Analysis of subsurface storage and streamflow generation in urban watersheds

Abstract

Subsurface storage as a regulator of streamflow was investigated as an explanation for the large proportion of pre-event water observed in urban streams during storm events. We used multiple lines of inquiry to explore the relationship between pre-event water proportion, subsurface storage, and streamflow under storm conditions. First, we used a three-dimensional model of integrated subsurface and surface flow and solute transport to simulate an idealized hillslope to perform model-based chemical hydrograph separation of stormflow. Second, we employed simple dynamical systems analysis to derive the relationship between subsurface storage and streamflow for three Baltimore, Maryland watersheds (3.8-14 km 2 in area) along an urban-to-rural gradient. Last, we applied chemical hydrograph separation to high-frequency specific conductance data in nested urban watersheds ( 50% impervious surface cover) in Dead Run, Baltimore County, Maryland. Unlike the importance of antecedent subsurface storage observed in some systems, we found that rainfall depth and not subsurface storage was the primary control on pre-event water proportion in both field observations and hillslope numerical experiments. Field observations showed that antecedent stream base flow did not affect pre-event water proportion or streamflow values under storm conditions. Hillslope model results showed that the relationship between streamflow values under storm conditions and subsurface storage was clockwise hysteretic. The simple dynamical systems approach showed that stream base flow in the most urbanized of three watersheds exhibited the largest sensitivity to changes in storage. This work raises questions about the streamflow generation mechanisms by which pre-event water dominates urban storm hydrographs, and the shifts between mechanisms in rural and urban watersheds.

Dynamics of nitrate concentration‐discharge patterns in an urban watershed

“…The observed threshold in dQ/Q behavior suggests that the event magnitude changes the dominant source of nitrate‐rich water to the stream. We have observed in previous work [ Bhaskar and Welty , ] and have shown here (Figure b) that smaller storm events are dominated by old water (groundwater), whereas larger storm events are dominated by new water (rainwater and overland flow). For small dQ/Q events in this watershed, if high‐nitrate‐concentration‐laden groundwater is the principal discharge source to the stream, this could explain why nitrate concentration increases as a function of discharge for these events.…”

“…A binary mixing model was used to separate old water (pre‐event water) from new water (event water) for storms where SC data were available, as detailed in Bhaskar and Welty []. An SC value of 0.025 mS/cm was assumed for event water, based on data from the National Atmospheric Deposition Program (NADP) Beltsville, MD station.…”

Urban base flow with low impact development

“…those in Figure B and Figure E). Conventionally, urbanized watersheds were found to have faster base flow recessions in Atlanta, Georgia (Rose and Peters, ) and Baltimore, Maryland (Bhaskar and Welty, ) compared with less urbanized counterparts, which are the opposite of the slower recession observed after urbanization in Tributary 104. A faster base flow recession was found in a Baltimore watershed even though it was estimated to have a dramatic reduction in ET compared with nearby forested watersheds (Bhaskar and Welty, ), and lower ET would be expected to lead to slower recessions (Wittenberg, ).…”

The direct and indirect effects of watershed land use and soil type on stream water metal concentrations

“…Salo and Mäkinen , ]. The positive effect of fine sediment soil across land use intensities implies that soil can act both as a sink and a source for metals, highlighting the importance of small particle size for metal retention [ Brunke and Gonser , ; Micó et al ., ; Bhaskar and Welty , ]. In addition, during base flow conditions, both the sedimentation and biological activity may have reduced the observed concentrations [ Sherrell and Ross , ].…”