DETERMINING CRITICAL WATER QUALITY CONDITIONS FOR INORGANIC NITROGEN IN DRY, SEMI‐URBANIZED WATERSHEDS¹

Abstract

Traditional approaches to establishing critical water quality conditions, based on statistical analysis of low flow conditions and expressed as a recurrence interval for low flow conditions (e.g., 7Q10), may be inappropriate for drier watersheds. The use of 7Q10 as a standard design flow assumes year-round flow, but in these watersheds, 7Q10 is zero or very small. In addition, the increasing use of multiple year dynamic water quality models at daily time steps can supercede the use of steady state approaches. Many of these watersheds are also under increasing urbanization pressure, which accentuates the flashiness of runoff and the episodic nature of critical water quality conditions. To illustrate, the conditions in the Santa Clara River, California, are considered. A statistical analysis indicates that higher inorganic nitrogen concentrations correlate strongly with low flow. However, peaks in concentrations can occur during the first storms, particularly where nonpoint source contribution is significant. Critical conditions can thus occur at different flow regimes depending on the relative magnitude of flow and pollutant contributions from various sources. The use of steady state models for these dry semi-urbanized watersheds based on 7Q10 flows is thus unlikely to accurately simulate the potential for exceeding water quality objectives. Dynamic simulation of water quality is necessary, and as the recent intense storm event sampling data indicate, the models should be formulated to consider even smaller time steps. This places increasing demand on computational resources and datasets to accurately calibrate the models at this temporal resolution. (KEY TERMS: TMDL; watershed modeling; low flow; runoff; loads; storm water; inorganic nitrogen.)

Socio‐ecohydrology and the urban water challenge

“…Consequently, many studies combine remote sensing products with variables simulated by physically based models (Norman et al, 1995;Bastiaanssen et al, 1998;Jiang and Islan, 2001;Su, 2002), although these currently include many uncertainties in model structure and assigned parameters that must be further improved (Hogue et al, 2006;Schoups et al, 2008). Arid and semi-arid watersheds also present unique challenges in streamflow modelling (Keller et al, 2004;Ackerman et al, 2005). During storm events, flow in arid watersheds is extremely episodic (Tiefenthaler et al, 2001).…”

Integrating hydrologic modeling and land use projections for evaluation of hydrologic response and regional water supply impacts in semi-arid environments

“…However, none of these studies focused on arid or semi-arid urban regions. Compared to humid regions, arid and semi-arid watersheds provide unique modeling challenges (Keller et al 2004;Ackerman et al 2005). During storm events, flow in arid watersheds is extremely flashy (Tiefenthaler et al 2001).…”

“…The model has been extensively applied in various practices, including simulation of streamflow (Brun and Band 2000;Bledsoe and Watson 2001;Hayashi et al 2004;Kim et al 2007;Choi and Deal 2008;Chung and Lee 2009), baseflow (Brun and Band 2000), streamflow temperature (Chen et al 1998a, b), as well as stream loadings of nutrients, pesticide, and sediments from agricultural and urbanizing watersheds (Tong and Chen 2002;El-Kaddah and Carey 2004;Hayashi et al 2004;Keller et al 2004;Luo et al 2007;Marcé and Armengol 2009). The size of the watersheds studied in these applications ranges from 14.7 km 2 (Des Moines Creek Watershed, Washington, US; Bledsoe and Watson 2001) to 1,000,000 km 2 (Upper Yangtze River Basin, China; Hayashi et al 2004), indicating the applicability of the HSPF to both small and large watershed systems.…”

Risk Based Critical Condition Analysis for the Red River Fecal Coliform TMDL

“…One of the approaches in such studies is to simulate the impact of rainfall for a selected period of time (1 year, 5 year, etc.) on river water quality, and identify combinations of river flows and rainfall conditions that have the greatest impacts on river water quality (Keller et al, 2004). In this approach, the selected time periods for model simulations do not always include the most critical conditions for the combinations of river flow and rainfall that can cause the greatest impact.…”