CAN WATERSHED RESTORATION PRACTICES REVERSE THE HYDROLOGICAL EFFECTS OF URBANIZATION?

Urbanization degrades headwater stream ecosystems. Increased runoff from impervious surfaces alters stream flow regimes, reduces habitat quality, and conveys chemical and thermal pollution to downstream channels. Moreover, storm sewers expand the drainage network of urban watersheds, directly connecting runoff sources to downstream channels. Stormwater management and watershed restoration practices have been widely implemented to mitigate urban stream ecosystem degradation. However, their effect on improving watershed hydrological function (e.g., storage) is still poorly understood. Our study examined stream responses to rainfall events in 11 headwater streams to understand how urbanization and watershed restoration activities affect watershed storage and stream-stormwater connectivity. Percent impervious cover across the 11 watersheds ranged from 0.4% to 78%; three of the urban watersheds had been implemented with a regenerative stormwater conveyances structure (RSC), an example of a watershed restoration practice rapidly gaining popularity in the mid-Atlantic region. We monitored high-frequency stage (depth) in each stream, and deployed tipping bucket rain gauges to observe precipitation patterns for a 1-year period. Using these datasets, we quantified several metrics related to storage and connectivity. Minimum rainfall thresholds for generating a runoff response declined with increasing impervious cover. However, we observed larger thresholds in one of the restored urban watersheds than expected, due to the added stormwater storage capacity provided by the RSC structure. The frequency and duration of stormwater-stream connections also increased with urbanization, and we again observed some limited positive effects in one of the restored urban watersheds. Variability in responses among the restored watersheds is likely controlled by the landscape context in which the restorations were implemented. Our study highlights the need for urban watershed restoration strategies to focus on both enhancing stormwater storage and reducing the drainage efficiency of the storm sewer network in order to recover watershed hydrological functionality.