Mapping global soil moisture and evapotranspiration coupling strength based on a two-system method and multiple data sources

Authors: ZHOU, J - Tianjin University; YANG, K - Tianjin University; DONG, J - Tianjin University; Crow, Wade; LU, H - Tianjin University; ZHAO, L - Southwest University; FENG, H - Hohai University; TIAN, J - Tianjin University; MA, X - Tianjin University; TIAN, X - Tianjin University

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/20/2025
Publication Date: 2/17/2025
Citation: Zhou, J., Yang, K., Dong, J., Crow, W.T., Lu, H., Zhao, L., Feng, H., Tian, J., Ma, X., Tian, X. 2025. Mapping global soil moisture and evapotranspiration coupling strength based on a two-system method and multiple data sources. Water Resources Research. 61(2). Article e2023WR036847. https://doi.org/10.1029/2023WR036847.
DOI: https://doi.org/10.1029/2023WR036847

Interpretive Summary: Accurately estimating the flux of water and energy between the land surface and the atmosphere is an important component of forecasting the onset and evolution of agricultural drought. However, such fluxes are notoriously difficult to measure, and available flux estimates are plagued by high inherent uncertainty. This uncertainty, in turn, hampers our ability to understand how such fluxes are coupled to land surface states like soil moisture. This paper develops and applies a new mathematical technique for removing the impact of random measurement error from estimates of the correlation between surface soil moisture and surface evaporation. This technique allows us to utilize existing remotely sensed estimates of both soil moisture and surface evaporation to produce a reliable, global-scale assessment of soil moisture/surface evaporation coupling strength and identify global regions where such coupling is poorly described in existing land surface models. Results from this analysis will eventually be used by the numerical weather prediction community to improve the representation of land/atmosphere feedbacks within land surface models and, thus, their ability to forecast agricultural drought.

Technical Abstract: Soil moisture (SM) exhibits varying controls on evapotranspiration (ET) and ET-regulated lower atmospheric processes under water- and energy-limited evaporative regimes. The SM/ET coupling strength is closely linked with land-atmosphere interactions and its reliability is an essential goal for earth system modeling. However, acquiring global maps of unbiased SM/ET coupling strength remains challenging giv-en significant levels of error present in globally available SM and ET products. Although, triple collocation (TC) provides a possible solution, it is demanding to apply globally since it requires access to three inde-pendent SM/ET data pairs – a requirement that is difficult to meet in practice. Here, we generate a new global map of SM/ET coupling strength based on a proposed two-system approach that requires only two independent SM/ET data pairs. Specifically, the two-system approach is first assessed over local ground sites versus a benchmark SM/ET coupling strength generated using a TC-based approach and three inde-pendent data pairs acquired from: ground-based observations, remote sensing (RS) retrievals, and land surface model (LSM) estimates. Subsequently, the two-system approach is extrapolated to various global-ly available datasets. The resulting multiple (two-system-derived) SM/ET coupling strength estimates, identified using the benchmark SM/ET coupling strength, are then integrated into a single global map. Results suggests that previous (TC-based) SM/ET coupling strength estimates are generally negatively bi-ased due to cross-correlated error present between RS products. Instead, our new two-system approach is shown to provide the most robust available description of linear global SM/ET coupling strength – thus offering an important reference for understanding land-atmosphere interactions and improving earth sys-tem models.