Quantifying flow interval–pollutant loading relationships in a rapidly urbanizing mixed-land-use watershed of the Central USA

Rapid urbanization and agricultural development have altered flow and pollutant loading regimes resulting in poorly understood pollutant transport regimes and inadequately conceived management practices. Four years of hydrologic and water quality data (i.e., stream flow, suspended sediment, nitrate, nitrite, total ammonium, total inorganic nitrogen, and total phosphorus) were collected using a five-site nested-scale study design in a representative mixed-land-use watershed. Cumulative nested sub-basin drainage areas ranged from 79 to 208 km2. Daily flow and load duration analyses were used to quantify land use impacts to flow-mediated pollutant loading at multiple flow intervals including high flows (0–10%), moist conditions (10–40%), mid-range flows (40–60%), dry conditions (60–90%), and low flows (90–100%). Pollutant loads ranged across four orders of magnitude for suspended sediments, and three orders of magnitude for nutrient loads during the high flow interval when nearly all of the total pollutant loads were transported (e.g., 99% of suspended sediments, 92% of total inorganic nitrogen, and 95% of total phosphorus loads). Results from stepwise multiple linear regression analyses showed significant relationships between land use, flow, and pollutant load at high flows, moist conditions, and mid-range flow intervals (R2 > 75.7; p < 0.035). No significant relationships (p < 0.05) were detected during dry conditions, or low flow intervals. Results highlight the need for consideration of combined flow and pollutant loading targets appropriate for watersheds modified by current and ongoing land use and point to a need for long-term and broad-scale efforts to develop achievable hydrologic and water quality recommendations especially in rapidly urbanizing mixed-land-use watersheds.