Critical Soil Moisture Derived From Satellite Observations Over Europe

다국어 초록 (Multilingual Abstract)

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.
Water evaporates at the Earth's surface given sufficient soil moisture and incoming radiation. Most water is transpired by plants during photosynthesis. In water limitation, the soil is too dry to support evapotranspiration (ET). When the soil gets wetter, ET will increase. At a characteristic soil moisture, the maximum ET is reached, so that further increases in soil moisture do not further increase ET: this is the critical soil moisture (CSM). When the soil gets wetter than the CSM, ET is controlled by the amount of incoming radiation (energy limitation).
In this study we determine this CSM in Europe at large spatial scales relevant for climate modelling using satellite data. By applying a novel detection method, we identify at which soil moisture values ET is transitions between water and energy limitation, which denotes the CSM. Results show that the CSM varies in space and is similarly sensitive to soil, climate and vegetation characteristics. This knowledge helps to improve climate models. Further, knowing the local soil moisture respective to the CSM can tell us something about how much available energy at the Earth's surface is used for ET, which aids to more accurately forecast the intensity of droughts and heat waves.


We introduce a novel correlation‐difference metric with which energy‐ or water‐controlled evaporation regimes can be determined
The resulting spatial pattern of water‐ versus energy‐limitation in Europe forms an observational constraint for climate models
Local critical soil moistures allow real‐time diagnosis of land‐atmosphere interactions with easily accessible satellite soil moisture