Journal of Hydrology

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1 Journal of Hydrology (2012) Contents lists available at SciVerse ScienceDirect Journal of Hydrology journal homepage: Analysing water level strategies to reduce soil subsidence in Dutch peat meadows E.P. Querner, P.C. Jansen, J.J.H. van den Akker, C. Kwakernaak Alterra, Wageningen University and Research, P.O. Box 47, 6700 AA, Wageningen, The Netherlands article info summary Article history: Received 29 August 2011 Received in revised form 15 February 2012 Accepted 17 April 2012 Available online 25 April 2012 This manuscript was handled by Philippe Baveye, Editor-in-Chief, with the assistance of Muhammad Ejaz Qureshi, Associate Editor Keywords: Hydrological modelling Pasture Subsidence Subsurface drains Water level control Water supply The survival of peat meadows in the Netherlands is threatened by soil subsidence, less favourable conditions for farming and rising costs of water management. To support policy-making, a study examined possible future strategies for these meadows in the west of the Netherlands. Future scenarios with different water level strategies and climate scenarios were modelled with the SIMGRO regional hydrological model. The analysis focused on water level control strategies, in combination with subsurface drains, with the aim of reducing subsidence and minimising the water supply in dry periods. Subsurface drains were found to be a good measure to reduce subsidence, but more water had to be supplied. Based on the simulated water level control strategies an optimal scenario was found; it minimises the negative effects of the increased water supply. A scenario simulating the anticipated climate change appeared to have a great impact on the peat meadows. In the future the subsidence rate will increase and more water will have to be supplied to maintain the target surface water levels. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction On large parts of the delta deposited by the Rhine and Meuse, in the area of the Netherlands now known as Holland, peat developed in the Holocene. From the 12th century onwards these peatlands were reclaimed, mainly for agriculture (Van de Ven, 1993; Bragg and Lindsay, 2003). Due to oxidation of the drained peat, the resulting peat meadows have been subsiding ever since. The subsidence, plus a rise in sea level means that now most of the peat meadows are below sea level. Throughout these low-lying Dutch polders, the water table is shallow and a dense network of engineered watercourses and pumping stations is needed to drain the land. The water management required to keep surface water levels in the polders low and thus groundwater tables, in order to maintain suitable conditions for agriculture, is becoming increasingly costly. It is also threatening the unique open and historical landscape, because though drainage of the area is essential to preserve the meadows, excessive drainage accelerates the subsidence of the peat, makes wetland nature areas too dry and leads to the inflow of undesirable saline groundwater. Within this landscape of peat meadows there are also deep polders (reclaimed lakes) to which groundwater from large parts of the surrounding peat meadows Corresponding author. Tel.: ; fax: addresses: erik.querner@wur.nl (E.P. Querner), peter.jansen@wur.nl (P.C. Jansen), janjh.vandenakker@wur.nl (J.J.H. van den Akker), ceesc.kwakernaak@ wur.nl (C. Kwakernaak). flows. In summer this lowers the levels of the surface water and groundwater even more, increasing the likelihood of peat subsidence. To prevent this, water must be supplied to keep the water tables sufficiently high. In this paper subsidence of drained peat soils is caused by oxidation of the organic matter of the peat soil, consolidation of the peat layer and permanent shrinkage of the upper part of the peat soil above the groundwater level. The survival of the peat meadows is also threatened because farming is under pressure as a result of agricultural reform and increasing urbanisation. Another important issue affecting their future is water quality, especially with the implementation of the EU Water Framework Directive (WFD), the water quality needs to be improved substantial. All these factors, coupled with the acknowledged high biodiversity of peatlands (Gulbinas et al., 2007) and their important function for conserving carbon (Van den Akker et al., 2008) made it necessary for a strategic study of the future of the peat meadows in the west of the Netherlands, to support policy making. In this paper we report on that study. Though the peatlands investigated are in the Netherlands, the problems encountered are relevant for other parts of the world with peat meadows in agricultural use. A study in peat meadows has to deal with certain special circumstances. The very shallow water tables prevailing in peat meadows mean that groundwater and surface water are closely interlinked. Among the key factors affecting the groundwater regime of these areas are the groundwater recharge pattern, drainage conditions and the hydraulic properties of the soil. In peatlands, /$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.

