Explaining the Surprising Simplicity of Continental Evapotranspiration

降雨时落到地面的大部分水通过地表蒸发和蒸腾作用返回大气，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，对蒸散量(ET)的准确估计，即蒸发量和蒸腾量的总和，对于理解与水循环有关的所有事物，包括水资源、地表气候、碳循环以及作物和生态系统的健康至关重要。ET可以直接测量，但这种测量需要复杂的设备，而这些设备只在少数几个站点提供。根据标准天气观测估算蒸散量的替代方法是存在的，但这种方法通常需要额外的信息，如土壤湿度和植被高度，这是不太常见的。这一奖项下的研究开发和测试了一种新的基于地面气温和湿度的ET估计方法，该方法可在整个全球地面气象站网络中获得，以及对地面净辐射的卫星观测。新的方法还消除了对特定地点进行校准的需要。该方法的前提是地表及其上方的空气处于表面通量平衡(SFE)状态，即地表空气温度和湿度已调整到陆地释放的热量和湿度，因此可以利用对空气温度和湿度的观测来推断ET。早期的工作已经证明了该方法是有效的，但还需要更多的工作来理解该方法的特性和适用范围。特别是，该项目试图了解为什么超临界流体萃取方法在内陆大陆地区如此有效，尽管它很简单。另一方面，该方法通常低估了海洋上的蒸发量(那里的蒸发量只是蒸发)，而高估了海岸线附近的蒸发量，该项目也解决了这些问题。这项工作是使用高分辨率大气模式-大气建模系统(SAM)进行的，通过比较来自SFE的ET与模型输出的实际ET进行比较。由于SFE得出的ET估计的社会价值，该工作具有更广泛的影响。如果成功，新方法将极大地扩大全球ET估计的可用性，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，该项目的首席调查员通过哈佛自然历史博物馆进行公共宣传，该项目为一名研究生提供支持和培训。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.