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Monday, October 13, 2014

Tracing Urban Stormwater

Our partners at Keene High School, Keene State College, and technician Errin have been monitoring Beaver Brook above and below the city of Keene (sites BBU and BBD, respectively). They also have sensors in an urban storm drain (site BWS) that contributes to Beaver Creek between BBU and BBD. We have enjoyed viewing the data, so thought that we would share.

Below is a time series from October 2013 showing water stage and specific electrical conductance at the three sites, highlighting three storms. The first is on Oct 4, which barely impacts the upstream site, has a clear signal in the storm drain, and that signal is also strong downstream of town. The rapid increase in stage and decrease in specific electrical conductance (SpEC) are typical storm responses within the LoVoTECS network. Groundwater is generally saltier than rain - thus the higher SpEC at low flows - and so direct runoff of rain dilutes a stream. But, the upstream site does not show a substantial stage or SpEC change after the Oct 4 event, which was about 0.5 inches of rain according to area CoCoRaHS observers. The two next waves of rain on Oct 6 and 8, were generally smaller than the Oct 4 amount, but the storm drain reacts strongly and so does the BBD downstream site. The upstream site shows some response to the Oct 6 and 8 rain, but much less than the other sites.

This clear impact of urban drainage on direct runoff to streams is novel. Because urban areas are so small relative to other land covers in a watershed, it is difficult to detect the hydrologic impact of urbanization in watersheds (see Hollis' critical view and O'Driscoll's recent review). Our partners and their sensors are seeing the impact.