2 60 E.P. Querner et al. / Journal of Hydrology (2012) the hydrology in the unsaturated zone interacts strongly with the groundwater and surface water locally. Also important are the drainage to local depressions and to ditches. Thus any development such as improved drainage or change in land use, whether natural or human, may impact the groundwater regime, possibly triggering a number of subsidiary impacts, such as excessive drying of the soil, soil subsidence and environmental degradation (Charman, 2002). Spatially distributed hydrological models are useful tools to support policy making. The dynamics of flow between aquifer systems and interconnected streams are explored using coupled stream aquifer interaction models that are capable of accounting for the interdependence of the functioning of groundwater and surface water (Bradley, 2002; Thompson et al., 2004). If peatland is to be conserved, its eco-hydrological functioning (groundwater flow pattern, groundwater quality and surface water conditions) must be assured (Wassen et al., 2006). It is therefore crucial to understand peatland hydrology. To analyse the complex situation in a polder with pumping stations to remove water and intake points through which water is supplied in dry periods, it is necessary to use a combined groundwater and surface water model. This entails analysing and assessing the groundwater and surface water system as a whole, not separately, and not decoupling the unsaturated zone from the saturated groundwater system (Freeze and Harlan, 1969; Van Bakel, 1988). To do so, an integrated modelling approach on a regional scale is required, combining both groundwater and surface water. It was for such practical situations that the MOGROW model was developed and refined (Querner, 1988 and Querner, 1997). The model simulates the flow of water in the groundwater and in the surface water. As it is physically-based, it is suitable for use in situations with changing hydrological conditions. Such a hydrological model can be used on a region of peat meadows for a scenario analysis focusing on feasible measures, in order to underpin decision-making. This paper describes a study in Zegveld polder, in which different water level strategies were simulated with the MOGROW model. Two climate change scenarios were considered, based on the predicted warmer conditions and drier summers for the Netherlands. The aim of the study was to find a more sustainable regime in which agriculture remains economically feasible, and to minimise subsidence to make the water management of peat meadows cheaper and less complicated now and in the future. The analysis focused on new water management strategies to reduce subsidence and create a simpler and more robust system. One way of reducing soil subsidence is to install subsurface drains, as by rapidly draining or supplying water, these reduce fluctuations in groundwater. However, a consequence may be that more water needs to be pumped out of or supplied to the polder. It is important to know the volumes of water involved. 2. The combined surface and groundwater flow model MOGROW To predict the effect of measures for a complex water system like the Dutch peat meadows and their adjacent polders and lakes, it is necessary to use a combined groundwater and surface water model. The MOGROW hydrological model used in the study area is a combination of the SIMWAT surface water flow model and the SIMGRO regional groundwater flow. SIMWAT (SIMulation of flow in surface WATer networks) describes the water movement in a network of water courses, using the Saint Venant equation (Querner, 1997). SIMGRO (SIMulation of GROundwater and surface water levels) is a distributed parameter model that simulates regional transient saturated groundwater flow, unsaturated flow, actual evapotranspiration, sprinkler irrigation, stream flow, and groundwater as a response to rainfall, reference evapotranspiration, and groundwater abstraction (Fig. 1). Practical applications of the model use in peatlands (Poland, Lithuania and The Netherlands) have been described elsewhere (Querner et al., 2010). To model the hydrology of a region, the system has to be schematised geographically, both horizontally and vertically. The horizontal schematisation allows different land uses and soils to be input per node, to make it possible to model spatial differences in evapotranspiration and moisture content in the unsaturated zone. For the saturated zone, various subterranean layers are considered. For the surface water, the major streams are used in the SIMWAT model for the flow routing (Querner et al., 1997). For a comprehensive description of MOGROW, including all the model parameters, see Querner (1997), case studies are described in Querner (1994a and 1994b). The two models were used within the GIS environment ArcView. A user interface, AlterrAqua, served to convert digital geographical information (soil map, land use, watercourses, etc.) into input data for the model. The results of the modelling are visualised and analysed together with specific input parameters Surface water flow In the Netherlands, the surface water system is often a dense network of engineered water courses. It is not feasible to explicitly account for all these water courses in a regional computer simulation model. As the surface water levels in the major water courses are important for the flow routing and to estimate the drainage or subsurface irrigation, in SIMWAT the major water courses, which are controlled by the Water Board, are modelled explicitly as a network of sections; the other water courses are treated as reservoirs and connected to this network. The model also includes structures such as weirs, pumps, culverts, gates and inlets, as these are necessary for the proper modelling of all water movements within a certain region Groundwater flow In SIMGRO, the finite element procedure is applied to represent the flow equation which describes transient groundwater flow in the saturated zone. A transmissivity is allocated to each node to account for the regional hydrogeology. A number of nodes make up a sub-catchment. Evapotranspiration