Microclimates in a desert city were related to land use and vegetation index

A heterogeneous patchwork mosaic of soil, vegetation, and built surfaces that result from a variety of urban land uses cause urban microclimates within cities. We studied the seasonal relationships of land use, urban plant cover and microclimate in Phoenix, Arizona, USA, metropolitan. Early morning (0500 HR) and afternoon (1500 HR) near-surface temperatures and humidities were measured along multiple transects in this desert city and outlying areas during June and December 1999. A Landsat thematic mapper normalized differential vegetation index (NDVI) image was used to quantify spatial patterns of plant density. Land use had the most pronounced effect on microclimate during the early morning hours of summer. Agricultural and residential land uses had the highest relative humidities, dew point temperatures, and NDVI, and the lowest air temperatures. Commercial and industrial land uses had highest temperatures and lowest NDVI. Temperatures were generally negatively correlated to NDVI, while humidity and dew point temperatures were generally positively correlated to NDVI. Distance from the urban core did not affect NDVI but had a significant negative effect on adjusted air temperature. In addition, a historical comparison of land use, NDVI and microclimate data collected during 1976 and again during 1999 along two transects revealed overall decreases in NDVI and relative increases in air temperature indicative of urban expansion. These findings show that microclimates in this desert city are caused by more than just variations in plant cover, and are likely an interactive effect of vegetation density and other non-vegetative urban surfaces.