Global models underestimate large decadal declining and rising water storage trends relative to GRACE satellite data.

Assessing reliability of global models is critical because of increasing reliance on these models to address past and projected future climate and human stresses on global water resources. Here, we evaluate model reliability based on a comprehensive comparison of decadal trends (2002-2014) in land water storage from seven global models (WGHM, PCR-GLOBWB, GLDAS NOAH, MOSAIC, VIC, CLM, and CLSM) to trends from three Gravity Recovery and Climate Experiment (GRACE) satellite solutions in 186 river basins (∼60% of global land area). Medians of modeled basin water storage trends greatly underestimate GRACE-derived large decreasing (≤-0.5 km3/y) and increasing (≥0.5 km3/y) trends. Decreasing trends from GRACE are mostly related to human use (irrigation) and climate variations, whereas increasing trends reflect climate variations. For example, in the Amazon, GRACE estimates a large increasing trend of ∼43 km3/y, whereas most models estimate decreasing trends (-71 to 11 km3/y). Land water storage trends, summed over all basins, are positive for GRACE (∼71-82 km3/y) but negative for models (-450 to -12 km3/y), contributing opposing trends to global mean sea level change. Impacts of climate forcing on decadal land water storage trends exceed those of modeled human intervention by about a factor of 2. The model-GRACE comparison highlights potential areas of future model development, particularly simulated water storage. The inability of models to capture large decadal water storage trends based on GRACE indicates that model projections of climate and human-induced water storage changes may be underestimated.

Characterizing drought events occurred in the Yangtze River Basin from 1979 to 2017 by reconstructing water storage anomalies based on GRACE and meteorological data.

The extreme change of water storage in the Yangtze River Basin (YRB) have a significant impact on identifying the characteristics of drought events in the basin. To quantify the historical hydrological drought characteristics, we put forward new framework to reconstruct the pre-2003 total water storage anomaly (TWSA) through the nonlinear autoregressive with exogenous input (NARX) model. The NARX model is developed by the Gravity Recovery and Climate Experiment (GRACE) based TWSA and the hydrometeorological data after removing the trend and seasonal signals from 2003 to 2017, then the full pre-2003 reconstructed TWSA signals were obtained by synthesizing hydrometeorological data driven NARX model results from 1979 to 2002 and GRACE-estimated seasonal cycle. We combined the reconstructed TWSA with GRACE observed TWSA to characterize the historical hydrological drought events (onset, end, duration, magnitude, intensity, and recovery) in the YRB. The results show that the drought-related extreme anomalies in total water storage can be captured successfully. From 1979 to 2017, 23 hydrological drought events were identified in the YRB with an average recovery time of 4.7 months. The longest drought lasted 28 months spanning from July 2006 to October 2008. The exceptional drought occurred in September 2011 reached to the largest deficit with a magnitude of -48.5 mm and minimum drought severity index (DSI) of -2.3. Comparing to the period of 1979-1999, the frequency, duration, and average recovery time of drought events increased significantly since 2000 in the YRB. Furthermore, we found that the duration and average recovery time of the drought events have an exponential relationship with the severity, which could help us to estimate the potential recovery time when drought events occur and predict water resources dynamic in the future.

Combining GRACE and satellite altimetry data to detect change in sediment load to the Bohai Sea.

The Gravity Recovery and Climate Experiment (GRACE) satellites provide a powerful tool for monitoring sediment mass change. However, signal leakage from nearby groundwater storage depletion in the North China Plain limits the potential capacity of GRACE to estimate sediment input from the Yellow River flows into the Bohai Sea. In the present work, we developed an improved approach based on forward modeling to reduce signal leakage from GRACE data and combined it with satellite altimetry to recover sediment load changes from 2003 to 2013 to the Bohai Sea. The total sediment input averaged 1.7 ± 0.8 Gt/yr, which agrees well with the estimate based on in-situ sediment data measured from the sediment cores (1.1 Gt/yr). Our method is also capable to describe sediment seasonal variations, with higher inputs in winter and spring, which confirm the output simulated by the sediment transportation model. We make presently tentative connections of seasonal variations to sediment resuspension driven by climatic monsoons contributed rough seas: although sediment load in rivers peaks in summer, low water discharge of the Yellow River leads to most of the sediment being deposited in a narrow area near the river mouth and not transported into the Bohai Sea; in winter and spring, huge waves provide favorable conditions for resuspension resulting in large amounts of sediment near the estuary being transported to the ocean along with northward waves. Moreover, our results indicate coastal erosion is also a nonnegligible resource of the sediment in the Bohai Sea. Comparing to the traditional approach, our study provides a new technological way to derive sediment in the Bohai Sea, which is capable of providing continuous measurements with improved timeliness at a lower cost.