is a function of the crop and the moisture content in the root zone. To calculate the actual evapotranspiration, it is necessary to input the measured values for net precipitation, and the potential evapotranspiration for a reference crop (grass) and woodland. The model derives the potential evapotranspiration for other crops or vegetation types from the values for the reference crop, by converting with known crop factors (Feddes, 1987). The network of watercourses, in terms of the size and density of the channels, is important for the interaction between surface and groundwater. The primary and secondary watercourses represent the larger channels, also considered in the surface water model. Additionally, smaller watercourses, ditches, trenches or subsurface drains can be present. In the model four different size categories of drainage ditches are used to simulate the interaction between surface water and groundwater, using a drainage resistance factor and the difference in level between groundwater and surface water (Ernst, 1978) Linking SIMWAT and SIMGRO The two modules are aligned by linking a node in the finite element grid in the groundwater module to a node in the surface water module. The MOGROW model has a groundwater part that

3 E.P. Querner et al. / Journal of Hydrology (2012) Fig. 1. Schematisation of water flows in the SIMGRO model. The main feature of this model is the integration of saturated zone, unsaturated zone and the surface water systems within a sub-catchment (Querner, 1997). reacts slowly to changes, plus a surface water part with a quick response. Therefore each part has been given its own time step: the surface water module performs several time steps during one time step of the groundwater module. The groundwater level is assumed to remain constant during that time and the interaction between groundwater and surface water is accumulated using the updated surface water level. The next time the groundwater module is called up, the accumulated drainage or subsurface irrigation is used to calculate a new groundwater level. 3. Description of the Zegveld study area and model application The Zegveld polder is in the green heart of the Netherlands, surrounded by the major cities (see Fig. 2). The study area of 45 km 2 lies north of the river Oude Rijn (some 1000 years ago the main channel of the river Rhine). The country slope is from about sea level along the Oude Rijn to about 2.5 m below MSL in the north-west. As a result, each of the 21 polders in the area has a different target surface water level. In the area modelled, the ground consists of peat up to 8 m thick. Along the Oude Rijn the peat soil is overlain by a layer of clay that can be as much as 1 m thick. The land use is predominantly pasture, mainly for dairy farming (Jansen et al., 2007 or Querner et al., 2008). About a 1000 years ago the peatland was around 1 m above sea level, higher than the Oude Rijn and thus free-draining. In the 12th century it began to be reclaimed; dikes, dams and windmills were used to keep the polders dry. There was also large-scale peatdigging in the area, for fuel; water flowed into the abandoned workings, forming numerous lakes which were reclaimed from the 16th century onwards and are now the lowest-lying polders (4 5 m below MSL). Fig. 3, a cross-section through the study area from south to north, shows the elevation of the ground and the different target surface water levels. The elevation declines from the Oude Rijn, ultimately reaching its lowest point: 2.65 m below MSL. The nature area with lakes is a metre or so higher than the surroundings. By the beginning of the 20th century the area had subsided by about 2 m. Since then, the ground has fallen by a further metre, mostly in the last 50 years. The water level management in this period has focussed mainly on maximising agricultural production, which meant avoiding waterlogging by lowering groundwater and surface water levels. In general the target surface water levels in the peat meadows are currently at depths of about 0.5 m in summer and 0.6 m in winter. Farmers whose fields are still too wet despite this have created their own pumped drainage area (small sub-polders). The result is a complex water system with numerous areas, all with different drainage levels. To predict the regional effects of water management strategies, therefore, this complex engineered Dutch polder system has to be analysed with a combined groundwater and surface water model. 4. Input data and schematisation The SIMGRO model application was set up for the Zegveld study area and its surroundings: an area of approx. 122 km 2 (Jansen et al., 2007). The finite element network covering the area comprised 14,136 nodes spaced about 75 m apart in the centre of the area (45 km 2 ) and spaced 250 m outside this area. The groundwater system was schematised in seven layers (Table 1). The first layer is a thin aquifer with a thickness of 1.5 m, in which the groundwater flows laterally through the top part of the peat layer towards the field ditches or trenches. The transmissivity (kd) of this layer for the study area is 2 3 m 2 d 1. The peat and clay layers of the polder was considered as an aquitard, ranging in thickness up to 6 m and with a hydraulic resistance of days. The main aquifer below extends over the whole modelling area and has a transmissivity in the range m 2 d 1. Underneath are four layers comprising alternating aquitards and aquifers. These layers are not relevant for regional groundwater flow, but the (specific) storage is considered to be important te release water in a dry summer. The spatially distributed features in the Zegveld polder were modelled using the available digital data. This included the topography (scale 1:10,000), the boundaries of the polders and the small sub-polders, together with land use; soil type; and hydrogeological parameters; the hydrographic network and positions of hydraulic structures. A summary of the input data is given in Table 1, together with the value or range used in this model application. Meteorological data were taken from a weather station 20 km east of the basin.

