Journal of Hydrology 446–447 (2012) 59–69
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Journal of Hydrology
journal homepage: www.elsevier.com/locate/jhydrol
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
a r t i c l e
i n f o
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
s u m m a r y
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.: +31 317 486 461; fax: +31 317 491 000.
E-mail addresses: [email protected] (E.P. Querner), [email protected]
(P.C. Jansen), [email protected] (J.J.H. van den Akker), ceesc.kwakernaak@
wur.nl (C. Kwakernaak).
0022-1694/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jhydrol.2012.04.029
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,
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E.P. Querner et al. / Journal of Hydrology 446–447 (2012) 59–69
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.
2.1. 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.
2.2. 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).
2.3. 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
E.P. Querner et al. / Journal of Hydrology 446–447 (2012) 59–69
61
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.
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.
3. Description of the Zegveld study area and model application
4. Input data and schematisation
The Zegveld polder is in the ‘‘green heart’’ of the Netherlands,
surrounded by the major cities (see Fig. 2). The study area of
45 km2 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,
The SIMGRO model application was set up for the Zegveld study
area and its surroundings: an area of approx. 122 km2 (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 km2) 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 m2 d1. 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 500–3000 days. The main
aquifer below extends over the whole modelling area and has a
transmissivity in the range 500–900 m2 d1. 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.
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E.P. Querner et al. / Journal of Hydrology 446–447 (2012) 59–69
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 d1. For water supply in summer the maximum capacity
for peat meadows is 2.5–3.0 mm d1. The groundwater flow to
nearby deep polders (reclaimed lakes) varies in space and time,
but in summer is around 0.2 mm d1.
The SIMGRO model was calibrated using groundwater levels
(period 1995–2003) 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 m2 d1. 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 1995–2003. 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.
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E.P. Querner et al. / Journal of Hydrology 446–447 (2012) 59–69
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
Flow resistance
Map of sub-catchments
G
L
G
W
W
33 m1/3 s1
Groundwater
Land use
Physical soil properties
Thickness of root zone per land use
Ground level
Transmissivity aquifers (layer 1 and 3)
G
L
L
G
L
N
N
U
N
N
Thickness of aquifers
Vertical resistance aquitard
L
L
N
N
Thickness of aquitard
Drainage resistance of major streams
Depth of ditches
Drainage resistance of ditches
Drainage resistance of subsurface drains
L
F
F
F
F
N
N
N
N
N
Digital data base
Wösten et al. (1985)
0.40–0.70 m
0–2.50 m-MSL
ly 1: 2–3 m2 d1
ly 3: 1500–2700 m2 d1a
ly 1: 1.5 m; ly 3: 30–40 m
ly 2: 500–1000 d
ly 4: 1000–3000 d
ly2: 6 m; ly 4: 4–18 m
100–600 d
1.0–1.2 m
30–110 d
27 d
F
N,W
Daily data
Measured data
Meteorological data, groundwater levels and discharges
a
2
1
Calibrated values, the original values from literature were 500–900 m d
.
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. 5.
5.1. 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
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E.P. Querner et al. / Journal of Hydrology 446–447 (2012) 59–69
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.
Precipitationa
Evapotr.b
Seepageb
Dischargea
Water supplya
Water supplyb
Difference in
balance
a
b
Summer
2000
Winter
2000/01
Summer
2001
Winter
2001/02
377
438
45
44
191
127
41
564
88
50
392
5
2
39
586
483
46
186
202
146
73
427
115
50
300
7
2
31
Measured.
Calculated by MOGROW.
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 2000.
5.2. Subsidence of the peatland
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 ¼ 23:54 ALGL 6:68
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).
E.P. Querner et al. / Journal of Hydrology 446–447 (2012) 59–69
– 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 (3.1–3.4) 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.
7.1. 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.
7.1.1. 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 1976. 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
65
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.
7.2. 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.
7.2.1. 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.
7.3. 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 (1991–2006).
