Effects of land use on thermal enrichment of urban stormwater and potential mitigation of runoff temperature by watershed-scale stormwater control measures

Abstract

Stream temperature is critical to the survival and reproduction of fauna in cold-water ecosystems. Urbanization causes abrupt increases in ambient stream temperature via stormwater inputs while concurrently decreasing groundwater recharge and stream baseflow. Relatively little is known about how various urban land uses affect thermal load discharged to streams or whether a network of green infrastructure retrofitted at the watershed scale can ameliorate these impacts. Runoff temperature and hydrology were monitored at 17 urban and two forested watershed outlets to assess 1) how urban land use affects stormwater thermal pollution and 2) if thermal pollution can be mitigated through infiltrating stormwater control measures (SCMs). Significantly and substantially larger runoff temperatures (i.e., event mean temperature, event maximum temperature) and thermal loads were observed from every urban land use (without SCMs) in comparison to forested (i.e., surrogate for pre-development) conditions. Event mean temperatures generally followed the land use trend of parking lot > light industrial > commercial > heavy industrial > multi-family residential, single family residential > low density residential > forest. Retrofitted bioretention cells and permeable pavements were not successful in mitigating runoff temperature in residential watersheds compared to (1) watersheds without SCMs or (2) restoring temperature to surrogate pre-development conditions. SCMs were seldomly able to mitigate enough runoff volume to have a significant thermal load reduction compared to watersheds without SCMs. These results suggest stormwater management and cold-water species management have not yet been successfully integrated. More work is needed to optimize green infrastructure SCMs design for sufficient treatment of runoff to protect cold-water ecosystems from urban development. Beyond SCM implementation, wholistic watershed management practices (e.g., riparian buffers, clustered imperviousness, underground storage/conveyance) are needed to stem thermal enrichment of streams and lakes by stormwater.