4 62 E.P. Querner et al. / Journal of Hydrology (2012) Fig. 2. Map showing the peatlands in the west of the Netherlands and the Zegveld study area. 5. Model Calibration Fig. 3. South North cross-section through the Zegveld study area as given in Fig. 2. The extend of the study area are the polders given in this figure. The area of interest, Groot Zegveld polder, is 1945 ha and shown in Fig. 2 in detail in Fig. 4. To model the surface water in the study area, each of the four different regions had its own water level control. In the model, the surface water network was schematised by 220 channel reaches (Fig. 4), the inlets for water supply and the weirs. At the east side of the Groot Zegveld polder (Fig. 4) there is a pumping station, where surplus water is pumped out of the polder. The maximum capacity of the two pumps is approximately 14 mm d 1. For water supply in summer the maximum capacity for peat meadows is mm d 1. The groundwater flow to nearby deep polders (reclaimed lakes) varies in space and time, but in summer is around 0.2 mm d 1. The SIMGRO model was calibrated using groundwater levels (period ) and the water balance of the Groot Zegveld polder for 2 years. Additionally it was necessary to calibrate the model on soil subsidence, as discussed below. On the basis of model runs and comparing measured and calculated phreatic groundwater levels, the transmissivities (kd values) of the 3rd layer (main aquifer) were adapted, following other modelling studies (Wendt and Haddink, 2003). For the entire modelling area the transmissivities were tripled, after which they ranged between 900 and 2700 m 2 d 1. Similar values have been reported in the literature on this area. The study area and its surroundings are quite flat (Fig. 2) as a result the regional groundwater flow is small. From a sensitivity analysis it appeared that changes in phreatic groundwater levels and pumping rates do not vary much when the transmissivity of the 3rd layer was changed drastically (Jansen et al., 2007), because of the high resistance of the first aquitard (2nd layer with clay and peat: Table 1). In the modelling area measurements from 25 piezometers were available with data for the period The differences between measured and calculated phreatic groundwater levels was expressed as RMSE. For 9 piezometers, out of the 13 in the study area, the RMSE was <0.1 m. For the remaining four locations it was between 0.1 and 0.2 m. Fig. 5 shows the difference for only 3 years in calculated and measured groundwater levels for a location in the study area given in Fig. 4 (Piezometer P3). The model simulated the measured water table dynamics reasonably well. The high groundwater levels in winter were about the same, but the calculated summer levels tended to be slightly shallower than the measured values.

5 E.P. Querner et al. / Journal of Hydrology (2012) Table 1 Overview of data input for the MOGROW model. The source of data is indicated by a code: G (GIS data), F (field data) and L (literature). The codes for the data required for the schematisation are N (nodes of finite elements), U (land use and soil unit) and W (water course). Parameter Source of data Required in schematisation Range of value or reference Surface water Invert levels and dimensions water courses, sluice gates and weirs G W Flow resistance L W 33 m 1/3 s 1 Map of sub-catchments G Groundwater Land use G N Digital data base Physical soil properties L N Wösten et al. (1985) Thickness of root zone per land use L U m Ground level G N m-msl Transmissivity aquifers (layer 1 and 3) L N ly 1: 2 3 m 2 d 1 ly 3: m 2 d 1a Thickness of aquifers L N ly 1: 1.5 m; ly 3: m Vertical resistance aquitard L N ly 2: d ly 4: d Thickness of aquitard L N ly2: 6 m; ly 4: 4 18 m Drainage resistance of major streams F N d Depth of ditches F N m Drainage resistance of ditches F N d Drainage resistance of subsurface drains F N 27 d Measured data Meteorological data, groundwater levels and discharges F N,W Daily data a Calibrated values, the original values from literature were m 2 d 1. Fig. 4. Groot Zegveld polder, showing the schematisation of the surface water in the SIMWAT hydraulic model (water courses, pumping station and intake points). P3 is the location of the piezometer, results are shown in Fig Water balance of surface water The water balance terms for the Groot Zegveld polder are shown in Table 2. There is a mix of measured and calculated parameters. The difference in the water balance per summer/winter half year varies from 31 mm to 73 mm. The differences can be attributed to errors in the estimated parameters or to changes in groundwater levels over the half-year periods (Table 2). The calculated water intake into the polder during the summer is around 30% less (60 mm) than the measured intake. This difference is mainly influenced by the uncertainty about the amount of water intake to flush the system in order to improve the water quality

6 64 E.P. Querner et al. / Journal of Hydrology (2012) Fig. 5. Calculated and measured groundwater levels for piezometer P3 (for the location see Fig. 4). The above relations between groundwater levels vs. subsidence were proved to have a high correlation coefficients. Therefore such a relation can be generally used for at least Dutch and comparable climatic conditions. Model results were used to estimate the ALGL. The soil subsidence per year was calculated from the ALGL and compared with the measured subsidence. Measurements were available from spring 1970 onwards. Fig. 6 shows that the calculated and measured subsidence correspond well. For the 35 years of measurements the soil subsidence was in the order of a substantial 0.4 m. The measured subsidence, recorded each year in spring, fluctuates slightly more than the calculated levels, because during very wet winters, as shown in Fig. 6, the ground tends to rise somewhat, as the waterlogged peat swells. In dry summers the temporary subsidence due to shrinkage can be several centimetres; for the peat to swell again, the winters must be wet (see Fig. 6). 