Hydroclimate-driven soil moisture declines during the North American megadrought

The recent megadrought in the southwestern U.S. is the most severe in over a millennium, intensifying pressure on water resources and compromising ecosystem function. However, this megadrought is often diagnosed using indirect methods such as tree-ring reconstructions, which can be spatially biased and imperfect proxies of climatic conditions.  Direct measurements of soil moisture provide quantitative records of soil water storage in the land surface, but it is only recently that the spatial and temporal scopes of these measurements have become large enough to diagnose the megadrought. By leveraging a dense network of in situ soil moisture measurements across depths, we quantified the trends in soil moisture during the megadrought and assessed its underlying drivers. The southwestern U.S. exhibited a pervasive drying trend of soil moisture during the megadrought, though there was significant spatial heterogeneity across basins. Reductions in mid-to-late season precipitation, along with widespread increases in VPD – vapor pressure deficit, were associated with long-term declines in soil moisture across all depths. Hydroclimate teleconnections were associated with soil moisture trends at larger spatiotemporal scales. Observed declines in soil moisture were not captured by a common microwave-based product but were better captured by gravimetry-based measurements. Our study highlights the importance of cool-season water inputs in the southwestern U.S., along with the future risks to water resources caused by rising VPD.