Multiple measures of water storage in monsoon Asia

Temporal water storage is a fundamental component of the terrestrial water cycle. Almost all precipitation falling on land is transferred via a series of short- to long-term storage locations, e.g. groundwater, to rivers, and eventually ends in the oceans or, through evapotranspiration, back in the atmosphere. The intermediate storage compartments are recharged during precipitation events and subsequently purge during phases of very little precipitation input. Methods to estimate water storage variations are often limited to specific, well-monitored locations and the findings from there are often difficult to generalize or to upscale. At the same time large scale monitoring represents an average of the entire system with very little prediction power for small areas. Thus, measures of storage from small systems can be difficult to compare to large systems and vice versa. In this recently published study (Schmidt et al., 2020) we compare three independent methods of estimating water storage variations for systems spanning over three orders of magnitude in basin area: 1) GRACE, 2) hydrograph recession curve analysis, and 3) quantifying precipitation-discharge hysteresis loops. We measured storage using all three methods for 242 watersheds in Asia spanning a size range from 10³ to 10⁶ km² and find that GRACE- derived storage correlates well with the quantification of hysteresis terms but recession curve derived dynamic storage does not correlate with hysteresis terms or GRACE-derived storage. Thus, we argue that precipitation-discharge hysteresis may be able to be scaled to GRACE-derived storage as an  independent estimate of storage to systems much smaller than the typical resolution of GRACE. Hysteresis-derived storage correlates well with mean monsoon rainfall in the upstream watershed while recession-derived dynamic storage does not. This suggests that hysteresis- and GRACE-derived storage may be input limited. In contrast, recession-derived dynamic storage does not correlate with topographic, climatic, or land cover metrics, suggesting that it may be limited by the rate at which water infiltrates into deep groundwater and then enters the river system. In addition, we find that recession-derived dynamic storage is a factor of seven lower than hysteresis-derived or GRACE derived storage. Recession-derived dynamic storage represents the annual variability in deep and saturated groundwater storage, a “leaky bucket” that is recharged from the top and “leaks” into rivers from deeper storage. The GRACE and hysteresis derived storage in turn integrates groundwater variations in addition to other storage units at or close to the Earth surface, such as snowpack, lakes, and soil moisture. These data may be able to be used to better quantify storage terms in hydrologic modeling and might help to improve GRACE data products.

Schmidt, A. H., Lüdtke, S., & Andermann, C. (2020). Multiple measures of monsoon-controlled water storage in Asia. Earth and Planetary Science Letters, https://doi.org/10.1016/j.epsl.2020.116415