6. Water level control Table 2 Water balance terms measured and calculated for the Groot Zegveld polder. Summer 2000 and to a small extend also by the uncertainty in the measured inflows (Jansen et al., 2009). The water intake for water quality control is carried out by the water boards on an irregular basis, therefore we could not incorporate this in the modelling experiments. This large error in the water balance was accepted since in all experiments similar inflows for water quality control are needed. Table 2 shows further that in the summer 2001 evapotranspiration was 45 mm more than in summer 2000, but the water supply was only 11 mm more. The summer of 2001 had a distinct dry period, but the precipitation was about 200 mm more than in the year Subsidence of the peatland Winter 2000/01 Summer 2001 Low groundwater level in summer results in aeration of the peat soil, followed by oxidation of peat, which shrinks, causing subsidence. We modified the hydrological model in order to obtain estimates of the peat subsidence. Using data measured on the experimental farm in Zegveld (for location see Fig. 4) and other locations over the last 30 years, Van den Akker et al. (2008). Available was data on surface water levels and on subsidence of peat soils in the northern part of the Netherlands and a set of data based on 30 years of measurements of surface water levels, groundwater levels (8 years) and subsidence of 14 parcels in four locations in the western part and one location in the northern part of the Netherlands. They derived a relationship between the groundwater level in summer and the subsidence. They related the peat subsidence to the average lowest groundwater level in summer (ALGL in metres), which is defined as the mean of the three deepest groundwater levels measured at 14-day intervals each summer. The relationship is: Peat subsidence ðmm=yrþ ¼23:54 ALGL 6:68 Winter 2001/02 Precipitation a Evapotr. b Seepage b Discharge a Water supply a Water supply b Difference in balance a b Measured. Calculated by MOGROW. Under current water management, the surface water level fluctuates slightly around the target level that is needed for agricultural purposes. When the water level is 0.02 m above the target, water is pumped out of the polders. When the level water is 0.02 m below the target, water is let into the polder. In recent years, it has been proposed to replace this rigid water level control by a more flexible or dynamic regime, with the aim of reducing water movement in and out of a polder without unduly affecting the peat subsidence. The permissible fluctuation of the surface water level for such a flexible water level regime is in the range of 0.1 m below or above the target level. The objective of a flexible water level regime is in general less water supplied and less water to pump out. Such a strategy could result in lower groundwater levels and that could mean more subsidence. For the dynamic water level control the present regime is used in principle, but the water level is raised or lowered an additional 0.05 m, depending on the groundwater level or expected rainfall. The manipulation is done when the groundwater level is more than 0.05 m below or above the target level of the surface water. Further criteria is that it is expected either 15 mm of precipitation in the next 3 days (in the case of the need to lower the surface water level) or that there will be no rain in the next 3 days, in the case of the need to raise the level (Jansen et al., 2009). 7. Scenario analysis and the results We investigated specific water management strategies for reducing peat subsidence and concomitantly decreasing pumping and water supply. Based on the present situation the following water management measures were considered: Fig. 6. Calculated and measured soil subsidence for a meadow on the Zegveld experimental farm shown in Fig. 4. Surface water levels are about 0.55 m below soil surface and all measurements were made in spring (Jansen et al., 2007).

7 E.P. Querner et al. / Journal of Hydrology (2012) A scenario (1) creating wetter conditions by higher surface water levels; A scenario without subsurface drains (2.1) and with subsurface drains (2.2); Four scenarios ( ) with water-level control options in combination with subsurface drains, and one optimal scenario 3.5; Two possible climate scenarios (4.1 and 4.2). In the present regime the target depths of the surface water are 0.6 m in winter and 0.5 m in summer. In this scenario, surface water level fluctuates about 2 cm above or below the target level. This fluctuation is the difference between the starting and stopping levels of the pumping station Higher surface water levels (strategy 1) Higher surface water levels are an interesting option for reducing the peat subsidence. In this scenario, the small-scale pumped drainage areas have been disregarded. In the scenario the surface water levels are raised an additional 0.2 m, to a depth of 0.4 m below ground in winter and 0.3 m below ground in summer. It is a scenario to reduce the subsidence drastically, less suitable for agriculture, but more suitable for nature Results In Fig. 7 the subsidence is shown for the present regime ( deep ) and for the scenario in which the surface water levels are 0.2 m higher ( shallow ), i.e. 0.4 m depth in winter and 0.3 m depth in summer. The results show that the subsidence is more pronounced in very dry summers such as 1959 and Under the present regime, the subsidence over the 45 years of calculation will be around 0.38 m. If the surface water level is raised 0.2 m, then the subsidence reduces to 0.22 m. In the calculations the drainage levels have not been lowered to follow the subsidence, thus the rate of subsidence decreases slowly. This is considered as a hypothetical situation. More realistic is the situation in which the drainage levels are lowered the same amount as the subsidence. If this is done, then the subsidence will be much more: it was estimated to be in the order of 0.50 m for the present regime and 0.31 m for the raised surface water level (Jansen et al., 2007). Maintaining the raised water levels required more water: 10 15% more than in the present regime. As a result of the raised surface water levels, about 20% of the area is too wet for agriculture, but these conditions are favourable for nature. In the remaining 80%, agriculture is still economically feasible Subsurface drains (strategy 2) Scenario 2.1 has no drains, but in scenario 2.2 the drains are installed below the surface water level. The drains reduce the dynamics of the groundwater