A rise in the water level in ditches by thirty centimeters can avert three millimeters of soil subsidence, but costs farmers in the Groene Hart 239 euros per hectare per year. These calculations are made by Wageningen researchers.
Alterra and Livestock Research have calculated for the first time, using the Waterpas simulation model, what it would cost farmers regionally if the water level rises. A rise of thirty centimeters in the ditch water level in the 1400-hectare Zegveld polder would lead to extra costs of 185 thousand euros per year for dairy farmers there. Many farmers would not be able to bear this, based on today's prices, contends researcher Jan van den Akker of Alterra. A dairy farmer with forty hectares would have to fork out nine thousand euros of his annual income if the water level rises.
A rise in the ditch water level by thirty centimeters would lead to a rise in the ground water level by fifteen centimeters in the peat district in the summer. That would result in lower crop yields; cows and machines would have less opportunity to be out on the land. One farmer could have more problems than another, depending on the ground level and the ditch water level at those areas in the polder.
On the other hand, the ground water level has to be as high as possible because peat above ground water breaks down in the presence of oxygen. If the average ground water level rises by ten centimeters, ground subsidence would be held back by 2.5 millimetres, says Van den Akker. He calculated that peat 2.5 millimetres thick could have emitted 5.5 tons of carbon dioxide per hectare. If farmers could be compensated by a carbon dioxide fund for averting ground subsidence, they would then benefit economically, says the soil expert.
This study is of interest to water boards managing the water level in the peat polders, as they can now calculate the costs and benefits involved in a rise in the water level.