Considering Streamflow Trend Analyses Uncertainty in Urbanizing Watersheds: A Baseflow Case Study in the Central United States

Assuming pro rata reductions in baseflow resulting from urban development may not be valid in all urbanizing watersheds. Anthropogenic offsets or compensatory contributions to baseflow (e.g., net exfiltration from sewer lines, wastewater effluents, and lawn irrigation) may mask or confound fundamental changes in hydrologic pathways. These offsets illustrate the complexities of urban flow processes and the need for improved understanding to mitigate urban development impacts. The authors used two dissimilar automated baseflow separation algorithms and Monte Carlo techniques to evaluate urban baseflow and estimation uncertainty using data from a representative urban watershed in the central United States. Three uncertainties affecting trend determinations were assessed, including algorithm structure, precipitation-runoff relationships, and baseflow algorithm parameterization. Results indicate that, despite ongoing population growth and development, annual streamflow metrics in the authors' representative watershed have not significantly increased or decreased (p > 0.05) from 1967 to 2010. However, several streamflow metrics featured shallow insignificant (p > 0.05) slopes in the direction hypothesized for an urbanizing (less pervious) watershed, including a downward slope for baseflow index (BFI) and increases in runoff volume coefficient. Median annual baseflow estimations differed by 29% between techniques (85.3 versus 118.9 mm yr(-1)). In the absence of direct tracer measurements, uncertainties associated with precipitation-runoff relationships, algorithm structure, and parameterization should be included in analyses evaluating alterations from baseline hydrologic conditions in urban watersheds. To advance application of separation algorithms for urban watersheds and support regulatory reductions in runoff volume, future work should include calibration of model parameters to available hydrogeologic and tracer data.