table fluctuations. In wet periods water drains quickly and the high groundwater levels fall, but during dry periods (in summer) the drains are used to supply water and the groundwater level does not fall too much. When drains are installed, the target level for the surface water is raised from a depth of 0.6 m to a depth of 0.5 m, because higher groundwater levels fall quickly and thus conditions are seldom too wet for agriculture Results The subsurface drains produce fast drainage for wet periods and during dry periods water infiltrates from them into the ground. In Fig. 8 the groundwater levels for a location without subsurface drains is compared with the levels when drains have been installed. The nearby deep polders and groundwater extraction result in downward flow of groundwater. Groundwater levels tend to be lower in summer due to this and the high evapotranspiration of the grasslands. Using Subsurface drains, in winter the groundwater level is about 0.1 m lower and in summer around 0.2 m higher (Fig. 8). The groundwater level fluctuates very little and the higher groundwater level results in less subsidence: a reduction of up to 50% in summer. In summer, as a consequence of the higher surface water level and infiltration of water from the subsurface drains, about 30% more water has to be supplied Water level control in combination with subsurface drains (strategy 3) We considered two water level control strategies: a dynamic regime (3.1 and 3.2) and a flexible regime (3.3 and 3.4). The criteria for both regimes were described in Section 6. Based on the results an optimal scenario was defined and presented as scenario 3.5. The SIMGRO model was run for a period of 15 years ( ). Fig. 7. Calculated subsidence of the soil surface for the period for deep surface water levels (0.5 m below ground in summer and 0.6 m below ground in winter) and shallow surface water levels (0.3 below ground in summer and 0.4 m below ground in winter) for 2 locations on the experimental farm shown in Fig. 4.

8 66 E.P. Querner et al. / Journal of Hydrology (2012) Flexible water level control For the flexible level management, the fluctuation around the target surface water level is about 10 cm. This scenario was considered because it may require less water to be supplied in dry periods and also less water needs to be pumped out during wet periods. Scenario 3.3 is without drains, scenario 3.4 is with drains. Fig. 8. Calculated ground water levels in 2001 without drains and with subsurface drains installed below the surface water level on the experimental farm (soil surface is 1.76 m below MSL) Dynamic water level control For the dynamic level management, it is assumed that the present water level control regime applies, but that water levels are lowered and raised depending on the groundwater level and the expected precipitation. Scenario 3.1 is without drains, scenario 3.2 is with drains. The point in the field at which the groundwater level is taken to represent the actual target level and requirements for water intake is about 1100 m north of the pumping station and intake point, but only 105 m from the a ditch (control point as shown in Fig. 4) Results Fig. 9 gives the results for the two scenarios and it shows the effects of the subsurface drains on groundwater and surface water levels. The water levels have been given relative to the Dutch ordnance datum, because the polder is not perfectly flat. The surface water level is varied according to the groundwater level at the control point. It is assumed that this point is representative for large parts of the polder. But the groundwater regime varies over the polder because of nearby deep polders or lakes. The actual pumping and water intake are given in Fig. 9. The groundwater level regime shown in Fig. 9 corresponds well with that of the scenario without dynamic level management: 2.1 (without drains) and 2.2 (with drains). When there are no subsurface drains the summer groundwater level is slightly higher, because the conditions for water intake are frequently met and surface water levels are raised an extra 5 cm. The raising of the surface water level results in higher groundwater levels. With subsurface drains, the criterion for water intake is met only occasionally Results Table 3 shows gives the scenarios with subsurface drains and different water level control methods. The results are changes in the subsidence, the water supply and the change related to the present situation and the regular water level control (scenario 2.1). It also gives an indication of how frequently the pump or inlet is operated at maximum capacity. When drains are installed, more water needs to be supplied, but the amount can be reduced by permitting surface water level to fluctuate more. Drains result in much less soil subsidence: it is reduced by 3.2 mm/yr in the flexible water management regime and by 4.4 mm/yr in the dynamic regime. Flexible water level management results in a clear decrease in the amount of water required, but in more subsidence than in scenario 2.1 (the present regime). A combination of flexible level management and subsurface drainage requires almost as much water as the present regime. The dynamic water level regime without drains appears to require much more water (an additional 41 mm) than scenario 2.1. With drains, the increase is only 11 mm. For all scenarios the amount of water discharged by pumping, mainly during winter, is more or less the same. The results presented in Table 3 are for a homogeneous peat layer. For the situation where the peat is overlain by clay the results are similar, except that the absolute soil subsidence is about 4 mm/ yr less. With drains, the reduction in subsidence is 2 3 mm/yr Optimal strategy Based on the results of the different strategies of water level control, a new scenario was formulated to capture their benefits and try to reduce the water supply and the soil subsidence. In this optimal scenario (3.5), two strategies were chosen, depending on the forecasted rainfall. For the regular strategy it was assumed that the expected rainfall in the next 5 days would be zero. For the flexible water level management strategy it was expected that rain would fall in the next 5 days, up to a maximum of 10 mm. The target level of the surface water level remained