Will cows still be able to graze outdoors?
Satellite measurements reveal that the soils of Zuid-Holland are subsiding rapidly at about one centimetre per year. What is going on and can this trend be stopped?
According to Prof. Ramon Hanssen of the Faculty of Civil Engineering and Geosciences (CEG), the peatlands have been subsiding at a rate of between 0 and 2 cm per year for about the last 800 years. His research group provides the data used for the Dutch soil subsidence map.
The first land reclamations that took place many centuries ago marked the beginning of this subsidence. The upper layer of the sodden peat dried up, started to rot, and shrank. After a time, water started to reinfiltrate these areas and the water level had to be lowered further. “There are polders in Delft where eight metres of soil have disappeared,” says Hanssen.
The fear is that this process will be accelerated by drier summers, leading to lower groundwater levels and exposing a thicker layer of soil to desiccation and compaction. In addition, the process of desiccation releases carbon dioxide and methane; about 1 to 2 per cent of the national greenhouse gas emissions.
After a dry, hot summer, the groundwater level can be as much as half a metre to a metre lower than normal, says groundwater expert Prof. Mark Bakker (CEG). The question for the water authorities is always: will the groundwater level be replenished by next spring? This is by no means guaranteed, because the rate of replenishment is slow. On average, 80 centimetres of rain falls each year in the Netherlands, about half of which ends up in the groundwater, while the rest flows to the sea or evaporates. To predict the groundwater level, Bakker collaborated with the consultancy firm Artesia and the University of Graz to develop a software package called Pastas. “Pastas can be used to determine whether a reduction in the groundwater level was caused by less rainfall and more evaporation, or, for example, by extracting groundwater or lowering the surface water level,” explains Bakker. According to Bakker, the desiccation of the peatlands of Zuid-Holland is mainly the result of human activity, and particularly drainage.
Farmers are hindered by high water levels and use drainage pipes installed under their fields to lower the groundwater to 70-80 cm below ground level and so improve the growth of grass and allow cows to graze and heavy machines to drive over the land.
However, the groundwater level subsequently also falls in nearby nature areas, to the detriment of the sometimes rare flowers and plants that grow there.
The droughts of the recent summers have increased this conflict of interests between nature and agriculture. More than ever, the water authorities have become a political arena where the goals of agriculture and nature conservation are at odds. Will cows still be able to graze in the meadows if the groundwater level cannot be lowered? “Maybe the typical Dutch landscape with a farm and cows grazing along a canal will disappear,” muses Hanssen.