ABSTRACT

All components of the water cycle are altered by human activities in cities, and the impacts of these changes on urban water and climate are still poorly understood. Urbanization affects climate, the amount of water in soil (soil moisture), and the type and amount of vegetation across the landscape. All of these factors strongly impact evapotranspiration (ET): the flux of water from land to the atmosphere. Urban ET is poorly predicted by hydrologic models that do not adequately represent human actions, such as irrigation. Yet, urban irrigation can have large effects on climate, soil moisture, and plant growth and survival. This study addresses the extent to which ET is limited by soil moisture, atmospheric water demand (a function of humidity and air temperature), or the density and distribution of vegetation within and across U.S. cities. Measurements will be made in three urban regions: Los Angeles, CA (a semi-arid city where irrigation has been declining due to drought response policy), Salt Lake City, UT (semi-arid but still heavily irrigated), and Tallahassee, FL (high rainfall and very high urban tree cover). These cities represent urban settings with different water cycle components. This project will advance knowledge and understanding of urban ET, improve basic climate and water cycle models, and to contribute to efficient water management in cities and urban landscapes.

Urban hydrologic data are still sparse relative to observations in natural and agricultural systems. To advance a generalizable understanding of urban hydrology, it is necessary to explore categorizable differences within and across cities in the balance of water supply (soil moisture as supplied by both irrigation and precipitation), plant demand for water uptake (as determined by the magnitude, distribution, and composition of leaf area), and atmospheric evaporative demand (net radiation and vapor pressure deficit). The project will examine similarities and differences in these fluxes within and across cities by quantifying irrigation efficiency, its variability, and its key drivers. The contribution of each component of the soil-plant-atmosphere system to ET fluxes is likely to vary in mesic vs. arid/semi-arid climates and according to local irrigation practices as well as urbanization processes that influence land and vegetative cover. By sampling cities that are hypothesized to span different combinations and ranges of irrigation practices and likely limits on urban soil moisture, vapor pressure deficit, and ET, the investigators will test a general framework that can be applied beyond these specific cities and measurement sites. Ultimately, this project will use the extensive datasets collected in this study for advancing mechanistic models of urban landscape ET as an alternative to empirical crop and landscape coefficient approaches. The results will be disseminated to stakeholders and extension specialists who are focused on improving turfgrass management, outdoor water management, and urban water policy. The investigators will also leverage programs for recruiting and retaining undergraduate and graduate students from under-represented groups to build a diverse, interdisciplinary team aimed at broadening participation in STEM.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

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