Evapotranspiration of the urban forest at the municipal scale in Los Angeles, CA

Abstract:

The severest drought on record in southern California and predictions of continued water shortages make it essential to understand urban water use. However, urban evapotranspiration (ET), which is an important part of municipal water budgets, remains a major uncertainty. Urban ET is difficult to measure and model, particularly in cities with diverse plant composition. The city of Los Angeles contains more than 6 million trees, most of which are non-natives that originate from multiple geographic regions, which further complicates predictions of urban forest transpiration. Previously, we made extensive in situ measurements of tree transpiration and turfgrass ET in greater Los Angeles area. Here, we utilize these data to systematize transpiration of different tree species based on physiological mechanisms underlying plant water relations. The resulting empirical model estimates Los Angeles urban forest ET from easy-to-collect plant characteristics and freely available environmental parameters. Plant characteristics are tree diameter, wood type (e.g. coniferous), phenological type (e.g. evergreen) and plant composition. Environmental parameters are vapor pressure deficit of the air, incoming solar radiation and reference ET (all available at http://cimis.water.ca.gov). By combining this model with existing surveys of urban trees in Los Angeles, we estimated that citywide ET of irrigated landscapes varies from 1.2 ± 0.5 mm/d in winter to 2.8 ± 1.1 mm/d in summer. On average, trees and turfgrass contributed 27% and 73% to total tree+turfgrass ET, correspondingly. To our knowledge, this model provides the first citywide estimates of Los Angeles ET differentiated by wood types and plant composition. These results will inform decision makers about species-specific water use by urban trees and assist with determining landscape designs that are beneficial for water conservation. This model may also be incorporated into a regional hydrologic model to provide spatially resolved ET at the municipal scale.