Are rootzone soil moisture dynamics and thresholds associated with surface layer?

Abstract

Identification of evapotranspiration regimes, primarily the water-limited and energy-limited regimes, separated by critical soil moisture (CSM) threshold, is fundamental to studying land-atmosphere interactions. To better understand the soil moisture (SM) dynamics happening synchronously in the soil column, we aim to estimate the rootzone (0-28cm and 0-100cm) CSM thresholds and associated regimes at a global scale, which was not previously attempted. We propose the use of covariability of soil diurnal temperature amplitude (derived from GLDAS) and SM (ERA5) to estimate the CSM, which enables overcoming the data uncertainty and multivariate dependencies of traditional methods. We find that transitional climatic regions, encompassing the western USA, Brazilian Savanna, Sahelian grassland, south African savanna, peninsular India, and Mediterranean region, are the global hotspots of frequent rootzone regime shifting with significant seasonality – wet regime prevails in the fall season, while dry regime takes over in other time of the year. The CSM value of 0-28cm and 0-100cm layers mostly in 0.2-0.35 and 0.25-0.4 m3m-3, respectively. We find that the landscape aridity and bioclimatic characteristics primarily determine the spatial distribution of CSM and associated regimes. Furthermore, we investigate the hydrological link between the surface and rootzone layers. We note that rootzone and surface CSM and regimes are strongly correlated, although the 0-28cm layer indicates a relatively stronger connection compared to the 0-100cm layer. The shallower (deeper) rootzone layer shows regimes similar to those on the surface for more than 80% (65-80%) of the time. We majorly observe that the strength of association between surface and rootzone regimes increases from arid (herbaceous vegetated regions) to humid (woody) regions and during wet to dry seasons. Overall, a strong association in regime dynamics between surface and subsurface layers suggests the potential applicability of remotely sensed surface SM as a surrogate to study rootzone regime responsiveness to soil-plant-atmosphere interactions.