Precipitation explains GRACE water storage variability over large endorheic basins in the 21st century
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Samantha Petch, Bo Dong, Tristan L. Quaife, Robert P. King, Keith Haines
When a satellite detects water rising and falling across an entire inland basin, what is really driving those changes? This study finds that rainfall, more than water leaving the ground, explains much of the year-to-year movement.
Introduction: Seasonal—interannual variations in surface water storage revealed by the Gravity Recovery and Climate Experiment (GRACE) satellites have received less attention than storage trends in the literature. We focus on six large endorheic basins and develop variability attribution diagnostics against independent precipitation and evapotranspiration (hereafter P and E) datasets. Methods: We generate a flux-inferred storage (FIS), representing the integral of the component flux anomalies into and out of a region, enabling a comparison between the P and E contributions to GRACE water storage anomalies on seasonal to interannual timescales. Additionally, a monthly budget closure approach is applied, giving self-consistent coupled water and energy exchanges from 2002 to 2020. Results: On seasonal timescales, P and E data show insufficient cancellation, implying over-large seasonal variations in surface storage. In most basins, P drives the seasonal storage cycle with E dampening storage amplitudes, although in the Caspian Basin, seasonal storage is driven by E, with P remaining seasonally constant when integrated over the whole drainage basin. Budget closure mostly adjusts E, which has larger uncertainties, in fitting the GRACE data. On year-to-year and multi-year timescales, there is a strong correlation between P-driven storage and the observed GRACE variability, which ranges between 0.55 and 0.88 across all basins, and this is maintained after budget closure. However, storage changes driven by P alone from GPCP are too large compared to GRACE, with E data from FLUXCOM generally having only very weakly compensating interannual variations. After budget closure, interannual E variability is substantially increased. Closed energy budgets often show interannual amplitudes, partly driven by radiation and partly by water budget variation through shared latent heat losses, although these have not been independently verified. Discussion: Although water flux trends cannot be detected with significance due to the large interannual variability, the strong agreement between multi-annual GRACE storage and precipitation variations, especially over the Caspian basin, lends no support to the suggestion that E changes driven by climate change are responsible for water storage trends seen by GRACE.
Transcript
When a satellite detects water rising and falling across an entire inland basin, what is really driving those changes? This study finds that rainfall, more than water leaving the ground, explains much of the year-to-year movement. Water stored at Earth’s surface can rise and fall from season to season and from one year to the next, but those changes have received less attention than long-term trends.
The study focuses on six large basins that have no river outlet to the sea, and compares their water changes with independent records of rainfall and water leaving the land. The central idea is like keeping track of money in a bank account: rainfall adds water, while water leaving the land takes it away.
Adding those changes over time gives an expected water balance to compare with the satellite’s record. The water exchanges are compared with the GRACE satellite record across seasonal-to-interannual timescales, showing how their contributions relate to observed storage changes.
A second, monthly budget-closure step adjusts the water exchanges, keeping the coupled water and energy exchanges consistent from 2002 to 2020. The comparison adds up the rainfall entering a region and the water leaving it, then turns those incoming and outgoing changes into an estimate of how stored water should vary.
That estimate can then be compared with changes in surface water detected from space. To make the comparison fully consistent, the observed water exchanges are adjusted while accounting for the uncertainty in each part of the balance.
The balancing keeps the result consistent with the satellite record over both short and long timescales, and the water and energy balances are solved together. The year-to-year changes in stored water detected from space can be clearly attributed to changes in rainfall across all these basins.
The original data for water leaving the land show very little year-to-year variation, and when variation appears, it generally reduces the storage changes rather than causing them. After the balance is corrected, water leaving the land shows more variation and plays a stronger role in reducing the storage changes.
Year-to-year changes in stored water closely follow changes in precipitation across these regions, while the original evaporation estimates show little matching variation. That makes precipitation the clearest explanation for much of the water loss and recovery recorded from space.
A strong positive match between rainfall-driven storage and the satellite record supports rainfall as the key driver of changing stored water. The match reaches plus zero point eight eight in the Caspian Basin and remains across every basin after the water balance is corrected.
After correction, the rainfall and water-loss contributions together almost perfectly reproduce both the size and timing of the year-to-year storage changes seen by the satellite. The starting rainfall and water-loss records do not balance well: their average mismatch is large in every basin.
Because of that mismatch, the study cannot sensibly assign the long-term storage trends seen by the satellite to one water exchange or the other. Even so, the records still reveal useful information about seasonal and year-to-year changes in stored water.
Long-term trends cannot be confidently assigned because the starting rainfall and water-loss records are often badly out of balance. In the Caspian Basin, however, falling rainfall is more than enough to explain falling stored water, and without the starting imbalance it would account for about seventy percent of the total loss over the period studied.
Those trends are not statistically decisive because year-to-year changes are so large, but they fit the broader conclusion that rainfall is the dominant driver of slower storage changes in the Caspian Basin. Across these large closed basins, precipitation closely tracks changing stored water, while evaporation data often miss important variation.
That makes rainfall the clearest guide to short-term water change, even though long-term trends remain harder to assign.
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