Author: Anggi Dewita
The groundwater beneath your feet is the world’s biggest source of freshwater. This hidden resource comprises 98% of the Earth’s fresh water and supports all living creatures on the surface more than we have imagined. It is estimated that groundwater provides nearly 50% of all drinking water worldwide, and 43% of the total water consumption is used for agricultural irrigation (Smith et al., 2016). That number keeps growing because of the increase in the human population and economic growth. Groundwater was estimated to give supplies around 42%, 27% and 36% of the water used for agricultural, industrial and domestic activities (Doll et al., 2012). Beyond human necessity, groundwater discharge sustains communities of animals and plants, ecosystem services, and ecological processes in the groundwater-dependent ecosystem whilst acting as a natural freshwater filter (Doddy et al., 2017).
Groundwater is naturally a renewable resource because it is a part of the hydrological cycle. However, with the current groundwater consumption, we expect a decline in groundwater storage in the future because the withdrawal rates are outstripping groundwater recharge (CDP, 2020). Groundwater is also equally vulnerable to current environmental issues, including climate change, land cover and land-use change, and overpopulation. Groundwater and surface water are parts of a complex hydrological cycle, and the interaction between them happens mainly through the recharge and discharge processes. Surface water percolates downwards into the ground and recharges the aquifer—groundwater discharges into the surface water bodies or diffusely across the landscape (Geoscience Australia, 2021). Any changes in the surface could impact the groundwater and vice versa. Based on this understanding, it is not surprising that climate change, often interpreted as surface-atmosphere interaction, becoming an emerging global problem that poses threats to water beneath the surface resources.
Between climate change and groundwater
Starting from the Industrial Revolution, human activities contributed greenhouse gases such as methane and carbon to Earth’s atmosphere. It marked the beginning of the Anthropocene Epoch Earth’s most recent geological period when anthropogenic activity impacted the planet’s ecosystem and climate significantly (National Geographic, 2021). Climate change refers to the alteration of climate patterns because of greenhouse gases in the atmosphere (Smerdon, 2017), and there are seven variables as indicators of climate change (World Meteorological Organization, 2021):
1. Surface Temperature
2. Atmospheric CO2
3. Sea Level
4. Ocean Acidification
5. Ocean Heat Content
6. Glacier Mass Balance
7. Sea Ice Extent
Broadly speaking, indicators of climate change that have an impact on our groundwater include Earth’s temperature, atmospheric CO2 levels, ocean acidification, and sea level rise. Additionally, derivative variables such as extreme weather and changes in weather patterns can also affect the quality and quantity of groundwater.
As a renewable resource, groundwater recharge occurs when water in the surface, either from precipitation or from lakes and rivers, infiltrates the soil and moves downwards to the aquifers. Recharge from the precipitation is the main element for groundwater availability because it influences the subsequent percolation and water availability on the surface water bodies (Barron et al., 2011). Nevertheless, the relationship between groundwater and climate is complex, as rainfall is not the only climate variable determining groundwater recharge. Any changes in humidity, temperature, and solar radiation can affect evapotranspiration. Thus, changing the surface water availability for recharge processes. In days without rainfalls, plants can also transpire using water directly from the aquifers if their roots reach the saturated zone below the water table (Barbeta & Penuelas, 2017).
Both annual mean temperature and precipitation present a robust relationship with yearly groundwater levels. During periods with high annual mean temperatures, the correlation between temperature and groundwater levels intensifies (Chen et al., 2004). Annual rainfall is the most important variable for recharging processes. In maintaining groundwater levels, the precipitation water must balance the summation of evapotranspiration and the water used (Baker et al., 2016). Still, in regions where the aquifer is located near the surface, temperature changes have a greater influence than rainfall (Kundzewicz et al., 2008).
Not only water quantity but climate change also influences groundwater quality. The salinity level of groundwater may increase because of the water loss during the drought or saltwater intrusion when the sea level rises. Seawater intrusion also causes the rise of unconfined groundwater levels, leading to inland flooding (Befus et al., 2020). Global warming can also lead to an increase in the intensity and duration of droughts, which can result in a decrease in groundwater. On the other hand, climate change also amplifies the likelihood of extreme rainfall and flooding events, which often lead to a rapid rise in groundwater recharge and contamination (Andrade et al., 2018). The changes in water quantity and quality not only impact human civilisation but also threaten biodiversity and the ecosystem balance that depends on the groundwater.
Complex feedbacks are seen between carbon dioxide (CO2) with vegetation. CO2 reduces the transpiration ability of plants, thus increasing the runoff. But CO2 can also increase plant growth, leading to a larger transpiration area that offsets the reduction in plants’ transpiration ability (Şen, 2015).
At a global scale, the groundwater system’s sensitivity to climate change is not well understood due to the complexities of interconnected systems, uncertainties, and climate systems’ dynamic behaviour (Cuthbert, 2019). The changes in climate variables projected using climate models under various climate scenarios will likely have various impacts on aquifers depending on the distance from the recharge areas(s), geological structure, aquifers sensitivity, and spatial variability in hydraulic properties (Green, 2016).
Future groundwater projection under climate change
Approximately 20 % of projected increases in global water scarcity can be attributed to climate change (Sophocleous 2004), with up to 270 million people vulnerable to water scarcity in 2050 under the 1.5°C warming compared to 2°C (Buis, 2019). Moreover, communities on small islands and low-lying coastal zones in developing countries also face the risk of groundwater contamination and coastal flooding (Befus et al., 2020).
Although categorised as a renewable resource, groundwater is also extremely vulnerable to changes. Humans’ dependence on groundwater will likely increase as water demands are stressed by population growth and climate change (Green, 2016). Around the world, humans are overexploiting groundwater, especially in arid and semi-arid areas (Hssaisoune et al., 2020). The over-pumping of groundwater may cause carbon leakage from the ground and release more organic carbon into the atmosphere, giving positive feedback and amplifying the global warming phenomenon (Macklin et al., 2018)
Population and economic growth are two factors that are amplifying climate change. When a country has a high Gross Domestic Product (GDP), it emits more carbon emissions because people gain access to and increase their energy requirements (Ritchie & Roser, 2020). Meanwhile, population and economic growth also lead to more extensive water use. Under this scenario, the water future demand coupled with climate change will unavoidably put huge pressure on the groundwater. Realising the critical roles of water to the environment, economy and society, for both groundwater and surface water requires strategic plans to manage and mitigate the negative impacts of any potential stressors.