Revisiting Sub‐Surface Drought Cascades With Daily Satellite Observations of Soil Moisture and Terrestrial Water Storage

Abstract

The increasing frequency, intensity, and duration of extreme heat and drought events in a warming climate make it crucial to understand the relationship between surface and subsurface water storage dynamics during these events. Changes in water storage can be studied globally using satellite observations. Microwave remote sensing observes the upper few centimeters of the soil, while satellite gravimetry detects changes in the entire column of terrestrial water storage. We use daily data of the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO), satellite-based surface soil moisture data and root zone products from Soil Moisture Ocean Salinity, Soil Moisture Active Passive, and European Space Agency Climate Change Initiative on a harmonized 1 global grid to study the evolution of water storage deficits across different soil layers. The joint analysis of the three types of data provides valuable insight into the hydrological dynamics in different soil depths and subsurface water storage compartments. To identify different dynamics, we compute the rate of change from de-seasonalized water storage anomaly time series to assess how quickly the system accumulates storage deficits during drought conditions and recovers from them for different integration depths in the subsurface. The results indicate characteristic patterns of the temporal dynamics of drought recovery with fast fluctuations and short recovery times for surface soil moisture, a prolonged behavior in the root-zone, and an even slower response in the entire water column. This highlights that the cascading propagation of drought dynamics from the surface to the subsurface can be quantified by remote sensing data with daily resolution at the global scale.