ALBERT

Description: Waternet is the executive agency of the regional water authority Amstel, Gooi and Vecht. Water authority Amstel, Gooi and Vecht manages the water levels (ditches) for 19 400 ha of peat meadows around the Netherlands capital Amsterdam. At present the ditches levels at about 40–60 cm beneath the peat meadow surface, resulting in a groundwater level between from 30 until 80 cm below peat surface and a subsidence of about 9 mm each year. A study was carried out on peat soil subsidence in the Amstel, Gooi and Vecht water authority water management area towards 2100: for short term effects (until 2027), midterm effects (until 2050) and longer term effects (until 2100). This study explores 4 scenarios: (1) present policy (maintain ditch waterlevel at maximum 60 cm below surface); (2) active rewetting, groundwater level at surface; (3) passive rewetting, subsidence is not compensated by lowering of water levels; (4) subsurface irrigation by submerged drains (infiltration in summer, drainage in winter). The scenarios are compared on farming, houses, public infrastructure, greenhouse gases and water management. At present, the total net benefit for farmers are EUR 7 million per year for the whole area, while the costs for the water authority are EUR 37 million per year for managing ditches, dikes and pumps. Costs for greenhouse gases are EUR 18 million (at a price of EUR 40 per ton CO2-eq). Active rewetting would reduce soil subsidence maximally from 2 to 0.5 m towards 2100 but reduces the benefits for farming, whilst the costs for water management stay alike. The costs for greenhouse gases however drops with EUR 3 million per year immediately because CO2-eq emissions drops. Best (financial) results (with respect to all stakeholders) on the long term are booked by passive rewetting with lower costs for water management, houses, public works and greenhouse gases. This scenario will eventually take away the farming possibilities, but not before 2050 and could be too slow to contribute strongly to Paris agreement goals. Best result with respect to climate for short and long term is active rewetting, which will drop the greenhouse gas emissions strongly (equivalent of EUR 2.3 million per year), reduce soil subsidence, but makes farming harder (drop from 7.1 up to EUR 2.5 million per year benefit) and brings no direct reduction of costs for the water authority. Best result on short term for farmers is submerged infiltration drains. However, the effect of this scenario on GHG emission is limited in this study.