For many communities in semi-arid and arid regions of Africa, groundwater is the only reliable source of freshwater for drinking and irrigation purposes where surface water is seasonal or perennially absent (MacDonald et al., 2012). The long-term sustainability of groundwater resources is dependent on replenishment by recharge, which results from effective precipitation infiltrating the subsurface where hydraulic gradients are downward (Taylor et al., 2013). Currently, natural inter-annual groundwater recharge occurs on a decadal timescale by episodic recharge (Taylor et al., 2013).
Global warming is expected to intensify the global hydrological cycle, through amplification of potential evapotranspiration and intensification of precipitation (Jiménez Cisneros et al., 2014). For the IPCC’s AR4 and AR5 GCM’s, the projected increases in extreme monthly rainfall under climate change are of much greater magnitude than changes projected for mean monthly rainfall (IPCC, 2012; Taylor et al., 2013). This is expected to result in more variable river discharges and soil moisture, exacerbating intra-annual freshwater variability and shortages (Owor et al., 2009) and threatening food security through reduced crop yields (Challinor et al., 2007).
A number of key studies provide evidence for groundwater recharge to be biased towards intense, heavy rainfall. Is it possible that under this climate change scenario, groundwater could offer a source of resilience for freshwater resources?
Owor et al. (2009) correlated decadal datasets of daily rainfall and groundwater levels for 4 stations in the seasonally humid Upper Nile Basin of Uganda. The results of a linear regression of observed groundwater recharge against both total rainfall depth (representing daily rainfall) and heavy rainfall exceeding a threshold depth of 10 mm day-1 showed that for three stations, the magnitude of observed groundwater recharge was more strongly correlated to the latter (Figure 1). This indicates that groundwater recharge is biased towards heavy rainfall events rather than daily rainfall levels. These higher co-efficient of determination (R2) values were also associated with lower root mean square error values (rmse), increasing the reliability of these conclusions.
Figure 1. Scatter plots displaying the relationship (linear regression) between groundwater level rise during observed recharge events and total rainfall depth (representing daily rainfall) (dashed lines) and heavy rainfall exceeding a threshold of 10 mm day-1 (solid lines). (Source)
Furthermore, the regression of observed recharge and sum of total rainfall (dashed lines on Figure 1) to positive intercepts (158-189 mm) on the x (rainfall) axis (i.e. where the dashed lines cross the x-axis) supports previous assertions (12) that annual rainfall exceeding 200 mm is required for recharge to occur in tropical Africa (Owor et al., 2009).
More recently, the bias of groundwater recharge to heavy precipitation events has been supported by Taylor et al. (2013). Their 55-year record of groundwater levels in the Makutapora Wellfield aquifer of semi-arid central Tanzania highlights the highly episodic occurrence of recharge events under heavy rainfall, which interrupts the multiannual recessions in groundwater levels. Figure 2b displays the records cumulative recharge distribution, showing that a small number of high recharge events account for a disproportionately high amount groundwater recharge; indeed the top 7 seasons of rainfall account for 60% of the total observed groundwater recharge (Taylor et al., 2013). Furthermore, the record suggests that unless seasonal rainfall exceeds 670 mm (i.e. exceeds the third quartile), little or no recharge occurs (Taylor et al., 2013). For nearly 2/3 of the record no recharge is observed, yet after this threshold the groundwater recharge increases exponentially with increasing seasonal rainfall, furthering the point that recharge is restricted to infrequent events of high intensity precipitation. As such, overall Figure 2 shows that the observed relationship between seasonal rainfall and groundwater recharge is non-linear, as recharge is disproportionately restricted to this anomalously intense seasonal rainfall.
Figure 2. a) Observed groundwater recharge from groundwater-level fluctuations and rainy season (November-April) rainfall (shaded region shows seasons with a month of statistically extreme rainfall, i.e. >95th percentile), solid line=median; dashed line=third quartile; b) Cumulative contribution of annual recharge (ranked from highest to lowest) to the total recharge received at the Makutapora Wellfield 1955-2010. (Source)
A key restriction of both Taylor et al.’s (2013) and Owor et al., (2009) papers are the number of groundwater aquifers analysed. With only 5 separate groundwater aquifers analysed between the two papers, this left me with the question – is groundwater recharge biased to extreme heavy rainfall events not just in these aquifers, but across Africa, and indeed the rest of the tropics?
For example, with Owor et al.,’s (2009) study, the differences in slope (evident from differences in the axis scales for each wellfield) primarily reflect variations in the storage properties of the monitored aquifers, since the recharge surface environments (e.g. soil type) of the aquifers are all similar (Figure 1). This demonstrates that factors such as the geology of the groundwater aquifer may influence its ability to transmit heavy rainfall through into storage, and thus not all groundwater aquifers may be biased towards heavy rainfall events.
However, a recent paper hot off the press addresses this question exactly. In order to demonstrate the ubiquity in the bias of tropical groundwater recharge to intensive threshold-exceeding precipitation, Jasechko and Taylor (2015) related long-term records of stable isotope ratios of O (18O/16O) and H (2H/1H) in tropical precipitation at 15 pan-tropical sites (including Africa, Asia and the Americas) to those of local groundwater. In the tropics these ratios are strongly determined by site-scale precipitation intensities; high-intensity rainfall is comparatively depleted in heavy isotopes (18O, 2H) (Jasechko and Taylor, 2015). As such, the comparison of rainwater isotope composition for all rainfall events with the isotope composition of groundwater recharge-generating rainfall enables tracing of the rainfall intensities that produce groundwater recharge.
