Satellite-Based Assessment of Land Surface Energy Partitioning-Soil Moisture Relationships and Effects of Confounding Variables

Land surface energetic partitioning between latent, sensible, and ground heat fluxes determines climate and influences the terrestrial segment of land-atmosphere coupling. Soil moisture, among other variables, has a direct influence on this partitioning. Dry surfaces characterize a water-limited regime where evapotranspiration and soil moisture are coupled. This coupling is subdued for wet surfaces, or an energy-limited regime. This framework is commonly evaluated using the evaporative fraction--soil moisture relationship. However, this relationship is explicitly or implicitly prescribed in land surface models. These impositions, in turn, confound model-based evaluations of energetic partitioning--soil moisture relationships. In this study, we use satellite-based observations of surface temperature diurnal amplitude (directly related to available energy partitioning) and soil moisture, free of model impositions, to estimate characteristics of surface energetic partitioning--soil moisture relationships during 10--20-day surface drying periods across Africa. We specifically estimate the spatial patterns of water-limited energy flux sensitivity to soil moisture (m) and the soil moisture threshold separating water and energy-limited regimes (theta*). We also assess how time evolution of other factors (e.g., solar radiation, vapor pressure deficit, surface albedo, and wind speed) can confound the energetic partitioning--soil moisture relationship. We find higher m in drier regions and interestingly similar spatial theta* distributions across biomes. Vapor pressure deficit and insolation increases during drying tend to increase m. Only vapor pressure deficit increases in the Sahelian grasslands systematically decrease theta*. Ultimately, soil and atmospheric moisture availability together play the largest role in land surface energy partitioning with minimal consistent influences of time evolution of other forcings.