Advancing Watershed Legacy Nitrogen Modeling to Improve Global Water Quality

Abstract

Despite widespread implementation of watershed nitrogen reduction programs across the globe, nitrogen levels in many surface waters remain high. Watershed legacy nitrogen storage, i.e., the long-term retention of nitrogen in soils and groundwater, is one of several explanations for this lack of progress. However scientists and water managers are ill-equipped to estimate how legacy nitrogen moderates in-stream nitrogen responses to land conservation practices, largely because modeling tools and associated long-term monitoring approaches to answering these questions remain inadequate. We demonstrate the need for improved watershed models to simulate legacy nitrogen processes and offer modeling solutions to support long-term nitrogen-based sustainable land management across the globe.

Keywords: hydrology; legacy nitrogen; nitrogen; nutrient management; watershed modeling.

Figure 1

Linked land-to-water components involving legacy N processes and management, and examples of each of these linked components.

Figure 2.

Minimum collective legacy N-related dynamics required to improve watershed models for legacy N simulations. These include additions and modifications in the soil profile, shallow aquifer, deep groundwater, and hydrological transport to the nearest surface water. Some widely used watershed models (e.g., SWAT) already integrate many processes into the soil profile (e.g., N accumulation and soil N mineralization) and shallow aquifer (see the asterisk). These processes, however, need refinements and more testing to ensure they are accurately represented. However, other shallow aquifer and all deep groundwater legacy N dynamics are typically absent in most watershed models. While hydrological flowpaths are also present, the potential for decadal lags in horizontal (to the stream) and vertical (through the soil and groundwater system) flows is also atypical in current watershed models and needs to be incorporated.