at a depth of 0.5 m throughout the year. For the regular strategy the permitted variation in water level remained ±2 cm and for the flexible strategy it was ±10 cm. When a drier period is expected the water level control is switched from flexible to regular, and thus the fluctuation is reduced to 2 cm. When the switch occurs and there is more water in the system (thus between 2 and 10 cm above the target level), this water will not be pumped out, but instead the surplus is stored in the system. If the water level is more than 2 cm below the target, then water is immediately supplied to regain the regular water level, and a limited amount of supplementary water is needed. The switch from flexible to regular should be not too late, as otherwise the groundwater levels will reach a critical lower level, causing increased soil subsidence. Under the optimal strategy, it appeared that less water is needed and the soil subsidence is less than under either of the two strategies (see Table 3). The soil subsidence is in the same order as the scenario without drains ( mm/yr). 8. Scenarios with climate change Fig. 9. Groundwater and surface water levels for 1996 for scenarios 3.1 (dynamic water level control) and for scenario 2.1 (regular water-level control). Climate change resulting in higher temperatures and drier summer will increase the rate of oxidation and degradation of peat soils substantial. According to Van den Hurk et al., 2006 the climate of the Netherlands is changing. How it will change depends mainly on the global temperature rise as well as on changes in the air

9 E.P. Querner et al. / Journal of Hydrology (2012) Table 3 Summary of the scenario calculations considering two water level control options in combination with subsurface drains. Scenario Water level man. Drains Soil subsidence (mm/yr) Supply summer (mm/yr) Capacity demand a Subs. Change b Intake Change b Intake Pumping 2.1 c Regular No Regular Yes ± ± 3.1 Dynamic No ± Dynamic Yes ± 3.3 Flexible No ± ± 3.4 Flexible Yes Optimal d Yes a It reflects the total number of days and the duration for a maximum water supply (and pumping capacity) during summer: + infrequent supply capacity needed and after long periods; ± moderate supply capacity needed; frequently maximum supply capacity necessary during consecutive days. b Related to the reference situation. c Considered as the reference situation. d Reduce soil subsidence and the amount of water supply using a combination of regular and flexible water level control. circulation patterns in our region (Western Europe) and the related changes in the wind. Based on the most recent results from General Circulation Model (GCM) simulations, the Dutch Royal Meteorological Institute has predicted a set of climate change scenarios, using the GCM results and methods given in the 4th IPCC report (Van den Hurk et al., 2006). In this analyses the relation between global warming, changes in air circulation above Western Europe and climate change in the Netherlands was mapped systematically, combining results from a large number of global and regional climate models and observational series. They related changes in projected global mean temperature and changes in the strength of the large scale atmospheric flow in the area around the Netherlands. Therefore, temperature and circulation were used to discriminate four different scenarios change in temperature, precipitation and potential evaporation. The construction of the extreme precipitation and temperature values and the potential evaporation values was carried out using an ensemble of Regional Climate Model (RCM) simulations and statistical downscaling on observed time series (Van den Hurk et al., 2006). The adopted global temperature rise was 1 (moderate scenario G) and 2 (warmer scenario W). Further the assumptions on air circulation response indicated the present westerly winds (G or W scenario) and more easterly winds resulting in much drier summer (G+ and W+ scenario). The four scenario s (G, G+, W and W+) have an equal chance of occurrence. The implications of the climate change scenarios for peat meadows are grave. For the moderate scenario (G), the rainfall in summer increases 3% but annual potential evapotranspiration also increases by 3%. For a temperature rise of 2, including the change in air circulation, the scenario W+, the average rainfall in summer will decrease by 19%. The annual potential evapotranspiration will increase by 15%. These two scenarios reflect the minimum or maximum changes expected for the near future (2050). These changes from a climate scenario was used to adjust the model input using it as a delta change approach. The expected change in precipitation, temperature and potential evaporation from the present situation was used to convert the meteorological data from the present situation to the 2050s. Using this approach rainfall patterns however are assumed not to change. The drier conditions will result in lower groundwater levels and subsequently increased subsidence. An increase in temperature results in more oxidation of the peat, resulting in a higher subsidence rates (Tate, 1987). A relationship was derived to estimate the increase based on the change in temperature, soil properties and biological activities. For the peatlands in the Netherlands under a temperature increase of 2 this factor amounts to 1.25 (Hendriks, 1991). It means that the subsidence equation presented in Section 5 was increased by 25% Results Under scenario G, the changes are small and therefore the increase in subsidence is about 15%, caused by the lower groundwater levels in summer and by the temperature rise and hence oxidation of the peat soil. Under scenario W+ the groundwater levels in summer are about 0.15 m lower, because of the decrease in precipitation and increase in evapotranspiration. Even though groundwater levels are lower in this scenario, the increase in water supply is around 43% and the subsidence is increased by 68%. For this situation the amount of water required may be not available, because other parts of the Netherlands will also need more water. 