Collaborative Research: Understanding the hydrologic consequences of urban irrigation across the U.S.

城市中人类活动改变了水循环的所有组成部分，而这些变化对城市水和气候的影响仍然知之甚少。城市化影响气候、土壤含水量（土壤湿度）以及整个景观中植被的类型和数量。所有这些因素都强烈影响蒸散发（ET）：从陆地到大气的水通量。水文模型对城市ET的预测很差，不能充分反映人类活动，如灌溉。然而，城市灌溉对气候、土壤湿度、植物生长和生存都有很大的影响。本研究探讨了土壤湿度、大气需水量（湿度和气温的函数）或美国城市内外植被的密度和分布对ET的限制程度。测量将在三个城市区域进行：加利福尼亚州的洛杉矶（半干旱城市，由于干旱应对政策，灌溉一直在减少），犹他州的盐湖城（半干旱但仍有大量灌溉），佛罗里达州的塔拉哈西（高降雨量和非常高的城市树木覆盖率）。这些城市代表了具有不同水循环成分的城市环境。该项目将促进对城市ET的认识和理解，改进基本的气候和水循环模型，促进城市和城市景观的有效水管理。与自然和农业系统的观测相比，城市水文数据仍然很少。为了促进对城市水文的一般性理解，有必要探索城市内部和城市之间在供水平衡（灌溉和降水提供的土壤水分）、植物对水分吸收的需求（由叶面积的大小、分布和组成决定）和大气蒸发需求（净辐射和蒸汽压亏缺）方面的可分类差异。该项目将通过量化灌溉效率、其可变性及其关键驱动因素，研究城市内部和城市之间这些通量的异同。土壤-植物-大气系统的每个组成部分对蒸散发通量的贡献可能在湿润气候与干旱/半干旱气候中有所不同，并根据影响土地和植被覆盖的当地灌溉做法和城市化进程而有所不同。通过对假设跨越灌溉实践的不同组合和范围以及城市土壤湿度、蒸汽压赤字和ET的可能限制的城市进行抽样，研究人员将测试一个可以应用于这些特定城市和测量地点之外的一般框架。最终，本项目将利用本研究收集的大量数据集来推进城市景观ET的机制模型，作为经验作物和景观系数方法的替代方法。研究结果将分发给专注于改善草坪草管理、室外水管理和城市水政策的利益攸关方和推广专家。研究人员还将利用从代表性不足的群体中招募和留住本科生和研究生的项目，建立一个多元化的跨学科团队，旨在扩大STEM的参与。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.