Modeling Runoff and Nitrogen Loads From a Watershed at Different Levels of Impervious Surface Coverage and Connectivity to Storm Water Control Measures

Abstract

Urban development of watersheds increases runoff and nitrogen loads by adding urban impervious surfaces and increasing the hydrologic connectivity of these surfaces to streams. Storm water control measures (SCMs) are designed to disrupt this connectivity by retaining water in biologically active depressions where nitrogen retention, transformation, and removal occur. This work applies a mechanistic, spatially distributed, hydroecological model (RHESSys) to a suburban watershed in Charlotte, NC, with 15% total imperviousness (TI) and 33% watershed area mitigated by SCMs. We developed emergent relationships between watershed‐scale predictors (TI and connectivity to SCMs) and water and nitrogen response variables (storm water runoff ratios and nitrogen load by species). Results showed that annual runoff ratios were insensitive to increases in connectivity to SCMs (varying by ~1% of rainfall) because SCMs did not substantially increase evaporation but that runoff ratios increased by an average 0.2% per 1% increase in TI due to decreases in transpiration in the watershed. Generally, nitrate loads increased with TI but decreased as more surfaces were mitigated by SCMs. However, these nitrate reductions corresponded to increased export of dissolved organic nitrogen and ammonium. Together, these results indicate that SCMs act as both removers and transformers of nitrogen at the watershed scale. SCMs showed a net assimilation of nitrogen in warm months and net release in cool months, which offset the timing of nitrogen export relative to inputs. This work highlights that using a hydroecological, process‐based model reveals both the emergent relationships between watershed condition and response and the processes controlling those relationships.

Stormwater Ponds in the Southeastern U.S. Coastal Plain: Hydrogeology, Contaminant Fate, and the Need for a Social-Ecological Framework

“…Given the complexity of environmental systems and the number of variables involved, site-specific models may help elucidate wet detention pond function. Knowledge of fate and transport mechanisms has been translated into models that aim to predict pollutant concentrations and loads through stormwater systems to inform engineering and management (Elliot and Trowsdale, 2007;Vezzaro et al, 2011Vezzaro et al, , 2012Fletcher et al, 2013;Bell et al, 2019). However, it has been reviewed that improvement of biophysical models will require more monitoring data for calibration and field testing, and improved understanding of fate processes and management outcomes for a wider selection of environmental contaminants (Bertrand-Krajewski, 2007;Elliot and Trowsdale, 2007;Gold et al, 2019).…”

The Regional Hydro-Ecological Simulation System for 30 Years: A Systematic Review

“…For instance, RHESSys has considered the change of water routing by roads and drainage networks, which potentially alters the transpiration, runoff and delivery of water to vegetations [42,43]. RHESSys is capable of identifying a cause-effect relationship between stressors and responses in a watershed, consequently, guiding more targeted managements and monitoring strategies for urbanized basins [44].…”

“…However, a green infrastructure could alleviate these effects. Applying RHESSys for urbanized areas in Charlotte, USA, for example, Bell et al [44] found that stormwater management measures allowed the basin to hold nitrogen during warm months while causing net nitrogen output during cold months. Shields and Tague [43] found that an increase in impervious layers in Santa Barbara cities may cause a shortage of local vegetation water usage.…”

“…Moreover, RHESSys is a useful tool for the temporally and spatially investigations of the sources of water pollution by quantitatively simulating various processes and elements related to water quality (Figure 8). Especially, RHESSys can incorporate a drainage network, which provides valuable information for urbanized watersheds [44].…”

A Rainwater Harvesting Accounting Tool for Water Supply Availability in Colorado

“…A pilot project on the Front Range of Colorado found the average annual HND to be 97% in a natural undeveloped catchment, with a runoff ratio of less than 3% [15]. Comparatively, runoff ratios in areas with residential land use typically range from 30% to 80% [16][17][18]. Per the pilot project statute, the postdevelopment runoff may be legally harvested for outdoor use because the volume was not part of the historically appropriated supply [14,19].…”