Fig. 7. Calculated subsidence of the soil surface for the period 1950–1995 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.
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E.P. Querner et al. / Journal of Hydrology 446–447 (2012) 59–69
ered 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).
7.3.1. 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).
7.3.2. 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.
7.3.3. Flexible water level control
For the flexible level management, the fluctuation around the
target surface water level is about 10 cm. This scenario was consid-
7.3.4. 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.
7.3.5. 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 (6.2–6.6 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
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E.P. Querner et al. / Journal of Hydrology 446–447 (2012) 59–69
Table 3
Summary of the scenario calculations considering two water level control options in combination with subsurface drains.
Scenario
2.1c
2.2
3.1
3.2
3.3
3.4
3.5
Water level man.
Regular
Regular
Dynamic
Dynamic
Flexible
Flexible
Optimald
Drains
No
Yes
No
Yes
No
Yes
Yes
Soil subsidence (mm/yr)
Capacity demanda
Supply summer (mm/yr)
Subs.
Change
10.7
6.2
10.0
6.3
11.7
7.5
6.4
4.5
0.7
4.4
1.0
3.2
4.3
b
Intake
Change
116
155
156
166
85
113
122
40
41
51
30
3
6
b
Intake
Pumping
+
±
±
±
±
+
+
±
+
±
+
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%.
8.1. 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 CO2
and NO2. 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.
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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.
9.1. 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 0.15–0.2 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.
9.2. 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.
9.3. 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.
9.4. 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. As a result, groundwater levels are
much lower in this scenario, so more water will be needed and
hence the subsidence is increased by 68%. In future, the water supply may be not available, because other parts of the Netherlands
will also need more water.
Acknowledgement
The projects referred to in this paper were carried out with support from the Dutch Ministry of Agriculture, Nature and Food Quality and the Provincial authority of Zuid-Holland. Joy Burrough
advised on the English.
References
Bradley, C., 2002. Simulation of the annual water table dynamics of a floodplain
wetland, Narborough bog, UK. J. Hydrol. 261, 150–172.
Bragg, O., Lindsay, R., 2003. Strategy and Action Plan for Mire and Peatland
Conservation in Central Europe. Publ. No. 18, Wetlands International,
Wageningen, 93 pp. <http://www.wetlands.org/pubsand/CEPP.htm>.
Charman, D., 2002. Peatlands and Environmental Change. John Wiley & Sons Ltd.,
Chichester, England, 288 pp.
Ernst, L.F., 1978. Drainage of undulating sandy soils with high groundwater tables. J.
Hydrol. 39, 1–50.
Feddes, R.A., 1987. Crop factors in relation to Makkink reference-crop
evapotranspiration. In: Hooghart, J.C. (Ed.), Evaporation and Weather.
Proceedings and Information No. 39, TNO Committee on Hydrological
Research, The Hague, 33–45.
Freeze, R.A., Harlan, R.L., 1969. Blueprint for a physically-based, digitally-simulated
hydrological response model. J. Hydrol. 9, 237–258.
Gulbinas, Z., Zingstra, H., Kitnaes, K., Querner, E.P., Povilaitis, A., Rašomavičius, V.,
Pileckas, M., 2007. Integrated water and biodiversity management in the Dovinė
River Basin. J. Ecol. Lithuania 53 (2), 64–69.
Hendriks, R.F.A., 1991. Decomposition and mineralisation of peat. Alterra,
Wageningen. Report 199 (in Dutch).
Jansen, P.C., Querner, E.P., Kwakernaak, C., 2007. Effects of Water Level Strategies in
Dutch Peatlands: A Scenario Study for the Polder Zegveld. Alterra, Wageningen,
Alterra Report 1516, 86 pp (in Dutch).
Jansen, P.C., Querner, E.P., van den Akker, J.J.H., 2009. The Use of Subsurface Drains
in Dutch Peatlands and the Consequences for Water Intake, Pumping, and Soil
Subsidence. Alterra, Wageningen, The Netherlands, Alterra Report 1872, 53 pp
(in Dutch).
