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 CO 2 -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 CO 2 -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.

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Soil subsidence is one of the major issues in the management area of the water authority Amstel, Gooi and Vecht, including emissions of greenhouse gases. This paper describes four different methods to calculate these emissions in agricultural peat meadows, based on (1) the mean lowest groundwater level, (2) the mean groundwater level, (3) the subsidence rates and (4) general numbers. The emissions were calculated in two polders (about 2600 ha peat meadow), these were comparable for all methods, ranging from 42 up to 50 kton CO 2eq yr −1 (based on data of 2015), which is about 14.5 up to 19 t CO 2 -eq ha −1 yr −1 . Besides, the greenhouse gas emissions were compared for different policy scenario's in one polder subunit (283 ha): (1) standard policy (lowering surface water level at the same rate as soil subsidence taking place), (2) passive rewetting (surface water level fixation), (3) subsurface irrigation by submerged drains, and (4) a maximum surface water level decrease of 6 mm yr −1 . Comparing the four policy scenario's in one polder subunit, greenhouse gas emissions were lowest in case of subsurface irrigation, decreasing greenhouse gas emissions by about 35 %-50 % in this polder compared to standard policy, meaning a decrease of about 5.5-9.3 t CO 2 -eq ha −1 yr −1 . This represents a value of about 550-930 EUR ha −1 yr −1 (at a price of EUR 100 per ton CO 2 -eq). The scenario passive rewetting leads to a decrease of about 12 %-21 %, or 2-3 t CO 2 -eq ha −1 yr −1 compared to standard policy. The estimation of the decrease in GHG emissions depends on the assumptions made. In this study it was assumed that subsurface irrigation halves soil subsidence. The water board will use the described procedures to estimate greenhouse gas emissions in the future to support water level management in areas with peat soils.

Water authorities responsible for water quantity and water quality management may strongly influence the magnitude of greenhouse gas emissions from the surface waters and the adjacent peat areas within their territories. Climate smart water management (reducing influx of organic matter and improving water quality) is therefore a potentially strong mitigation tool. We hypothesize that climate smart water management has a stronger mitigation potential than reducing emissions from the operational management of a Water Authority. Based on literature data on greenhouse gas emissions from ditches and agricultural peatlands, we present a case study of a Dutch Water Authority -Amstel, Gooi and Vecht (operated by Waternet). We estimate that greenhouse gas emissions from the 195 km 2 large peat area within its territory are 470 kt CO 2 -eq per year. An additional 231 kt CO 2 -eq yr −1 is emitted from the water bodies within the 102 km 2 large water area territory. Both emissions are considerably higher than the estimated climate footprint of the operational management of the water board (∼ 62 kt CO 2 -eq per year in 2017). While Waternet strives to have a net zero emission of greenhouse gases related to its operational management by 2020, we postulate that measures (to be taken before 2030) such as the prevention of organic matter and nutrients entering surface waters, the removal of organic carbon from ditches and higher groundwater levels in agricultural peatlands, may reduce greenhouse gas emissions in ditches and agricultural peat meadows with 26 and 27 kt CO 2 -eq per year, respectively. Measures that are taken to reduce greenhouse gas emissions in water bodies are expected to have a positive impact on water quality as well.