Effects of urbanization and infiltration-based watershed restoration on the hydro-ecology of headwater streams
Fanelli, Rosemary Margaret
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Urbanization profoundly alters the hydrologic routing of a landscape resulting in the degradation of downstream aquatic ecosystems. To mitigate these effects, watershed managers implement infiltration-based storm water control measures (SCMs), designed to convert stormwater runoff into groundwater recharge. However, the ability of infiltration-based SCMs to restore hydrological processes, and to reverse damage to the downstream ecosystem, remains poorly understood. To address this research gap, I examined the hydro-ecological effects of urbanization and SCM implementation in 11 headwater watersheds spanning an urbanization-restoration gradient (4 forest, 4 urban-degraded, and 3 urban watersheds restored with SCMs) near Annapolis, Maryland, USA. Regenerative stormwater conveyances (RSCs) were the type of SCM examined in the study. I used high-frequency precipitation, stream stage, and baseflow discharge collected at the watershed outlets to develop metrics characterizing watershed storage and stream responses to precipitation. I then conducted water quality sampling, temperature monitoring, and quantified aquatic insect community composition in the downstream ecosystems. Finally, I employed high-frequency groundwater monitoring in one of the SCMs to identify potential mechanisms controlling their hydrological function. The hydrological effects of urbanization were clearly observed across the study watersheds, but only one of the three restored watersheds modulated hydrology (e.g., a larger minimum runoff threshold relative to the other urban watersheds). However, baseflow in this stream was low compared to the forested streams, suggesting that enhanced infiltration of stormwater runoff did not recharge storage zones that support stream baseflow. Aquatic insect diversity and the percentage of sensitive taxa declined with increasing urbanization, with no significant effect of restoration. Water quality remained poor in both urban-degraded and urban-restored streams, with higher conductivity values, lower dissolved oxygen, and warmer stream temperatures than in forested streams. These water quality issues likely hampered recovery of sensitive taxa in downstream ecosystems. Groundwater monitoring in one of the SCMs indicated that high runoff delivery rates from the watershed limited infiltration within the SCM, which allowed the conveyance of untreated runoff to the downstream channel. The centralized design of RSCs, and their placement in areas of topographic convergence above channel heads, likely limits their effectiveness for restoring hydrological processes to urban watersheds.