Effective global soil profile depth and water holding capacity inferred from GRACE time-variable gravity

Abstract:

Soil depth and soil water holding capacity are fundamental controls on land surface processes and terrestrial ecology, and are critically important in understanding links between water and climate globally. However, field surveys of soil water below 1-meter are sparse, and sub-surface water storage variability is nearly impossible to monitor over a global domain.  NASA's GRACE (Gravity Recovery and Climate Experiment) mission has been observing global terrestrial water storage anomalies at consistent, monthly intervals over a longer than 10-year record, allowing us to measure dynamic soil water storage as a component of that signal. Here, we combine 1-degree scaled GRACE water storage observations with estimates of surface and snow water storage to derive a global, sub-surface water storage anomaly time series with uncertainty. We convolve this result with global maps of soil porosity from the FAO/UNESCO Harmonized Soil Database and a simple model of soil saturation to produce an observation-based estimate of effective soil profile depth with global coverage. Our results show 1-degree global soil storage requirements typically ranging between 2 to 100 cm, for a total terrestrial soil water storage capacity of roughly 15x10³ km3.  The observed storage indicates effective soil depths between 0.5 and 10 meters, with a global mean value of 1.5 +/- 0.5 meters. Effective soil depths are correlated with land cover type, temperature and precipitation. Comparisons with previous statistical and empirical methods suggest greater capacity and greater depth using the GRACE-based method. This methodology offers a new tool for ongoing monitoring of global soils, and has implications for hydrological fluxes in Earth System models which usually assume a globally uniform soil depth.