Journal Article

Roles of Hydrology and Transport Processes in Denitrification at Watershed Scale

Plaza, F., Liang, X., Wen, Y., Lin, J.‐S.

2024

Overview

Rainfall runoff and leaching are the main driving forces that nitrogen, an important non‐point source (NPS) pollutant, enters streams, lakes, and groundwater. Hydrological and transport processes thus play a pivotal role in NPS nitrogen pollution. Existing hydro‐environmental models for nitrogen pollution often over‐simplify the within‐watershed processes. It is unclear how such simplification affects the pollution assessment regarding the formation and distribution of denitrification hot spots—which is important for the design of land‐based countermeasures. To study this problem, we developed a model, DHSVM‐N, and its variant, DHSVM‐N_alt. DHSVM‐N is developed by integrating nitrogen‐related processes of SWAT into a comprehensive process‐based hydrological model, the Distributed Hydrology Soil and Vegetation Model (DHSVM). DHSVM‐N includes detailed representations of nitrate transport process at a fine spatial resolution with good landscape connectivity to accommodate interactions between hydrological and biogeochemical processes along the flow travel pathways. Because of the lack of spatially distributed observational data for validation, a model‐to‐model comparison study is conducted. Through comparison studies on a representative catchment using SWAT, DHSVM‐N and DHSVM‐N_alt, we quantify the critical roles of hydrological processes and nitrate transport processes in modeling the denitrification process. That is, the capabilities to give reasonable soil moisture estimates and to account for essential processes that take place along flow pathways are keys to simulate denitrification hot spots and their spatial variation. Furthermore, DHSVM‐N results show that terrestrial denitrification from hotspots alone can reach as high as 36% of the annual stream nitrate export of the watershed. Plain Language Summary Impacts on water quality of streams and lakes caused by non‐point nitrogen sources from agricultural activities have been the focus of investigations of nitrogen pollution. Typically, such investigations are carried out by employing existing hydro‐environmental models even though they often adopt simplified hydrological and transport processes. These simplified models may serve their purposes well, but when within‐watershed dynamics, such as the denitrification and its hotspot spatial distribution within a watershed, play important roles, the existing models could be subjected to large errors or uncertainties. To improve the modeling of watershed nitrogen transport, a new model, DHSVM‐N, which incorporates essential hydrological and transport processes, is developed by combining the widely used SWAT and DHSVM models. Because of the lack of spatially distributed observational data for validation, a model‐to‐model comparison study is conducted. DHSVM‐N and SWAT are found to give very different results in the levels and spatial distribution of denitrification, as well as nitrate export to streams. Key Points Key processes determining denitrification hot spots and spatial distribution are identified via systematic analysis using three models Landscape unit connectivity, routing and runoff flow paths are critical hydrological components for modeling denitrification hot spots A process‐based model, DHSVM‐N, is developed for studying nitrogen‐related processes and pollution at watershed scale

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Denitrification / DHSVM and SWAT models / Environment models / Environmental assessment / Environmental modeling / Groundwater / habitat connectivity / Hot spots / Hydrologic models / Hydrologic processes / Hydrology / hydro‐environmental modeling / Lakes / landscape connectivity / Leaching / Modelling / Moisture content / Nitrates / Nitrogen / nitrogen pollution / Nitrogen sources / Nitrogen transport / observational studies / pollutants / Pollution / Pollution dispersion / Pollution sources / Rainfall / Rainfall runoff / Rainfall-runoff relationships / Rivers / runoff / Soil moisture / Soil moisture estimation / soil water / Spatial discrimination / Spatial distribution / spatial patterns and hot spots / Spatial resolution / Spatial variations / Stream pollution / Streams / Transport processes / travel / Vegetation / Water pollution / Water quality / Watersheds