Terrestrial water storage regime and its change in the endorheic Tibetan Plateau

Analogous to flow regime, this study proposed a new statistical framework to assess inter-annual and intra-annual terrestrial water storage (TWS) regime and its changes from the aspects of magnitude, variability, duration and components. The framework was applied to two endorheic basins, Inner Basin (IB) and Qaidam Basin (QB), in the Tibetan Plateau and their eight sub-regions. Our major findings are as follows: (1) TWS in the IB (2.09-2.35 mm/a, P < 0.05) and QB (0.05-0.52 mm/a, P > 0.1) increased in all seasons from 1989 to 2019 with regional climate warming and wetting. TWS showed high increase rates (>4.50 mm/a, P < 0.05) in northeastern IB but decrease rates (<-0.90 mm/a) in southern IB. Seasonal total storage in groundwater, lake, permafrost and glacier (GLPIA) also increased in both the IB (2.55-2.68 mm/a, P < 0.05) and QB (0.05-0.43 mm/a). Seasonal soil water storage (SWA) decreased in the IB (-0.39 to -0.26 mm/a) and slightly increased in the QB (0.002-0.08 mm/a); (2) Intra-annual TWS followed approximately a cosine curve. After mutation, monthly TWS showed a higher positive magnitude change (>50 mm), accompanied by a longer duration and higher variability in the IB and its northeastern sub-regions. There was a large reduction in low storage (-18.25 mm) combined with higher variability in southeastern IB; (3) SWA change dominated the storage surplus in summer (82%) and storage deficit in autumn (-78%) and winter (-51%) in the IB, while GLPIA change dominated the storage surplus in spring (57%). In the QB, TWS change was mainly contributed by SWA change in spring (94%) and by GLPIA change in summer (73%), autumn (-62%) and winter (-58%). Component contribution rates showed a significant change in spring and winter but not much change in summer and autumn, indicating that the TWS components were more sensitive to climate change in the cold season.