Groundwater pumping poses worldwide threat to riverine ecosystems

Almost 20% of the catchment areas where groundwater is pumped suffer from a flow of streams and rivers that is too low to sustain their freshwater ecosystems. This number is expected to increase to 50% by 2050. This is the conclusion of a study that appears in Nature today. The study is a collaboration between Utrecht University, the water institute Deltares, the University of Freiburg (Germany) and the University of Victoria (Canada).

Strong population growth and economic development over the last 50 years have led to a large increase in demand for fresh water, mainly for the irrigation of food crops. Currently, about half of the world’s irrigation water is pumped as groundwater from underground aquifers. In many irrigation areas, more groundwater is pumped than is naturally replenished through precipitation. This is causing falling groundwater levels worldwide.

Threat to underwater life

An important consequence of water table decline is a reduced flow of groundwater to streams and rivers. This is a problem particularly during the dry season, when streamflow is almost entirely dependent on the influx of groundwater. This poses a major threat to riverine freshwater ecosystems. The water level decreases and the water temperature rises too much for organisms living underwater, such as fish, plankton and water plants.

Global flow

To find out to what extent freshwater ecosystems have been affected by groundwater pumping, the team of scientists used a novel hydrological model that was able to simulate the flow of groundwater and its interaction with surface water globally. Using this unique model, they calculated the influx of groundwater to the world’s network of streams and rivers during periods with low streamflow.

Changing influx

How has the influx changed since the 1960s due to increasing groundwater pumping? How will it change over the 21st century? Around the year 2010, 17-21% of the river basins suffered a drop in groundwater influx to streams and rivers large enough to threaten freshwater ecosystems. This will apply to 42-79% of catchment areas by 2050.

Ecological harm

‘We find that catchment areas in the central United States and the Indus river basin have already been seriously affected,’ says Inge de Graaf, an Assistant Professor at the University of Freiburg and lead author of the study, which she already started as a PhD candidate in Utrecht. ‘We expect that if groundwater pumping continues on the same footing as now, river basins in Southern and Eastern Europe, North Africa and Australia will also reach their ecological limits in the coming decades. Climate change will only bring forward the time at which this occurs, as we expect it to lead to less precipitation and thus more groundwater pumping in areas that are dry already.’

Ticking time bomb

Co-author Marc Bierkens, Professor of Hydrology at Utrecht University and researcher at Deltares, adds: ‘What is striking about our results is that a small drop in the water table can cause a major reduction in groundwater influx to streams and rivers. This shows that riverine freshwater ecosystems are extremely sensitive to water table decline.’ The study also shows that it often takes decades for groundwater pumping to lead to a noticeable reduction of groundwater influx. ‘Groundwater pumping can thus be considered a ticking time bomb whose ecological effects become visible only years later,’ says Bierkens.

Conservative estimates

In their projections of the future, the researchers only took account of climate change and assumed that the number and intensity of groundwater pumping activities remain the same as those of the last ten years. However, it is much more likely that groundwater pumping will increase due to population growth and economic development. The scientists therefore believe that their estimates of the future effects of groundwater pumping on freshwater ecosystems may even be on the conservative side.

Publication

Environmental flow limits to global groundwater pumping

Inge de Graaf, Tom Gleeson, Rens van Beek*, Edwin Sutanudjaja*, Marc Bierkens*

Nature 2019

* = from Utrecht University

This research was co-financed by NWO.