Large-scale Riverine Reaction and Transport of Anthropogenic Nitrate in the Community Land Model

Abstract:

Increasing amount of reactive nitrogen added to the biosphere, as the direct impact of rapidly growing demands for food and energy, has gravely altered the global nitrogen and water cycles. Streams and rivers play a major role in nitrogen transformation, transport between terrestrial water systems, and export to the coastal zones. However, our knowledge of nitrogen budgets across the land, and especially in river networks, is limited due to lack of long-term and spatially continuous in situ measurements of natural and anthropogenic nitrogen loadings. This study presents the first-order estimates of large-scale stream nitrate reaction and transport by implementing a nitrogen routing scheme in the Model for Scale Adaptive River Transport (MOSART) coupled with the latest version of the Community Land Model (CLM5). The new scheme numerically solves the conventional one-dimensional advection-diffusion-Reaction equation for explicitly resolving riverine nitrate concentration across the watershed. While the model could be applied and verified at regional to global scales, simulations are conducted at ~5-km resolution over a 5,200 km² watershed in Michigan (i.e., the Kalamazoo River watershed for which rich Nitrogen data of loading and application are available from multiple sources), and forced by the North America Land Data Assimilation System phase II (NLDAS2) data. The simulated nitrate concentration is broadly evaluated using the national water quality data from the United States Geological Survey and Storet databases, as well as several private field measurements. We find promising representation and simulation of stream nitrate dynamics, achieved through the incorporation of the new Nitrogen transport scheme. The study opens up opportunities for improved large-scale nitrogen transport modeling and enhancing our understanding of nitrogen-related interactions within hydrology and biogeochemistry.