Abstract:

Hypoxia has been recognized as one of the most severe water quality problems. The dramatic increase in the number of hypoxia-experiencing water bodies is primarily caused by eutrophication, which itself is caused by nitrogen as the leading nutrient. The community Noah land surface model with multi-parameterization (Noah-MP) was recently extended to simulate the terrestrial nitrogen cycle by embedding soil nitrogen dynamics from the Soil and Water Assessment Tool and plant nitrogen uptake processes from the Fixation and Uptake of Nitrogen (hereafter, Noah-MP-CN). Noah-MP-CN is capable of simulating potential non-point nitrogen source runoff and its impacts on crop productivity and water resources. To advance water quality predictions, we developed a riverine nitrogen transport model by coupling Noah-MP-CN and the Routing Application for Parallel computation of Discharge (RAPID), a vector-based river routing model (hereafter, Noah-MP-CN-RAPID). RAPID is efficient in representing the horizontal water movement in river networks containing thousands of reaches. The model development allows us to simulate nitrogen transport at large river networks, and examine the impacts of terrestrial nitrogen dynamics on nitrogen export in complex watersheds with varying land use practices and climate and hydrologic conditions. We assess the model efficacy in simulating riverine nitrogen fluxes for the Texas Gulf region from 2008 to 2018 against the USGS gauge observations. We then discuss the model’s sensitivity to land management practices and climate conditions. To conclude, the Noah-MP-CN-RAPID shows premise as a useful model for advanced water quality predictions. Noah-MP-CN-RAPID could be adopted in other basins and applicable at continental to global scales.