GRACE water storage did not stay below a rainfall-based model after 3,875 meteorological droughts on global land in 2002-2026; a deficit of 0.12 SD or more is unlikely

Question: after multi-year meteorological droughts, does total terrestrial water storage (TWS) stay lower than precipitation predicts (non-recovery), over global land in 2002-2026? Earlier campaign studies found no persistent deficit in US runoff and US well levels; this study tests TWS globally. Approach: CSR RL06.3 GRACE/GRACE-FO mascons, GPCP v2.3 precipitation and GHCN_CAMS temperature on 2,274 land cells of 2.5 degrees. A drought is a run of 12 or more months with SPI-12 below 0 that includes 6 or more months at -1 or less. For each drought, a reference model (trend, seasonal terms, precipitation anomalies weighted over 3, 12 and 36 months, temperature) was fitted on all months except that drought and its 36-month post-drought window. The metric R is the mean standardised residual in the post-drought window. The null comes from 244 common time shifts of all drought windows with refitted models. Results: for 3,875 droughts in 1,980 cells, mean R was -0.011 SD with a linear trend (placebo 95% range -0.055 to +0.064, p = 0.17) and -0.022 SD with a spline trend (-0.077 to +0.072, p = 0.42). In 13 variants, mean R was -0.024 to +0.013 SD (p 0.09-0.59). An injected 0.25 SD deficit was detected (p = 0.004). After correction for the 65-67% recovery of the leave-one-event-out design, the 95% upper bound on a uniform deficit is 0.11-0.12 SD (about 0.35 cm). Post-drought years 2-3 had a small negative mean (-0.05 to -0.07 SD) that was not significant. No group of aridity, TWS trend, severity, latitude or period passed a Benjamini-Hochberg correction; cells with long-term storage decline had no deficit. Limitations: 24-year record, few droughts per cell, a trend that can absorb slow deficits, a coverage rule that removes 14% of droughts (mostly long; a variant without it gave the same result), 2.5-degree precipitation, no human water-use terms, and a design change after the planned spline fit failed.

GRACE · terrestrial water storage · drought recovery · hydrological drought · non-recovery · GPCP · placebo test · time-shift null · global land · water availability · mascons · GRACE-FO