Explaining the Surprising Simplicity of Continental Evapotranspiration

大部分以雨的形式落到地面的水通过地表蒸发和蒸腾作用回到大气中，蒸腾作用是植物从根部吸收水分并从叶子中释放水分的过程。因此，准确估计蒸散（ET），即蒸发和蒸腾的总和，对于理解所有与水循环有关的事情都是至关重要的，包括水资源、地表气候、碳循环以及作物和生态系统的健康。ET可以直接测量，但这种测量需要精密的设备，而这种设备只在少数几个地点可用。存在根据标准天气观测估计ET的替代方法，但这些方法通常需要额外的信息，例如土壤湿度和植被高度，而这些信息不太常见。它们也可能涉及不基于第一性原理的校准，并且可能具有很强的现场特异性。该合同下的研究开发并测试了一种基于地面空气温度和湿度的新的蒸散发估算方法，这种方法可以在整个全球地面气象站网络中使用，同时还可以通过卫星观测地面净辐射。新方法还消除了特定地点校准的需要。方法的前提是,地表和海洋上空的空气表面通量平衡状态(医药),这意味着表面空气温度和湿度调整的热量和湿气的土地,以便观察的空气温度和湿度可以用来推断等。先前的工作已经证明,这个方法是可行的,但需要更多的工作来理解的属性方法及其适用性的限制。特别是，该项目试图理解为什么SFE方法尽管简单，但它在内陆大陆地区的效果如此之好。另一方面，该方法通常低估了海洋上的ET（其中ET只是蒸发），而高估了海岸线附近的ET，该项目也解决了这些问题。这项工作是使用一个高分辨率大气模式，即大气模拟系统（SAM），通过比较SFE得到的ET与模式输出的实际ET进行的。由于sfe估算的ET的社会价值，这项工作具有更广泛的影响。如果成功，新方法将大大扩大全球ET估算的可用性，从而为解决与水循环有关的重要社会问题提供宝贵的资源。此外，该项目的首席研究员通过哈佛自然历史博物馆进行公众宣传，并为研究生提供支持和培训。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。

Most of the water that falls to the ground as rain returns to the atmosphere through surface evaporation and through transpiration, the process through which plants take up water in their roots and release it from their leaves. Accurate estimates of evapotranspiration (ET), the sum of evaporation and transpiration, are thus essential for understanding all things related to the water cycle, including water resources, surface climate, the carbon cycle, and the health of crops and ecosystems. ET can be measured directly, but such measurements require sophisticated equipment which is only available at a few sites. Alternative methods exist to estimate ET from standard weather observations but such methods generally need additional information, such as soil moisture and vegetation height, which is less commonly available. They may also involve calibrations which are not based on first principles and can be strongly site specific.Research under this award develops and tests a new ET estimation method based on surface air temperature and moisture, which are available across the full global network of surface weather stations, along with satellite observations of surface net radiation. The new method also eliminates the need for site-specific calibration. The premise of the method is that the land surface and the air above it are in a state of surface flux equilibrium (SFE), meaning that the surface air temperature and humidity have adjusted to the heat and moisture given off by the land, so that observations of air temperature and humidity can be used to infer ET. Earlier work has demonstrated that the method works, but more work is needed to understand the properties of the method and the limits of its applicability. In particular, the project seeks to understand why the SFE method works as well as it does, despite its simplicity, over inland continental areas. On the other hand, the method typically underestimates ET over oceans (where ET is just evaporation) and overestimates it near coastlines, and the project also addresses these issues. The work is conducted using a high-resolution atmospheric model, the System for Atmospheric Modeling (SAM), by comparing ET derived from SFE with the actual ET from the model output.The work has broader impacts due to the societal value of SFE-derived ET estimates. If successful, the new method will vastly expand the availability of ET estimates around the globe, thereby providing a valuable resource for addressing societally important problems related to the water cycle. In addition the Principal Investigator of the project conducts public outreach through the Harvard natural history museum, and the project provide support and training to a graduate student.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.