Urbanization in Arid Central Arizona Watersheds Results in Decreased Stream Flashiness

There is broad consensus that urbanization results in dramatic changes in stream hydrology, such as higher peak flows and greater flashiness. However, this has not been definitively tested for aridlands, which are characterized by these very same hydrograph properties. We analyzed streamflow records from 19 watersheds of central Arizona, USA, to determine how hydrograph characteristics varied with urban development. Using linear mixed effects models, which factored in imperviousness along with other watershed characteristics, we evaluated influences on streamflow regime metrics calculated from daily and subdaily flow data. We found that flashiness, coefficient of variation, zero-flow days, and hydrograph rise and fall rates decreased with extent of imperviousness-the opposite pattern to that observed in previous studies primarily 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. We also found strong relationships between these hydrologic metrics and mean area-weighted discharge, watershed area, and annual precipitation. Like in humid systems, we did observe more high flow events in the urban desert streams compared to nonurban desert streams. However, this was only at the lower flood threshold; there was no 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.