IPCC: “Increased precipitation intensities have enhanced groundwater recharge, most notably in tropical regions (medium confidence)”

Eight UPGro papers have been cited in the new IPCC Working Group Report (August 2021)

As widely reported, the UN’s Intergovernmental Panel on Climate Change (IPCC) has laid out a stark warning on the threat of climate change across the world. The IPCC does not conduct science itself but rigorously examines the latest scientific evidence to be able to put forward a rigorous analysis of how our global climate is changing and the role that human activities play.

One of the long-standing areas of uncertainty has been what impact climate change will have on rainfall, and the water cycle more generally, particularly in the tropics. This is essential for understanding likely impacts on ecosystems, agriculture, water supply and the wider risks to societies and economies.

The following UPGro papers have been cited to help build a picture of what is happening to groundwater and climate across Africa, and the tropics more generally, to support the conclusion that increased precipitation is increasing groundwater recharge:

• Comte, J.-C. et al., 2016: Challenges in groundwater resource management in coastal aquifers of East Africa: 15 Investigations and lessons learnt in the Comoros Islands, Kenya and Tanzania. Journal of Hydrology: Regional 16 Studies, 5, 179–199, doi:10.1016/j.ejrh.2015.12.065.
• Cuthbert, M.O. et al., 2019b: Observed controls on resilience of groundwater to climate variability in sub-Saharan 35 Africa. Nature, 572(7768), 230–234, doi:10.1038/s41586-019-1441-7.
• Dunning, C.M., E. Black, and R.P. Allan, (2018): Later Wet Seasons with More Intense Rainfall over Africa under Future Climate Change. J. Climate, 31, 9719–9738,https://doi.org/10.1175/JCLI-D-18-0102.1
• Jasechko, S. and R.G. Taylor, 2015: Intensive rainfall recharges tropical groundwaters. Environmental Research 16 Letters, 10(12401), 5
• Kotchoni, V.D.O. et al., 2019: Relationships between rainfall and groundwater recharge in seasonally humid Benin: a comparative analysis of long-term hydrographs in sedimentary and crystalline rock aquifers. Hydrogeology Journal, 27, 447–457.
• Kolusu, S.R. et al., 2019: The El Niño event of 2015–2016: climate anomalies and their impact on groundwater resources in East and Southern Africa. Hydrology and Earth System Sciences, 23(3), 1751–1762, doi:10.5194/hess-23-1751-2019.
• Shamsudduha, M. and R.G. Taylor, 2020: Groundwater storage dynamics in the world’s large aquifer systems from GRACE: uncertainty and role of extreme precipitation. Earth System Dynamics, 11(3), 755–774, doi:10.5194/esd-11-755-2020Taylor, R.G. et al., 2013a: Ground water and climate change. Nature Climate Change, 3(4), 322–329, 57 doi:10.1038/nclimate1744. 58
• Taylor, R.G. et al., 2013b: Evidence of the dependence of groundwater resources on extreme rainfall in East Africa. 59 Nature Climate Change, 3, 374–378, doi:10.1038/nclimate1731.

Well done to the dozens of researchers involved in this work. This science is essential for leading to positive action to protect lives and livelihoods.

The following is extracted from Chapter 8, on the Water Cycle:

Land-use change and water extraction for irrigation have influenced local and regional responses in the water cycle (high confidence). Large-scale deforestation has likely decreased evapotranspiration and precipitation and increased runoff over the deforested regions. Urbanization has increased local precipitation (medium confidence) and runoff intensity (high confidence). Increased precipitation intensities have enhanced groundwater recharge, most notably in tropical regions (medium confidence). There is high confidence that groundwater depletion has occurred since at least the start of the 21st century as a consequence of groundwater withdrawals for irrigation in agricultural areas in drylands (e.g., the United States southern High Plains, California Central Valley, North China Plain, and northwest India). {8.2.3.4, 8.3.1.7, Box 10.3, FAQ8.1}

Page 8-7

8.3.1.7.4 Groundwater
As the world’s most widespread store of freshwater (Taylor et al., 2013b), groundwater is estimated to supply between a quarter and a third of the world’s annual freshwater withdrawals to meet agricultural, industrial and domestic demands (Döll et al., 2012; Wada et al., 2014; Hanasaki et al., 2018). Attribution of changes in groundwater storage, observed locally through piezometry (Taylor et al., 2013b) or estimated from GRACE satellite measurements (Rodell et al., 2018) at regional scales (> 100.000 km2 8 ) (Figure 8.10), is often complicated by non-climate influences that include land-use change (Favreau et al., 2009) and human withdrawals (Bierkens and Wada, 2019).

Following a global review of groundwater and climate change (Taylor et al., 2013b) and AR5 WGII, evidence of an association between heavy or extreme precipitation and groundwater recharge has continued to grow, especially in tropical (Asoka et al., 2018; Cuthbert et al., 2019a; Kotchoni et al., 2019) and sub tropical regions (Meixner et al., 2016). Stable-isotope ratios of O and H at 14 of 15 sites across the tropics trace groundwater recharge to intensive monthly rainfall, commonly exceeding the ~70th intensity percentile (Jasechko and Taylor, 2015). Further, heavy rainfall recharging groundwater resources is often influenced by climate variability such as ENSO and PDO (Taylor et al., 2013c; Kuss and Gurdak, 2014; Asoka et al., 2017; Cuthbert et al., 2019b; Kolusu et al., 2019; Shamsudduha and Taylor, 2020). Additionally, increases in groundwater storage estimated from GRACE for 37 of the world’s large-scale aquifer systems from 2002 to 2016 are generally found to result from episodic recharge associated with extreme (>90th percentile) annual precipitation.

Page 8-45

Coastal aquifers are the interface between the oceanic and terrestrial hydrological systems. Global sea level 21 rise (SLR) causes fresh–saline-water interfaces to move inland. The extent of seawater intrusion into coastal aquifers depends on a variety of factors including coastal topography, recharge, and groundwater abstraction from coastal aquifers (Comte et al., 2016).

Page 8-46

“Changes in seasonality (see also Box 8.2) are projected with a later monsoon onset (high confidence) over the Sahel and a late cessation (medium confidence), suggesting a delayed wet season as a regional response to global GHG forcing (Biasutti, 2013; Akinsanola and Zhou, 2018; Dunning et al., 2018).” Page 8-88

Page 8-88