The results of the study were arresting: the comparison revealed that groundwater recharge in the tropics is near-uniformly (14/15 sites) biased to intensive monthly rainfall, commonly exceeding the ~70th precipitation intensity decile. Figure 3 shows that 14 out of 15 sites have mean groundwater δ18O values lower than the (i.e. groundwater depleted in 18O- and 2H relative to) long-term amount-weighted precipitation δ18O. This isotopic data confirms that groundwater recharge is biased towards 18O- and 2H- depleted rainfall – in other words, more intense rainfall.
Figure 3. Groundwater and long-term amount-weighted precipitation isotope compositions at 15 pan-tropical sites. 14/15 sites have 18O- and 2H-depleted groundwater relative to long-term amount weighted precipitation. Errors marks average +/- one standard deviations. (Source)
This analysis revealed that this bias is pan-tropical, occurring in a wide range of hydrological environments, for a variety of aquifer types, soil types. This confirms that a wide range of geological, climatological and land-use conditions are able to transmit intensive rainfall exceeding the median to shallow aquifers – thus the bias of recharge to heavy rainfall is not restricted to the few aquifers addressed in Owor et al.’s (2009) and Taylor et al.’s (2013) analyses.
For each site, Jasechko and Taylor (2015) matched the mean groundwater isotope compositions with those of the amounts-weighted precipitation under varying precipitation intensity thresholds, in order to trace the threshold precipitation intensities required to produce groundwater recharge. The intersection of groundwater δ18O and the amount-weighted precipitation δ18O under varying precipitation intensities provides an estimation of precipitation intensity thresholds required to initiate groundwater recharge. At all the sampled locations across tropical Africa, the groundwater isotope compositions were consistent with monthly rainfalls exceeding the ~70th percentile intensity (i.e. the range of >30th to >90th percentile) (within 1 standard deviation) (Figure 4). The authors conclude that under the primarily humid conditions experienced at the sites, these potential intensity thresholds correspond to rainfall intensities of ~100-300 mm/month, suggesting that rainfall intensities below these thresholds do not contribute substantially to groundwater recharge. *
Figure 4. Long-term amount-weighted precipitation δ18O using data exceeding precipitation intensity thresholds and local groundwater δ18O values. (Source)
Overall, under the predicted regime of decreasing frequency of low- and medium-intensity rainfall events, and increasing frequency of very heavy precipitation events, groundwater may be a plausible solution to the negative effects this will have on the reliability of river discharge and soil moisture. Groundwater resources are better distributed than surface waters and account for over 90% of accessible freshwater worldwide (Shiklomanov and Rodda, 2003). They present low-cost strategies to adapt to changing freshwater availability and demand (Owor et al., 2009), through strategies such as groundwater-fed irrigation and sustenance of domestic and industrial water suppled (Jasechko and Taylor, 2015).
However, there are a number of important uncertainties with the three papers reviewed in this post. A key uncertainty overall concerns whether soil infiltration capacities are able, in practice, to transmit the modelled increases in recharge generated by heavy rainfall. Although Jasechko and Taylor (2015) demonstrated the ubiquity of soil types that are biased towards heavy rainfall in groundwater recharge, the relationship between precipitation and groundwater recharge remains poorly resolved in many regions due to a lack of long-term observational data (Taylor et al., 2013).
Furthermore, substantial uncertainty remains as to whether potential rises in groundwater recharge under climate change will be offset by increased evapotranspiration associated with warmer atmospheres (Owor et al., 2009; Kingston et al., 2009). The observed thresholds of the three discussed papers reflect the requirement of intense rainfall to overcome high rates of PET that prevail in the tropics in order to generate recharge (Taylor et al., 2013). The conversion of precipitation into groundwater recharge at low latitudes is constrained by continuously high PET rates (Jasechko and Taylor, 2015), estimated locally to be 160 mm month-1 during the monsoon season in the Makutapora Wellfield in Tanzania for example (Taylor et al., 2013).
A last point is that the resilience of groundwater to climate change can be undermined by other influences on groundwater storage such as human overuse, changes in the total volume of precipitation, and land-use change. Taylor et al. (2013) observed that rates of groundwater level decline in Tanzania have increased substantially from ~0.5 m yr-1 (1955-1979) to ~1.7 m yr-1 since 1990. As Figure 5 shows, multiannual declines in groundwater can be strongly linked to increases in monthly groundwater abstraction, that has increased from 0.1 to 0.9 million m3 over the 55 year period in order to supply potable water to the national capital, Dodoma (Taylor et al., 2013).
Figure 5. 55-year record of groundwater levels (top panel) and monthly groundwater abstraction (lower panel) from the Makutapora Wellfield, central Tanzania. (Source)
*An important uncertainty associated with this conclusions is that the analysis assumed all intensive rainfalls under increasingly higher decile thresholds contribute equally to groundwater recharge – however, Jasechko and Taylor (2015) point out that some (limited) data reveal that the proportion of very heavy, statically extreme rainfalls (e.g. >95th percentile) converted to recharge can be substantially greater than less intensive rainfalls. As such, the problem here essentially lies with the decile system, which groups together rainfall intensities above each decile.