The hidden resource: groundwater’s role in achieving water security
Groundwater is critical to global water security. This was the clear message Professor Richard Taylor delivered at a seminar in Oxford on 13 November.
Knowledge gaps in groundwater science
Water stored underground provides around 36% of the world’s domestic supplies, and 42% of all irrigation water. Use of groundwater could also prove a useful adaptation to climate variability and change, Taylor said.
Despite its strategic importance, our understanding of the magnitude of groundwater resources across the world is strikingly poor. A groundbreaking 2012 study by Taylor and colleagues showed that the volume of groundwater in Africa is 10-100 times greater than water found above the surface. Far greater investment in groundwater monitoring and science is needed, if this resource is to be harnessed to achieve water security.
Climate change impacts on groundwater recharge
While knowledge on the current state of groundwater is patchy, still less is known about how these resources will change into the future. It is unclear how the trend towards more frequent and heavy rainfall events expected as global temperature rises will impact groundwater recharge.
The 2007 IPCC Fourth Assessment Report referred to just one study on the impact of climate change on groundwater, which projected a dramatic 70% reduction in groundwater recharge in some parts of Brazil and Africa. The model used assumed that more intensive rainfall would more often exceed the capacity for water to infiltrate soils.
However, Taylor provided stark evidence to the contrary, suggesting that more heavy rainfall events may in fact lead to greater groundwater recharge. This more optimistic outlook was based on a study of 55 years of observational data of rainfall and groundwater levels in semi-arid Tanzania.
The records from Tanzania show that recharge is closely associated with extreme seasonal rainfall, with only seven rainfall events during the 55 years accounting for 80% of the recharge. Increased use of groundwater could therefore prove a useful adaptation to climate variability and change, suggested Taylor.
Sustainability of groundwater use
In Bangladesh, China and India, tapping into groundwater reserves for irrigation has enabled these countries to dramatically increase food production to meet the needs of their expanding populations.
However, groundwater is being used faster than it can be replenished in some parts of the world, warned Taylor, including north-west India, the California Central Valley, and the North China Plain. Indirect impacts on groundwater, for example due to irrigation demand, can outweigh any direct impacts of climate change on recharge rates. The sustainability of resources therefore remains a key concern.
Groundwater depletion is not always inevitable and in some cases, groundwater abstraction can actually lead to greater recharge. Taylor’s research shows that in parts of the Bengal Basin in Bangladesh, water pumped out of the ground is almost completely replenished by the yearly monsoon. The subsurface effectively acts as a storage reservoir, with pumping during the dry season making space for greater storage and recharge during rainy periods.
Whether abstraction leads to groundwater depletion or increased recharge depends on the geology and soils, with sandy soils being favourable to recharge in the Bangladesh case.
The inadequacy of global water scarcity metrics
Current metrics of freshwater availability are based solely on river flows. According to Taylor, these measures are fundamentally flawed as they ignore groundwater, and therefore warp perceptions of water security.
Water scarcity metrics are also unhelpful when it comes to planning for adaptation to climate change. Using water more efficiently, and increasing storage, can both help buffer increasing varibility in flows. “We need to think of storage more holistically”, said Taylor. This means considering not only constructed storage such as reservoirs, but also the water stored naturally beneath the Earth’s surface.
This blog is based on a talk given by Richard Taylor, Professor of Hydrogeology at University College London, as part of the Water Security, Growth and Development seminar series. Download the presentation slides