Vegetation response to precipitation variability is an important climate-ecosystem-hydrology feedback. Anthropogenic impacts coupled by changes in seasonal and annual precipitation patterns can have a dramatic and large spatial effect on ecosystem structure and functioning, especially in water limited environments. While the natural Sonoran Desert is water limited, Phoenix metropolitan area is constantly being irrigated to support human activities. The aim of this research is to study how urban areas differ from their natural surroundings ecosystems in their phenology and response to summer water inputs. Rain use efficiency (RUE), inter-annual and intra-annual vegetation phenology, and aboveground net primary production (ANPP) of the two land-cover types and their response to summer precipitation have been analyzed. In addition, a soil water balance model is used to simulate the Horton index (H) as a measure land-cover response to climate variability. Results show that the urban environment has a year-round constant, high productivity with high variability in RUE. The desert has lower productivity and responds strongly to summer water. Furthermore, the desert ecosystem convergences towards H=1 and RUE∼133 MJ/m^2*hr mm^-1. Based on the RUE and ANPP it was calculated that 295 mm of water input are necessary to sustain the urban tree biomass. Unlike natural ecosystems, urban areas RUE do not converges to a common maximum value, suggesting that inter annual variability in hydrological partitioning over urban and desert land-cover is consisted with the water use efficiency concept.