Spatial and Temporal Effects of Irrigation on Neighborhood-Scale Thermal Environments

Despite its reputation as an arid desert city, Phoenix Arizona receives substantial irrigation for many parks and neighborhoods through a network of dams, lakes and canals that bring snow melt from the high country, through the desert, to city neighborhoods. Several times each month, certain neighborhoods receive this irrigation, which floods yards and parks with several inches of water that seeps into the soil over the period of 2 to 4 hours. In stark contrast, other parts of the city are characterized by Xeriscaped yards with very little irrigation. This paper takes advantage of this intra-urban contrast to explore the spatial and temporal effects of irrigated landscapes on local microclimates. Specifically, we analyze several years of hourly temperature and dew point data for weather stations located in irrigated and non-irrigated neighborhoods throughout the metropolitan area. These data are linked to records of timing and spatial extent of flood irrigation to identify the observed impact of such irrigation on near-surface urban air temperatures and humidity. We supplement this analyis with a study of data from dedicated urban traverses prior to and immediately subsequent to irrigation events in and around floor-irrigated parcels of land. While flood irrigation does represent an extreme form of irrigation, this study does provide information useful in determining upper bounds for the spatial and temporal extent of effects resulting from irrigation and landscape variations more generally.