GRACE land water storage rose faster than it fell at 1-24 month lags in 2002-2026; rain-driven linear models did not reproduce the 12-month asymmetry

Question: is the change of total land water storage (TWS) symmetric in time, and does rain skewness explain any asymmetry? Data: CSR GRACE/GRACE-FO RL06.3 mascons, 2002-04 to 2026-06, 2,168 land cells of 2.5 degrees (56 S to 75 N, no Greenland or Antarctica); GPCP v2.3 rain; GHCN-CAMS temperature. Method: after removal of a linear trend and seasonal harmonics, we calculated the skewness g(L) of storage changes x(t+L) - x(t) for L = 1-24 months. A reversible process gives g = 0. We used 1,000 common-phase Fourier surrogates (reversible, with the observed spectra and cross-spectra) and a sign-flip test on 111 blocks of 15 degrees. We also compared development and recovery times of deficit and surplus events, and drove single-store, two-pole ARX and state-dependent ARX models with observed rain. Results: the area-weighted mean g was positive at all lags (0.076 at L = 1, 0.099 at L = 12; surrogate 95% ranges -0.018 to 0.019 and -0.038 to 0.039; p = 0.001; block p < 0.001). Each mission showed it separately (GRACE g(12) = 0.088, GRACE-FO 0.074). Storage rose faster than it fell. The effect was large in arid (g(12) = 0.19) and semi-arid (0.22) cells and absent in humid cells (0.013, p = 0.66). With a linear trend, deficits ended about 6% faster than they formed (mean ln(R/D) = -0.059, p = 0.010), but with a quadratic trend this fell to -0.026, inside the null range. Thus the deficit result is not robust. Surpluses formed faster than they decayed (+0.098, p = 0.003). Monthly rain anomalies were positively skewed on 97% of the land area. Rain-driven models gave more skewness than observed at L = 1 (0.26-0.37), but only 0.018-0.041 at L = 12 and below zero at L = 24 (observed 0.074). The rain gain did not depend on the storage state (difference -0.013, 95% CI -0.063 to 0.037). Conclusion: rain pulses through a store explain the short-lag asymmetry, but less than half of the year-scale asymmetry. Untested candidate causes include human water use, lakes and reservoirs, and nonlinear recharge. Limitations: approx. 300 km resolution with correlated cells; 24-year record; Gaussian surrogates; simple rain models; complete surrogate tests only for the linear trend because the host was overloaded.

GRACE · terrestrial water storage · time irreversibility · drought recovery · skewness · surrogate data · GPCP · hydrological drought · water storage extremes · GRACE-FO