Evapotranspiration of urban landscapes in Los Angeles, California at the municipal scale

Evapotranspiration (ET), an essential process in biosphere-atmosphere interactions, is highly uncertain in cities that maintain cultivated and irrigated landscapes. We estimated ET of irrigated landscapes in Los Angeles by combining empirical models of turfgrass ET and tree transpiration derived from in situ measurements with previously developed remotely sensed estimates of vegetation cover and ground-based vegetation surveys. We modeled irrigated landscapes as a two-component system comprised of trees and turfgrass to assess annual and spatial patterns of ET. Annual ET from vegetated landscapes (ET_veg) was 11106 53 mm/yr and ET from the whole city (vegetated and nonvegetated areas, ET_land) was three times smaller, reflecting the fractional vegetation cover. With the exception of May and June, monthly ET_land was significantly higher than predicted by the North American Land Data Assimilation System. ET_veg  was close to potential ET, indicating abundant irrigation inputs. Monthly averaged ET_veg varied from 1.560.1 mm/d (December) to 4.360.2 mm/d (June). Turfgrass was responsible for 70% of ET_veg. For trees, angiosperm species (71% of all trees) contributed over 90% to total tree transpiration, while coniferous and palm species made very small contributions. ET_land was linearly correlated with median household income across the city, confirming the importance of social factors in determining spatial distribution of urban vegetation. These estimates have important implications for constraining the municipal water budget of Los Angeles and improving regional-scale hydrologic models, as well as for developing water-saving practices. The methodology used in this study is also transferable to other semiarid regions for quantification of urban landscape ET.