A strong role of shallow soil moisture in land-atmosphere coupling

There is a common perception in the Earth science community that plant rooting is deep and thus shallow soil moisture (0-5 cm), that can be observed from satellites, is limited in driving land surface hydrology. Here, we ask: to what degree does shallow soil moisture drive land surface behavior, especially land surface temperature (LST) and evapotranspiration? Can satellites observe hydrologically meaningful soil moisture information? Here, we argue that shallow soil moisture is playing a large role in land-atmosphere coupling with support from several methods. First, ECOSTRESS LST is correlated with soil moisture at USCRN sites, revealing that the strongest coupling often exists between shallow soil moisture layers and LST across a range of land cover types. Deeper soil layers appear to be more important drivers during dry periods. Next, isotopic tracer studies are collated that show a prevalence of vegetation that preferentially takes up upper soil layer moisture, despite their deeper rooting. Finally, we show that microwave L-band sensors carry information about soil moisture deeper than the commonly defined 5 cm heuristic in many conditions. Specifically, under wetter conditions, shallow and deeper soil moisture are highly vertically correlated. Under drier conditions, microwave emission tends to originate from deeper layers, thus providing deeper sensing capabilities. Indeed, it is critical to observe deeper layer soil moisture, for example, for studying forested surfaces, evaluating groundwater resources, and initialization of weather forecasts. Nevertheless, we argue that shallow soil moisture has a proportionally strong role in land surface behavior.