CAREER: Understanding Soil Moisture and Its Changes in a Warming World

About This Grant

The water held in soil is a critical resource for sustaining ecosystems and agriculture and, through bare-ground evaporation and transpiration through plant leaves, a primary source of surface humidity over land and driver of the land-based hydrological cycle. But despite its central importance for continental climate and hydroclimate the basic science of soil moisture, at least at regional to continental scales, is not well developed. There are several reasons for the lack of basic science understanding, among them the complexity of factors that determine soil moisture, including physical climate factors like precipitation and the surface energy available for evaporation, and biological factors that determine how much water is taken up by plant roots and transpired through leaves. The development of a basic science is also hampered by lack of observations, and by a tendency for research to focus on more applied science issues like the use of soil moisture as an input for subseasonal weather forecasting. The Principal Investigator (PI) of this CAREER award seeks to develop the basic science of soil moisture at regional to continental scales by addressing two questions: first, what are the key controls on the spatial variability of soil moisture in present-day climate? One issue here is why the latitudinal profile of continental soil moisture has a characteristic "W" shape, with a maximum at the equator and minima in the subtropics of the Northern and Southern Hemispheres. Second, how will soil moisture change in a warming world? The question is motivated by the drying trends found in many of the world's semi-arid regions including the US Southwest, which raise concerns about future water resources as well as increases in the severity of heat waves and the risk of wildfire. But evidence of a drier future is inconclusive as climate models show subtantial disagreements in their projections of soil moisture change under warming, as well as important discrepancies with the observed record. Issues to be addressed under this award include why the soil moisture response to warming is muted compared to the precipitation response and what factors determine whether a region gets drier or wetter as a result of warming. The research questions are addressed through the construction of a hierarchy of soil moisture models starting from the simplest configuration and incrementally adding complexity to determine the most parsimonious version that can account for the behaviours of interest. The simplest configuration is the model of Stahl and McColl (2022) in which the moisture budget of a thin layer of soil (meaning moisture added by precipitation and removed by evaporation, transpiration, runoff, and drainage) is boiled down to a formula involving only precipitation and sunlight received at the surface. Despite its simplicity and neglect of important factors such as vegetation physiology the model successfully reproduces the annual cycles of soil moisture seen in a variety of soil moisture regimes over the globe. Complexity is added to the model through more realistic representations of factors including the nonlinear effects of soil saturation fraction on evaporation, runoff, and drainage. In addition to the work with simple models, which are compared to observations and climate model output, the PI conducts experiments with a high-resolution atmospheric model (a version of SAM, the System for Atmospheric Modeling, see AGS-2218827) coupled to a land surface model. The educational component of the CAREER project uses art to teach students about the water cycle and the balance of sources, sinks and storage that accounts for the presence of water on land. The effort builds on a pilot course developed by the PI in collaboration with the Harvard Art Museum in which students are asked to consider the water balance implications of images of water-dominated and water-depleted landscapes such as the Salton Sea and the surrounding drylands that were, until recently, submerged by it. The course, which is intended for a general audience and requires no specific math or physics background, is scaled up under this award to class sizes of 100 or more and is further developed into a class that can be taught indepedently at other universities. The class is developed so that it can be taught online and also tailored to museums near other colleges and universities, most of which have art work that has appropriate depictions of water. 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.