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. In many dry areas,
more groundwater is pumped than is naturally replenished through precipitation. This is causing
falling groundwater levels worldwide. An important consequence of water table
decline is a reduced flow of groundwater to streams and rivers.
This is a major threat to the freshwater ecosystems in these streams and rivers.
The water level decreases and the water temperature rises too much for
organisms living underwater, such as fish, plankton and water plants. To investigate how strongly freshwater
ecosystems have been affected, we used our global, new hydrological model that
includes the interaction between groundwater and stream flow and the effects of human water use. With this model, we were able
to calculate the flow of groundwater to rivers all over the world, and how it
is reduced by groundwater pumping. A global groundwater surface water model
that is needed to perform these analyses did not exist
and was developed as part of the PhD research of Inge de Graaf
at Utrecht University. Developing such a model and
understanding what simulations to perform is a real team effort
requiring expertise on global hydrology and high-performance computing. Our calculations show that for almost 20% of
the water sheds where groundwater is pumped this pumping has resulted in exceeding
the minimal flow requirements. This means that the water flow in
streams and rivers has become too low to sustain their ecosystems.
And we expect freshwater ecosystems in more than half of these areas to be
threatened by 2050. What is striking about our results is that only a small drop
of groundwater level is necessary before it results in a critical
decline in environmental stream flow. Freshwater ecosystems are therefore
extremely sensitive to a decline in groundwater levels. Moreover, the effects of groundwater extraction
often only becomes noticeable after decades. This programme already exists in Central United States
and for the Indus river basin in Asia. Based on our calculations, we expect this
also to occur within decades in Southern and Eastern Europe
North Africa and Australia, if groundwater pumping continues on the same
footing as now. To prevent this from happening is not easy. But I believe
it can be done. We somehow need to pump less groundwater while maintaining
food security. One direction is to experiment with food
crops dat use less water, or can even be grown on salt or brackish groundwater.
Other possibilities is to use groundwater smarter. Many countries with groundwater
decline also have floods in other parts of the season. And if they can use this excess
water in one part of the season and infiltrate it into the groundwater
system we can use it in the dry season for irrigation.