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

城市中的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。城市化会影响气候、土壤中的水量（土壤湿度）以及景观中植被的类型和数量。所有这些因素都强烈影响蒸散（ET）：从陆地到大气的水通量。城市ET预测不佳的水文模型，不充分代表人类活动，如灌溉。然而，城市灌溉对气候、土壤湿度、植物生长和存活有很大的影响。这项研究解决了ET受到土壤湿度，大气需水量（湿度和空气温度的函数）或美国城市内和城市之间植被密度和分布限制的程度。将在三个城市地区进行测量：加利福尼亚州的洛杉矶（一个半干旱城市，由于干旱应对政策，灌溉量一直在下降）、犹他州的湖城（半干旱但仍有大量灌溉）和佛罗里达州的塔拉哈西（降雨量高，城市树木覆盖率很高）。这些城市代表了具有不同水循环组成部分的城市环境。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，并为城市和城市景观的有效水管理做出贡献。 相对于自然和农业系统的观测，城市水文数据仍然稀少。为了促进对城市水文学的普遍理解，有必要探索城市内部和城市之间在供水平衡（由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（净辐射和蒸汽压赤字）方面的可分类差异。该项目将通过量化灌溉效率、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相似性和差异。土壤-植物-大气系统的每一个组成部分对ET通量的贡献可能会有所不同，在中温带与干旱/半干旱气候，并根据当地的灌溉做法，以及城市化进程，影响土地和植被覆盖。通过对假设跨越不同灌溉实践组合和范围的城市进行采样，以及对城市土壤水分，蒸汽压赤字和ET的可能限制，研究人员将测试可应用于这些特定城市和测量地点之外的一般框架。最终，本项目将使用本研究中收集的大量数据集，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案。结果将传播给利益相关者和推广专家谁是重点改善草坪管理，室外水管理和城市水政策。调查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计划，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NSF的法定使命，并被认为值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估来支持。

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.