Seasonal and Decadal Groundwater Changes in African Sedimentary Aquifers Estimated Using GRACE Products and LSMs

Abstract

Increased groundwater abstraction is important to the economic development of Africa and to achieving many of the Sustainable Development Goals. However, there is little information on long-term or seasonal groundwater trends due to a lack of in situ monitoring. Here, we used GRACE data from three products (the Centre for Space Research land solution (CSR), the Jet Propulsion Laboratory's Global Mascon solution (JPL-MSCN), and the Centre National D'etudes Spatiales / Groupe de Recherches de Géodésie Spatiale solution (GRGS)), to examine terrestrial water storage (TWS) changes in 12 African sedimentary aquifers, to examine relationships between TWS and rainfall , and estimate groundwater storage (GWS) changes using four Land Surface Models (LSMs) (Community Land Model (CLM2.0), the Variable Infiltration Capacity model (VIC), the Mosaic model (MOSAIC) and the Noah model (NOAH)). We find that there are no substantial continuous long-term decreasing trends in groundwater storage from 2002 to 2016 in any of the African basins, however, consistent rising groundwater trends amounting to~1 km 3 /year and 1.5 km 3 /year are identified in the Iullemmeden and Senegal basins, respectively, and longer term variations in ∆TWS in several basins associated with rainfall patterns. Discrete seasonal ∆TWS responses of ±1-5 cm/year are indicated by GRACE for each of the basins, with the exception of the Congo, North Kalahari, and Senegal basins, which display larger seasonal ∆TWS equivalent to approx. ±11-20 cm/year. The different seasonal responses in ∆TWS provide useful information about groundwater, including the identification of 5 to 9 month accumulation periods of rainfall in many semi-arid and arid basins as well as differences in ∆TWS responses between Sahelian and southern African aquifers to similar rainfall, likely reflecting differences in landcover. Seasonal ∆GWS estimated by combining GRACE ∆TWS with LSM outputs compare inconsistently to available in situ measurements of groundwater recharge from different basins, highlighting the need to further develop the representation of the recharge process in LSMs and the need for more in situ observations from piezometry.

Groundwater and resilience to drought in the Ethiopian highlands

“…poor functionality rates, with less than 50% of hand pumps equipped boreholes fully functional in Ethiopia (Kebede et al 2017) with similar results for other African countries (Mwathunga et al 2017, Owor et al 2017. Reasons for poor functionality are complex with poor siting, maintenance and corrosion (Foster 2013, Fisher et al 2015, Bonsor et al 2018 often being cited as greater predictors of failure than drought. The implications are clear, ensuring that households have access to a well sited borehole equipped with a handpump, and investing in maintenance to improve functionality, is an excellent way to improve access to secure water through extended dry seasons and drought.…”

“…Groundwater offers storage of a different magnitude than annual rainfall or river flow (MacDonald et al 2012) and across the African continent appears resilient to droughts of several years. No significant longterm regional decline in groundwater storage has been observed over the past 20 years (Bonsor et al 2018) despite shorter term variability (Kolusu et al 2019) and longer term decline (>1000 years) associated with increasing aridity from longer climate process across northern African aquifer basins (e.g. Gossel et al 2004, Edmunds 2009).…”

The El Niño event of 2015–2016: climate anomalies and their impact on groundwater resources in East and Southern Africa

“…Several studies have shown recharge to be episodic in semi-arid regions of Africa (Meyer, 2005;van Wyk et al, 2011;Taylor et al, 2013;Cuthbert et al, 2017) and elsewhere (Jasechko and Taylor, 2015;Cuthbert et al, 2016), highlighting the need to understand patterns and drivers of climate variability, both temporally and spatially, that influence recharge. Bonsor et al (2018Bonsor et al ( ) analysed recent (2002Bonsor et al ( -2016 trends in, and seasonality of, groundwater storage within 12 African sedimentary basins implied from Gravity Recovery and Climate Experiment (GRACE) satellite data. Here, we employ evidence from both in situ observations (piezometry) and GRACE satellite data to examine the effect of large-scale inter-annual climate anomalies on groundwater across spatial scales for locations and domains that represent the rainfall anomaly gradient over EASE and SA associated with characteristic El Niño response, exemplified by the event of 2015-2016.…”

Climate-groundwater dynamics inferred from GRACE and the role of hydraulic memory

“…It has been demonstrated that there is relatively poor correlation between GRACE and GHMs/LSMs in the evaluation of ∆TWS, with significant discrepancies at the basic level of whether storage trends are increasing or decreasing (Scanlon et al, 2018). These findings have been confirmed with reference to regional piezometric groundwater measurements from tropical aquifers in Africa (Bonsor et al, 2018). Thus, the application of GRACE 90 data to ∆GWS implies three distinct areas of uncertainty: in the processing of the GRACE signal, accuracy of GHM/LSM model projections and mutual consistency of the observed (GRACE) and modelled (GHM/LSM) data (Long et al, 2015).…”

“…Several studies have used GRACE data to examine storage changes within a particular GW system e.g. Bonsor et al, 2018;Chen et al, 2016Chen et al, , 2010Henry et al, 2011;Z. Huang et al, 2015;Ramillien et al, 2014;Shamsudduha et al, 2017Shamsudduha et al, , 2012Tiwari et al, 2009;Xavier et al, 2010;Yeh et al, 2006).…”