Abstract
Spherical harmonics (SH) and mascon solutions are the two most common types of solutions for Gravity Recovery and Climate Experiment (GRACE) mass flux observations. However, SH signals are degraded by measurement and leakage errors. Mascon solutions (the Jet Propulsion Laboratory (JPL) release, herein) exhibit weakened signals at submascon resolutions. Both solutions require a scale factor examined by the CLM4.0 model to obtain the actual water storage signal. The Slepian localization method can avoid the SH leakage errors when applied to the basin scale. In this study, we estimate SH errors and scale factors for African hydrological regimes. Then, terrestrial water storage (TWS) in Africa is determined based on Slepian localization and compared with JPL-mascon and SH solutions. The three TWS estimates show good agreement for the TWS of large-sized and humid regimes but present discrepancies for the TWS of medium and small-sized regimes. Slepian localization is an effective method for deriving the TWS of arid zones. The TWS behavior in African regimes and its spatiotemporal variations are then examined. The negative TWS trends in the lower Nile and Sahara at −1.08 and −6.92 Gt/year, respectively, are higher than those previously reported.
Highlights
Obtaining reliable terrestrial water storage (TWS) estimates from the Gravity Recovery and Climate Experiment (GRACE) is challenging but crucial for studying hydrological cycles and climate change, especially in regions with limited freshwater resources, such as Africa
Comparing region of interest (ROI) of similar size and different climate domains reveals that the small-sized arid ROI presents larger errors than the small dry sub-humid ROI
The scale factor obtained from the CLM4.0 is smaller for the lower Nile than that for
Summary
Obtaining reliable terrestrial water storage (TWS) estimates from the Gravity Recovery and Climate Experiment (GRACE) is challenging but crucial for studying hydrological cycles and climate change, especially in regions with limited freshwater resources, such as Africa. GRACE was launched in March 2002 into a near-polar orbit at an elevation of 450 km to measure the temporal Earth gravity field caused by geophysical phenomena [1,2]. The traditional SH solutions suffered from measurement errors represented by non-physical longitudinal “stripes” and obtained as the result of the north-south ground track orientation. These stripes obscure the east-west gravity gradient and are associated with high-degree SH—the most valuable coefficients for studying geophysical phenomena [27]
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