Mapping Global Soil Moisture and Evapotranspiration Coupling Strength Based on a Two-System Method and Multiple Data Sources

Abstract

Soil moisture (SM) is a key state variable in the climate system through its control on evapotranspiration (ET) and ET-regulated lower atmospheric processes. The SM-ET coupling strength (SECS) is thus closely linked with land-atmosphere interactions and its reliability is crucial for Earth system modeling. However, acquiring global maps of unbiased SECS remains challenging given significant levels of error present in globally available SM and ET products. Triple collocation (TC) provides a possible solution; however, it is difficult to apply globally since it requires access to three independent SM-ET data pairs—a requirement that is difficult to meet in practice. Here, we generate a global SECS map based on a new two-system approach that requires only two independent SM-ET data pairs. This two-system approach is first validated over local ground sites versus a ground-inclusive benchmark SECS. Subsequently, it is applied to generate a global map of SECS with input from various independent globally available SM-ET data pairs (identified using the benchmark SECS). Results suggest that previous TC-based SECS estimates are generally negatively biased due to cross-correlated error present between RS products. Instead, our generated new SECS map is shown to provide more robust mapping of global SECS—thus offering an important reference for improving Earth system models.