9. Discussion and conclusions This study aimed to improve understanding of the impact of subsurface drains in combination with water level control on subsidence and water supply needs in peatland with agricultural land use. Subsidence of drained peat soils is caused by oxidation of the organic matter of the peat soil, consolidation of the peat layer and permanent shrinkage of the upper part of the peat soil above the groundwater level. Though the study area was typical peatland in the west of the Netherlands, the results are relevant for other peatlands in the world which are used for agriculture. In such areas, soil subsidence is a critical issue, and the present study provides useful information on the options for water level control in combination with subsurface drains. In order to facilitate decision-making about the future management of Dutch peatland, we investigated the consequences of various strategies for the subsidence and the amount of water required or needing to be pumped out. Another critical issue for peatland is the subsidence of the peat surface and the corresponding release of the greenhouse gas CO 2 and NO 2. A sustainable situation of no subsidence in agricultural peatlands is not possible: some subsidence is inevitable. In order to preserve the peatlands in the western part of the Netherlands as much as possible, the water management should be adapted and focus on methods to keep the groundwater and surface water levels in summer as high as possible. Minimising the subsidence means maintaining shallow groundwater levels, but then conditions for agriculture will be wetter. Installing subsurface drains proves to be a good measure to improve the conditions for farming and to raise the often too deep groundwater levels in summer. The higher groundwater levels in summer reduce soil subsidence; however, more water needs to be supplied in summer and more water needs to be pumped. The extra pumping increases the costs.

10 68 E.P. Querner et al. / Journal of Hydrology (2012) It can be concluded that subsurface drains in combination with a raising the target surface water level by 0.1 m reduces the soil subsidence considerably but the consequence is that more water must be supplied. By permitting greater surface water fluctuation using flexible water level control, (0.1 m above or below the target level) the extra water supply can be reduced, even if using subsurface drains. Using flexible water level control, however, the soil subsidence increases. Dynamic water level control, such as defined in this study, where the surface water is regulated depending on the groundwater level, results in more or less the same subsidence as the present regime with or without subsurface drains; however, much more water is let in in summer. This makes this kind of dynamic water level control very unattractive. The weak point in the dynamic water level control is the steering according to the groundwater level. The time taken for the groundwater level to react to changes in the surface water level is much too long. However, better dynamic water level control is possible. The optimal strategy is a combination of regular and flexible strategy, depending on the forecasted rainfall. This strategy needs less water and results in less soil subsidence than the dynamic or flexible strategies (see Table 3). The soil subsidence in the optimal strategy is in the same order as the reference scenario with drains, while the water supply was about the same as the reference scenario without drains Soil subsidence With subsurface drains the soil subsidence decreases, because the surface water level can be raised and therefore the groundwater levels also rise and in summer water from the drains can easily infiltrate into the ground. As a result the lowest groundwater level in summer will be m higher. The largest reduction in soil subsidence is achieved by using subsurface drains and raising the target surface water by 0.1 m. The reduction in subsidence is 4.5 mm/yr for the present situation and 4.4 mm/yr for the dynamic water level control. Dynamic water level control without subsurface drains regularly leads to a higher surface water level, as a result of which the groundwater level also rises slightly. The soil subsidence decreases by 0.7 mm/yr. A larger fluctuation of the surface water level (flexible water level control) results in a lower groundwater level in the summer and more soil subsidence. When the fluctuation in water level is a maximum of 0.1 m around target level, the soil subsidence increases by 1.0 mm/yr. Using dynamic water level control combined with subsurface drains and a 0.1 m rise in surface water level decreases the soil subsidence by 3.2 mm/yr Water discharge If there are subsurface drains and surface water level is raised, neither a flexible nor a dynamic surface water level greatly affects the total water pumped out of the polder. With dynamic level management and raised surface water level, more water is pumped out in winter than under the present regime, regardless of whether or not there are subsurface drains Water supply For areas with a regular or flexible water level control, where subsurface drains are used and the surface water level is raised 0.1 m, the amount of water needed increases by one third. The change in the present water level control without subsurface drains (a fluctuation of about 0.02 m around target level) to a flexible water level control (a fluctuation of about 0.1 m) reduces the water supply by about a quarter. Changing the present water level control without subsurface drains (fluctuation of about 0.02 m) to a flexible water level control (fluctuation of about 0.1 m) and subsurface drains, requires no extra water. Compared with present water-level management, dynamic level management without subsurface drains requires 35% more water; with drains it requires 44% more water. The number of days that water must be supplied is the largest of all the scenarios examined Climate change The anticipated climate change scenarios show that climate change can have a great impact on the peat meadows. The subsidence rate will increase and more water will be needed to maintain the target surface water levels. For scenario G, there will be smaller changes and therefore the increase in subsidence is small. For the scenario W+ the groundwater levels in summer will be about 0.15 m lower, because of the decrease in precipitation and the increase in evapotranspiration. 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