A plot scale evapotranspiration model for urban landscapes in Los Angeles, CA

Abstract:

Southern California is experiencing the worst drought on record and facing more dramatic decreases in water supply due to regional climate change trends. In Los Angeles, evapotranspiration (ET) from irrigated landscapes is likely to be a major contributor to the urban water budget. However, data on water use by irrigated urban plants is very scarce and ET patterns from urban vegetation are not well quantified. Methods currently used for estimating water use by urban lawns are mainly based on Penman-Monteith model, which depends on a set of empirical coefficients that are hard to define, as well as a simplified crop coefficient approach that has low accuracy. Previously, we performed continuous in-situ measurements of transpiration from 126 urban trees in the Los Angeles area using sapflux sensors and ET from 8 turfgrass lawns using portable chambers. Here, we synthesize the results of those measurements as well as literature data and construct biologically meaningful equations for assessment of ET from urban landscapes. The resulting empirical model is reasonably accurate yet relatively simple for managers to apply with basic weather and tree inventory data. It explains 72% of tree transpiration variability and 79% of turfgrass ET variability. The parameters of the model currently include vulnerability of branches to cavitation for trees, turfgrass area and percent tree canopy cover for lawns, and incoming solar radiation, vapor pressure deficit of the air and soil water content for defining environmental conditions. To give an illustration, the model estimate of the highest ET from a vegetated area in Los Angeles during a typical summer day is 5 – 8 mm/d (high transpiring street trees with turfgrass groundcover or unshaded turfgrass) and the lowest ET is 0.6 – 0.8 mm/d (low-transpiring trees without groundcover). The next step is to substitute vulnerability to cavitation, the only model parameter that requires laboratory equipment, with parameters that can be more easily applied by non-specialists. The results of this study will inform the general public and decision makers and help develop modifications to urban landscape design and management practices for efficient outdoor water conservation. The equations will be also used with a region-specific hydrologic model to simulate spatially resolved ET at larger scales.