Abstract
Abstract. GRACE (Gravity Recovery and Climate Experiment) satellite data monitor large-scale changes in total terrestrial water storage (ΔTWS), providing an invaluable tool where in situ observations are limited. Substantial uncertainty remains, however, in the amplitude of GRACE gravity signals and the disaggregation of TWS into individual terrestrial water stores (e.g. groundwater storage). Here, we test the phase and amplitude of three GRACE ΔTWS signals from five commonly used gridded products (i.e. NASA's GRCTellus: CSR, JPL, GFZ; JPL-Mascons; GRGS GRACE) using in situ data and modelled soil moisture from the Global Land Data Assimilation System (GLDAS) in two sub-basins (LVB: Lake Victoria Basin; LKB: Lake Kyoga Basin) of the Upper Nile Basin. The analysis extends from January 2003 to December 2012, but focuses on a large and accurately observed reduction in ΔTWS of 83 km3 from 2003 to 2006 in the Lake Victoria Basin. We reveal substantial variability in current GRACE products to quantify the reduction of ΔTWS in Lake Victoria that ranges from 80 km3 (JPL-Mascons) to 69 and 31 km3 for GRGS and GRCTellus respectively. Representation of the phase in TWS in the Upper Nile Basin by GRACE products varies but is generally robust with GRGS, JPL-Mascons, and GRCTellus (ensemble mean of CSR, JPL, and GFZ time-series data), explaining 90, 84, and 75 % of the variance respectively in "in situ" or "bottom-up" ΔTWS in the LVB. Resolution of changes in groundwater storage (ΔGWS) from GRACE ΔTWS is greatly constrained by both uncertainty in changes in soil-moisture storage (ΔSMS) modelled by GLDAS LSMs (CLM, NOAH, VIC) and the low annual amplitudes in ΔGWS (e.g. 1.8–4.9 cm) observed in deeply weathered crystalline rocks underlying the Upper Nile Basin. Our study highlights the substantial uncertainty in the amplitude of ΔTWS that can result from different data-processing strategies in commonly used, gridded GRACE products; this uncertainty is disregarded in analyses of ΔTWS and individual stores applying a single GRACE product.
Highlights
Satellite measurements under the Gravity Recovery and Climate Experiment (GRACE) mission have, since March 2002 (Tapley et al, 2004), enabled remote monitoring of large-scale (i.e. GRACE footprint: ∼ 200 000 km2), spatiotemporal changes in total terrestrial water storage ( TWS) at 10-day to monthly timescales (Longuevergne et al, 2013; Humphrey et al, 2016)
Rainfall derived from Tropical Rainfall Measuring Mission (TRMM) satellite observations over the same period are shown on the bottom panel (d)
The analysis shows that leakage from Lake Victoria to the Lake Victoria Basin (LVB) for GRCTellus is substantially greater than GRGS product by a factor of ∼ 2.6
Summary
Satellite measurements under the Gravity Recovery and Climate Experiment (GRACE) mission have, since March 2002 (Tapley et al, 2004), enabled remote monitoring of large-scale (i.e. GRACE footprint: ∼ 200 000 km2), spatiotemporal changes in total terrestrial water storage ( TWS) at 10-day to monthly timescales (Longuevergne et al, 2013; Humphrey et al, 2016). M. Shamsudduha et al.: Recent changes in terrestrial water storage in the Upper Nile Basin glacier and ice melting, sea-level rise) and anthropogenic (e.g. abstraction-driven groundwater depletion) influences on TWS. GRACE-derived TWS provides vertically integrated water storage changes in all water-bearing layers (Wahr et al, 2004; Strassberg et al, 2007; Ramillien et al, 2008) that include (Eq 1) surface water storage in rivers, lakes, and wetlands ( SWS), soil moisture storage ( SMS), ice and snow water storage ( ISS), and groundwater storage ( GWS). GRACE measurements have become an important hydrological tool for quantifying basin-scale
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