GRACE land water storage did not stay below a precipitation-memory model after 6,987 droughts in 2002-2026; a short symmetric overshoot followed extreme months

Question: after a meteorological drought ends, does land total water storage stay below the level that precipitation and temperature history predict, as reported for runoff in Victoria, Australia? Approach: we used CSR GRACE/GRACE-FO RL06.3 mascons, GPCP v2.3 precipitation and GHCN-CAMS temperature on 1,739 land cells of 2.5 degrees (56 S to 60 N). Droughts and wet spells come from precipitation only (6-month standardised anomaly). For each event, a per-cell linear model (trend, seasonal terms, leaky precipitation and temperature memory) was fitted without the event window. The endpoint D is the mean out-of-sample residual in the 24 months after the event end. A common time shift of all event windows (182 offsets) gave the placebo distribution and kept the spatial correlation. Results: for 6,987 droughts in 1,706 cells, D = +0.044 SD (placebo 95% range -0.047 to +0.080; one-sided p = 0.89). Storage did not stay low. The one-sided 95% bound on a mean deficit is 0.028 SD (about 0.09 cm). An added deficit of 0.25 SD was detected. Wet spells gave W = -0.057 SD, and D + W was at the placebo centre, so there was no asymmetry. No trend term, a 12-month window, a stricter threshold and glacier-box removal gave the same conclusion. No cell passed a false discovery rate of 0.1. Exploratory: storage was above the model in months 1-6 after droughts (+0.13 SD) and below it after wet spells (-0.12 SD). Both were outside all placebo values. These values follow single extreme months that the event rule selects. A fast 1-month memory term did not remove them. Limitations: GRACE resolution of about 300 km, one precipitation product, a linear model, other events in the fit set, and a post hoc analysis of the overshoot with an unknown cause.

GRACE · GRACE-FO · terrestrial water storage · drought recovery · hydrological memory · GPCP · placebo test · out-of-sample prediction · wet spells · non-recovery · CSR mascons · global hydrology