Correlating Vegetation, Water Use, and Surface Temperature in a Semiarid City: A Multiscale Analysis of the Impacts of Irrigation by Single-Family Residences

Urban heat islands (UHIs) and the vegetation that mitigates them vary across space and time, but little research has investigated this coupled natural-human system using both spatial and temporal analyses. Focusing on semiarid, water-scarce Tucson, Arizona, we examined whether outdoor water use by residents of single-family homes (a practice that uses close to half of residential water supplies) contributes to urban "greenness" and the mitigation of UHI effects. Specifically, we investigated how different types of residential development mediate vegetation-water use-temperature interactions. Our data sets include Landsat-derived normalized difference vegetation index (NDVI) and surface temperatures, parcel-level zoning and assessor data, and residential water use records at the quarter section level (0.63km ²). We analyzed these data at multiple spatial and temporal scales. Spatial analysis results demonstrate that cooling from vegetative evapotranspiration is mediated by development factors as well as by topography and wind patterns. Findings also suggest that outdoor water use aside from irrigation, particularly the use of swimming pools, promotes cooling without elevating the NDVI. Temporal analysis reveal that most residential areas maintained or increased greenness despite declining 1995-2008 water use due most likely to long-term regional climate cycles. Only high-density developments with little undeveloped ground cover and few natural drainage channels exhibit a strong relationship between household water use and NDVI trends. These results suggest that the preservation of natural drainage channels and limitation of impervious surfaces, as well as the siting of development in naturally cooled microclimates, may be sustainable strategies for UHI mitigation in water-scarce regions.