Utrecht University: Extreme droughts almost three times more frequent with climate change

The three consecutive very dry summers in 2018, 2019 and 2020 confirmed the urgency to increase the scientific knowledge on how droughts affect the availability of fresh water in the Netherlands and thus the important functions water has for nature and the economy of our country. This led to the project Extreme droughts and the Dutch water sector: impacts and adaptation. In a subproject, a consortium consisting of Utrecht University, Hydrologic, KWR and KNMI is studying the effects of changes in land use and climate on groundwater levels in the Netherlands.

Shortage of precipitation or river water
In hydrological terms, drought consists of two components: a shortage of precipitation, reflected in a shortage of soil moisture, low groundwater levels and low stream discharges, on the one hand, and a shortage of water supply in the major rivers on the other. And these two components affect different parts of our country. “Too little water in the rivers causes problems in the lowlands, so west of Utrecht and in the northern provinces,” says Niko Wanders, hydrologist at Utrecht University. ” In the higher sandy parts of Netherlands, however, droughts are caused by a precipitation deficit.”

The years 2018 and 2019 were examples of years with two different types of extreme drought. Wanders: “In 2018, the shortage of precipitation was the major problem, which resulted in low groundwater levels and stream discharges.” This is detrimental to agriculture and nature, where the recovery of nature in particular can take a long time.

Impacts in different parts of the country also differ. In the high sandy soils, in some places it took three years for groundwater levels to return to the values they had at before the drought of 2018. “In 2019 we saw 15% less water in the Rhine at Lobith,” says Wanders. “This so-called discharge shortage mostly affects the shipping sector.”

Extremely dry year every 15 years
The researchers used a new large-ensemble technique to look at the consequences of extreme drought. To do so, they ran the KNMI climate model 2000 times for the current climate conditions and for a situation of two degrees Celsius warming. Wanders: “This produced two weather plumes for an entire year. We then linked this model to two models that estimate the consequences for water management: the PCR-GLOBWB hydrological model of the UU for the upstream discharge in the major rivers and the National Hydrological Model for the Dutch hydrological system This is how we discovered what the current climate and the climate with two degrees of warming will do to the water supply in the Rhine and groundwater levels in the Netherlands.”

Years of extreme drought occur almost three times more often with a changing climate, it turns out. “In the model of our current climate, such low river discharges occur once every 40 years. In a two degrees warming world, which we are heading for if we do not keep to the Paris agreements, a similar extremely dry year will occur every 15 years. This shows that the supply of fresh water to the Netherlands will no longer be sure thing and this could also have consequences for the water reserves in the Netherlands.

Adjustments in the water system needed
Hydrologist Ruben Boelens from Hydrologic used the National Hydrological Model to find out how this change in the Rhine discharge in combination with a large precipitation deficit will affect the water system of the Netherlands. He analyzed the consequences on groundwater levels, water levels of the IJsselmeer and available water for irrigation.

Boelens also determined whether large-scale land-use changes and adjustments to the water system can limit the negative effects of drought. “The timing of the drought is important. And it is also important to find out which areas are always affected by drought and which only occasionally. In our analysis, we saw that the effects of droughts can be partly offset by large-scale land-use changes. But drastic adaptations in the water system aimed at water retention will have an even greater effect. These can significantly increase local water resources in the future.”

Utrecht University