Soil and Atmospheric Controls on the Land Surface Energy Balance: A Generalized Framework for Distinguishing Moisture‐Limited and Energy‐Limited Evaporation Regimes

Abstract The relationship between evaporative fraction (EF) and soil moisture (SM) has traditionally been used in atmospheric and land‐surface modeling communities to determine the coupling strength between land surfaces and the atmosphere in the context of the dominant evaporation regime (energy or moisture limited). However, observation‐based analyses suggest that EF‐SM relationship in a given region can shift subject to other environmental factors, potentially influencing the determination of the dominant evaporation regime. This implies more complex dependencies embedded in the conventional EF‐SM relationship and that in fact it is a multidimensional function. In this study, we develop a generalized EF framework that explicitly accounts for dependencies on other environmental conditions. We show that large scatter in observed EF‐SM relationships is primarily due to the projection of variations in other dimensions and propose a normalization of the EF‐SM relationship accounting for the dimensions and dependencies not included in the conventional relationship. In this first study, we focus on bare soil conditions in order to establish the basic theoretical framework. The new generalized EF framework provides new insights into the origin of transition between energy‐limited and moisture‐limited evaporation regimes (marked by a critical SM), linked to soil type and meteorological input data (primarily wind speed and air temperature, but not solar radiation) dominating the evolution of land surface temperature and thus the relative efficiency of surface energy balance components during surface drying. Our results offer new opportunities to advance predictive capabilities quantifying land‐atmosphere coupling for a wide range of present and projected meteorological input data.