Factors influencing export of dissolved inorganic nitrogen by major rivers: A new, seasonal, spatially explicit, global model

Abstract

AbstractSubstantial effort has focused on understanding spatial variation in dissolved inorganic nitrogen (DIN) export to the coastal zone and specific basins have been studied in depth. Much less is known, however, about seasonal patterns and controls of coastal DIN delivery across large spatial scales. Understanding seasonal patterns of DIN export is critical to efforts to predict impacts of coastal eutrophication, such as algal blooms and hypoxic areas, which are often seasonal phenomena. Here we describe, test, and apply a global model that predicts seasonal DIN export to coastal regions for >6000 rivers using the Nutrient Export from Watersheds (NEWS2) model. NEWS2‐DIN‐S used spatially explicit, seasonal N inputs and was calibrated with measured DIN yield (kg N km−2 season−1) for 77 rivers, distributed globally. Of the characteristics considered, DIN‐transport efficiency was positively related to runoff and negatively related to temperature (r2 = 0.34–0.60, depending on season p < 0.0001), likely due to flushing effects and increased retention by plants and soils, respectively. NEWS2‐DIN‐S incorporated these insights and performed well in predicting DIN yield (Nash‐Sutcliffe Efficiency = 0.54–0.65, depending on season). Catchments were effective in retaining DIN and average export rates were lower during the growing season (3–5% of total nitrogen inputs) compared to other seasons (6–10%) for major latitude bands. Model output was insensitive to changes in the magnitude of N inputs, suggesting that refinement of seasonal N input budgets will not substantially improve model performance. Rather, better representation of land‐to‐river N transfers could improve future models because of strong landscape N attenuation.