A new study has revealed that almost a quarter of the world's population lives in places where groundwater is being used up far too quickly. DW spoke to Dr. Marc Bierkens, a professor of hydrology at Utrecht University.
DW: Professor Bierkins, could you tell us what groundwater is and why we should be concerned about it.
Marc Bierkens: Groundwater is the water that is stored in the pores and fissures of the geological formations below the ground. If you dig a hole in the western part of the Netherlands, within one meter you will find that the hole will fill with water - that's groundwater.
Has it recently arrived or is it water that has been there for a long time?
That differs, depending on where you are. If you are tapping shallow groundwater in my country, the Netherlands, it might be a few months to a few years old. But if you go to the Sahara, they have actually found water there that has been dated up to a million years old. So, it is quite variable, but on average it's a little older than the water you find in the rivers. It's preferred for drinking water because it's usually much safer and much cleaner than surface water, and less bound to be polluted.
Irrigating crops with groundwater has improved production on Indian farms, but the region's water tables are declining
Why is it cleaner than the other water available?
A lot of that water has travelled through the soil… there is air, sand grains and fissures. The water slowly percolates through that, sort like a sponge. And it is being filtered before it reaches the actual groundwater body.
Looking at your findings, can you tell us what you've discovered?
We looked at groundwater-carrying layers all over the world, these are called aquifers. For each aquifer, we checked how much rainwater percolates down to the groundwater per year. We also worked out how much water was extracted from the ground. If there is more water extracted than replaced, you are depleting those resources. It's like taking more money out of your bank account than you earn.
Your study shows that some regions are headed for real trouble if action isn't taken right away. Can you give me an example of a place where the problem is particularly bad?
The area that really stands out is north-western India and north-eastern Pakistan. Other areas are north-eastern China, the central United States, the central valley of California, places in Mexico and Saudi Arabia and Iran. Those are the hotspots of the world.
Can you tell us more about the situation in India?
In India, the western part is quite dry during the growing season. Now I have to explain, this area is tropical. There used to be just one growing season there in the past, only during the rainy season. But they figured out how to irrigate there so they are taking water from surface water, but also from groundwater and use it to irrigate crops.
This means they can have two, sometimes three harvests per year. They can do that by continually irrigating those crops - including during dry periods. So you see an extension of irrigated areas and that's all driven by population growth.
Population numbers have soared in those areas. There's not enough surface water - not enough water in rivers and lakes - to supply them. So they have been taking it out of the groundwater. With this, they slowly entered a situation of overuse. There are areas there where the groundwater tables are dropping more than a meter a year.
Looking at the situation here in Europe, what were your findings in this part of the world?
Here the situation is quite different. If you talk about Germany and the Netherlands, this is an area where, on average, there is much more precipitation than evaporation. So, we have high rates of groundwater recharge. On average, we have sufficient groundwater to feed our needs. We also don't need to irrigate our crops so much because we have enough rainfall.
But there are other parts of Europe, not as severe as India, but look at south-eastern Spain, where there is not much rainfall. It's quite dry. They have been using groundwater as well, to irrigate crops. That has also led to groundwater depletion. Other areas include the lower parts of the Danube, Romania and Bulgaria. There you have irrigated crops that use groundwater. And, at least in the summer, much more groundwater is pumped up there than recharges during that period. So, there you also have some problems with groundwater depletion.
What about places where they were using their groundwater in ways that were not sustainable, but are now meeting demand without overusing available resources?
In the southeast of Spain, legislation has come into place to limit the extraction rights of farmers, which means they have to start growing crops that are much less dependent on water. In the central United States, in the high plains or the Ogallala aquifer, they've been working on recharge projects. They have been redirecting surface water into those aquifers, thereby increasing groundwater recharge. That has reversed the decline of water tables, at least in the northern part of that aquifer.
Dr. Marc Bierkens is a professor of hydrology at Utrecht University in the Netherlands. He is a co-author on the report 'Water balance of global aquifers revealed by groundwater footprint' along with with Tom Gleeson of McGill University in Montreal.
Interview: Saroja Coelho
Editor: Mark Hallam