Critical Soil Moisture Derived From Satellite Observations Over Europe

Evapotranspiration (ET) is a crucial quantity through which land surface conditions can impact near-surface weather and vice versa. ET can be limited by energy or water availability. The transition between water- and energy-limited regimes is marked by the critical soil moisture (CSM), which is traditionally derived from small-sample laboratory analyses. Here, we aim to determine the CSM at a larger spatial scale relevant for climate modeling, using state-of-the-art gridded data sets. For this purpose, we introduce a new correlation-difference metric with which the CSM can be accurately inferred using multiple data streams. We perform such an analysis at the continental scale and determine a large-scale CSM as an emergent property. In addition, we determine small-scale CSMs at the grid cell scale and find substantial spatial variability. Consistently from both analyses we find that soil texture, climate conditions, and vegetation characteristics are influencing the CSM, with similar respective importance. In contrast, comparable CSMs are found when applying alternative large-scale energy and vegetation data sets, highlighting the robustness of our results. Based on our findings, the state of the vegetation and corresponding land-atmosphere coupling can be inferred, to first order, from easily accessible satellite observations of surface soil moisture.