Monitoring groundwater storage changes in complex basement aquifers: An evaluation of the GRACE satellites over East Africa

Abstract

Although the use of the Gravity Recovery and Climate Experiment (GRACE) satellites to monitor groundwater storage changes has become commonplace, our evaluation suggests that careful processing of the GRACE data is necessary to extract a representative signal especially in regions with significant surface water storage (i.e., lakes/reservoirs). In our study, we use cautiously processed data sets, including GRACE, lake altimetry, and model soil moisture, to reduce scaling factor bias and compare GRACE‐derived groundwater storage changes to in situ groundwater observations over parts of East Africa. Over the period 2007–2010, a strong correlation between in situ groundwater storage changes and GRACE groundwater estimates (Spearman's ρ = 0.6) is found. Piecewise trend analyses for the GRACE groundwater estimates reveal significant negative storage changes that are attributed to groundwater use and climate variability. Further analysis comparing groundwater and satellite precipitation data sets permits identification of regional groundwater characterization. For example, our results identify potentially permeable and/or shallow groundwater systems underlying Tanzania and deep and/or less permeable groundwater systems underlying the Upper Nile basin. Regional groundwater behaviors in the semiarid regions of Northern Kenya are attributed to hydraulic connections to recharge zones outside the subbasin boundary. Our results prove the utility of applying GRACE in monitoring groundwater resources in hydrologically complex regions that are undersampled and where policies limit data accessibility.

Recent changes in terrestrial water storage in the Upper Nile Basin: an evaluation of commonly used gridded GRACE products

“…This inconsistency suggests that the "true" GRACE TWS signal is weakened during processing and that the combined SWS + SMS signal is greater than TWS, mathematically resulting in a positive estimate of GWS. In contrast to the assertions of Nanteza et al (2016), applying the GRCTellus CSR solution, we find that this uncertainty prevents robust resolution of GWS from GRACE TWS in these complex hydrogeological environments of eastern Africa. Despite substantial efforts to improve groundwaterlevel monitoring and to collate existing groundwater-level records across Africa, we recognize that understanding of in situ GWS remains greatly constrained by limitations in current observational networks and records.…”

“…For example, GRCTellus underrepresents greatly (63 %) the reduction of 83 km 3 in bottom-up TWS, whereas GRGS and JPLMascons GRACE products underrepresent this by 17 and 4 % respectively. Previous studies in the Upper Nile Basin have relied upon a single GRACE product such as GRCTellus CSR (Nanteza et al, 2016) and GFZ (version (RL04) (Awange et al, 2014) without considering uncertainty in the seasonal amplitude of TWS associated with the processing of different GRACE products. Over a longer period (2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012) in the Upper Nile Basin, all GRACE products correlate well with bottom-up TWS but, similar to the unintended experiment, variability in amplitude is considerable (Fig.…”

“…1) surface water storage in rivers, lakes, and wetlands ( SWS), soil moisture storage ( SMS), ice and snow water storage ( ISS), and groundwater storage ( GWS). Over the last decade, GRACE measurements have become an important hydrological tool for quantifying basin-scale TWS (Güntner, 2008;Xie et al, 2012;Hu and Jiao, 2015) and are increasingly being used to assess spatio-temporal changes in specific water stores Shamsudduha et al, 2012;Jiang et al, 2014;Castellazzi et al, 2016;Long et al, 2016;Nanteza et al, 2016) where timeseries records of other individual freshwater stores are available (Eq. 1).…”

“…Consequently, the ability of low-resolution GRACE gravity signals to trace GWS in these hard-rock environments is unclear. A recent study (Nanteza et al, 2016) applies NASA's GRCTellus (CSR GRACE) data over large basin areas (> 300 000 km 2 ) of eastern Africa and argues that GWS can be estimated with sufficient reliability to characterize regional groundwater systems after accounting for SWS by satellite altimetry and SMS data from the GLDAS LSM ensemble (Rodell et al, 2004).…”

Land Cover, Land Use, and Climate Change Impacts on Endemic Cichlid Habitats in Northern Tanzania

“…Irrigation and other water use such as drinking water in the region almost entirely draws water from sources originating from groundwater [42,47]. With precipitation being a significant driver of the water storage behavior of the region encompassing our study areas [83], climate change scenarios for the region suggest an increase in water stress in the future. In addition, interannual high precipitation events associated with large-scale circulations (i.e., IOD, ENSO) will play increasingly important roles in mitigating accumulated water storage deficits [83].…”