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Flashiness of urban and desert streams in arid Arizona watersheds (2003-2016)

There is general consensus that urbanization results in dramatic changes in stream hydrology, such as higher peak flows and increased flashiness (the rate at which water levels rise and fall). However, this has not been thoroughly assessed for aridlands, which are characterized by these very same hydrograph properties. This study analyzed streamflow records from 19 watersheds of central Arizona, USA to determine how hydrograph characteristics varied with urban development. This study evaluated influences on streamflow regime metrics calculated from daily and sub-daily flow data using linear mixed effects models, which factored in imperviousness along with other watershed characteristics. Results indicate that flashiness, coefficient of variation, zero flow days, and hydrograph rise and fall rates decreased with extent of imperviousness, which is the opposite pattern to that observed in previous studies predominately in humid regions. Engineered retention basins are one explanation for this observation, though novel urban sources of dry weather flows are likely also playing a role. This study also identified strong relationships between these hydrologic metrics and mean area-weighted discharge, watershed area, and annual precipitation. As in more humid systems, more high flow events in the urban desert streams compared to non-urban desert streams were observed. However, this was only at the lower flood threshold, and there was not an increase in larger floods with urban development. Overall, the urban stream syndrome manifests differently in this arid system: urbanization increases water retention and leads to less variable flows in stream ecosystems. Data and analysis code are included with this data set.