Spatiotemporal variability in water sources of urban soils and trees in the semiarid, irrigated Salt Lake Valley.
Plant transpiration is a critical process in ecohydrology and urban water budgets. Urban forests in the semiarid cities of the western United States are composed largely of non-native species sustained by irrigation. Given that the major end use of water in these cities is usually landscape irrigation, the efficiency of irrigation practices is essential in future water conservation measures. The dependence of urban trees on irrigation is driven not only by the availability of alternative water sources (e.g., precipitation) but also by patterns of infiltration and mobility of soil water. We used hydrogen and oxygen stable isotopes to identify spatiotemporal dynamics of trees, of varied sizes and species, and shallow soil water apportioning of irrigation versus winter snowmelt in urban parks in the semiarid Salt Lake Valley. We estimate for this ecosystem that, on average, >70% of tree transpiration and soil moisture during the summer originate from irrigation, with lower proportions of snowmelt (<30%) from the preceding winter. This implies that a fraction of summer transpiration is originated from out of phase precipitation inputs. Irrigation fractions are dominant throughout the growing season for soil and stem water, but snowmelt fractions increase in stem water towards the end of summer. Tree size, location, and species were not strongly associated with water sources under well-irrigated conditions. These results show that urban tree management practices should consider the impact of both irrigation practices and residual precipitation inputs, highlighting the importance of examining interannual time scales when calculating the local water budget.