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 (ETveg) was 1110 ± 53 mm/yr and ET from the whole city (vegetated and nonvegetated areas, ETland) was three times smaller, reflecting the fractional vegetation cover. With the exception of May and June, monthly ETland was significantly higher than predicted by the North American Land Data Assimilation System. ETveg was close to potential ET, indicating abundant irrigation inputs. Monthly averaged ETveg varied from 1.5 ± 0.1 mm/d (December) to 4.3 ± 0.2 mm/d (June). Turfgrass was responsible for ∼70% of ETveg. For trees, angiosperm species (71% of all trees) contributed over 90% to total tree transpiration, while coniferous and palm species made very small contributions. ETland 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.

Although the water management sector is often characterized as resistant to risk and change, urban areas across the United States are increasingly interested in creating opportunities to transition toward more sustainable water management practices. These transitions are complex and difficult to predict – the product of water managers acting in response to numerous biophysical, regulatory, political, and financial factors within institutional constraints. Gaining a better understanding of how these transitions occur is crucial for continuing to improve water management. This paper presents a replicable methodology for analyzing how urban water utilities transition toward sustainability. The method combines standardized quantitative measures of variables that influence transitions with contextual qualitative information about a utility's unique decision making context to produce structured, data‐driven narratives. Data‐narratives document the broader context, the utility's pretransition history, key events during an accelerated period of change, and the consequences of transition. Eventually, these narratives should be compared across cases to develop empirically‐testable hypotheses about the drivers of and barriers to utility‐level urban water management transition. The methodology is illustrated through the case of the Miami‐Dade Water and Sewer Department (WASD) in Miami‐Dade County, Florida, and its transition toward more sustainable water management in the 2000s, during which per capita water use declined, conservation measures were enacted, water rates increased, and climate adaptive planning became the new norm.

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In western North America, massive infrastructure systems convey water over hundreds of miles to support cities, farms and economic growth. The systems were twentieth-century solutions to seasonal water scarcity, but today imported water supplies are strained. Cities are looking to water conservation and local sources of supply as future solutions. But integrated planning remains a challenge. Here, we assess economic implications of transitioning to local water supply reliance. We enhance an integrated model (Artes) of urban water management in Los Angeles County (LA County) with annualized costs for water supply sources (local and imported) and water conservation. We model cost-effective options for mitigating the effects of imported water cuts affecting nine million people within a hundred agencies, and denote associated policy options. The approach allows for a more holistic assessment that improves upon current studies, which typically compare nominal and annualized prices of different options. The concept of urban water supply trains, which include the multiple steps of acquiring, treating, distributing, and discharging or reusing water, is presented to understand emerging cycles of water supply. The analysis enhances current studies that often emphasize single agency perspectives, but also shows how existing assumptions and infrastructure shape what appear to be cost-effective options.Systematic studies of urban water management necessarily include many aspects of operations [1][2][3][4] . Engineered pipe and channel networks move surface water and runoff. Water supply utilities build and maintain systems to keep taps running. Complex networks of agencies oversee the acquisition, distribution and use of urban water and wastewater 5,6 . Landscapes and outdoor water use are especially important drivers of water demand in California, as half of urban water use goes to irrigation and trees [7][8][9][10] . Economics and social attitudes shape the expectations of residents and utilities for water supply. In many cities across the globe, climate change and population growth will test operating assumptions in the current systems 11 .Los Angeles County is an immense laboratory for exploring the future of urban water in seasonally dry climates (Fig. 1). The county currently receives 55-60% of its annual water supplies from imported sources, which include northern California through the Sacramento-San Joaquin Delta, the Colorado River basin, and the higher-altitude Owens Valley. The remainder comes from local sources, including groundwater pumping, recycled water (nonpotable or indirect recharge) and stormwater capture 5,12,13 . Of the water imported to southern California by the giant Metropolitan Water District of Southern California (MWD), Los Angeles County agencies receive approximately 40%. The City of Los Angeles receives additional imports from the Owens Valley. A shift to becoming primarily reliant on local sources is significant in a region famous for its efforts to import water [14][15][16] .Previously, to u...

Los Angeles, which relies on large infrastructure systems that import water over hundreds of miles, faces a future of reduced imports. Within Los Angeles and its hundreds of water agencies, the capacity to adapt to future changes is influenced by laws, institutions, and hydrogeology. This paper presents a systems analysis of urban water management in metropolitan Los Angeles County to assess opportunities for increasing local water reliance. A network flow model was developed to investigate management tradeoffs across engineered, social, and environmental systems. With an aggressive regional demand target, increased stormwater capture (300%), and prioritized water reuse from existing facilities, imported water supplies can be cut by 30% while maintaining landscapes, economic productivity, and groundwater resources. Further reducing imports (by 40-50%) is possible through actions to promote additional reuse, recharge, conservation, and groundwater access. Reducing imported water without significant conservation results in likely groundwater overdraft. Fragmented networks of agencies in Los Angeles create an uneven landscape of vulnerability to water shortages. The paper discusses model applications, research needs, and policy implications of results for dry-climate cities.