Explaining the Surprising Simplicity of Continental Evapotranspiration

Harvard University, Cambridge MA

Investigators

Abstract

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.