Querner, E.P., 1988. Description of a regional groundwater flow model SIMGRO and
some applications. Agric. Water Manage. 14, 209–218.
Querner, E.P., 1994a. The combined surface and groundwater flow model MOGROW
applied to the Hupselse Beek drainage basin. FRIEND: flow regimes from
international experimental and network data. In: 2nd Int. Conf., Braunschweig,
Germany. October 1993. IAHS Publ. 221, pp. 381–389.
Querner, E.P., 1994b. Aquatic weed control options evaluated with the integrated
surface and groundwater flow model MOGROW. In: Kirby, C., Whyte, W.R.
(Eds.), Int. Conf. on Integrated River Basin Development, Wallingford.
September 1994, pp. 109–120.
Querner, E.P., 1997. Description and application of the combined surface and
groundwater model MOGROW. J. Hydrol. 192, 158–188.
Querner, E.P., Morábito, J.A., Manzanera, M., Pazos, J.A., Ciancaglini, N.C., Menenti,
M., 1997. The use of hydrological models in the irrigated areas of Mendoza,
Argentina. Agric. Water Manage. 35, 11–28.
E.P. Querner et al. / Journal of Hydrology 446–447 (2012) 59–69
Querner, E.P., Jansen, P. C., Kwakernaak, C., 2008. Effects of water level strategies in
Dutch peatlands: a scenario study for the polder Zegveld. In: Farrell, C., Feehan,
J. (Eds.), Proc. 13th International Peat Congress: After Wise Use – The Future of
Peatlands. Tullamore, Ireland, 8–14 June, 2008, pp. 620–623.
Querner, E.P., Mioduszewski, W., Povilaitis, A., Slesicka, A., 2010. Modelling peatland
hydrology: three cases from Northern Europe. Polish J. Environ. Studies 19 (1),
149–159.
Tate, R.L., 1987. Soil Organic Matter. Biological and Ecological Effects. John Wiley,
New York, 291 pp.
Thompson, J.R., Sørenson, H.R., Gavin, H., Refsgaard, A., 2004. Application of the
coupled MIKE SHE/MIKE 11 modelling system to a lowland wet grassland in
southeast England. J. Hydrol. 293, 151–179.
Van Bakel, P.J.T., 1988. Operational aspects of surface water management in relation
to the hydrology of agricultural areas and nature reserves. Agric. Water Manage.
14 (1–4), 377–387.
Van de Ven, G.P., 1993. Leefbaar Laagland (Living in the Dutch low lands). Stichting
Matrijs, Utrecht, The Netherlands.
Van den Akker, J.J.H., Kuikman, P.J., de Vries, F., Hoving, I., Pleijter, M., Hendriks,
R.F.A., Wolleswinkel, R.J., Simões, R.T.L., Kwakernaak, C., 2008. Emission of CO2
from agricultural peat soils in the Netherlands and ways to limit this emission.
In: IPS 13th Int. Peat Congress, Tullamore, Ireland, pp. 645–648.
Van den Hurk, B., Klein Tank, A., Lenderink, G., Ulden, A. Van, Oldenborgh, G.J. Van,
Katsman, C., Brink, H. van den, Keller, F., Bessembinder, J., Burgers, G., Komen, G,
Hazeleger, W., Drijfhout, S., 2006. KNMI Climate Change Scenarios 2006 for the
Netherlands. KNMI, The Netherlands, KNMI Scientific Report WR 2006-01.
69
Wassen, M.J., Okruszko, T., Kardel, I., Chormanski, W., Swiatek, D., Mioduszewski,
W., Bleuten, W., Querner, E.P., El Kahloun, M., Batelaan, O., Meire, P., 2006. EcoHydrological Functioning of the Biebrza Wetlands: Lessons for the Conservation
and Restoration of Deteriorated Wetlands. In: Bobbink, R., Beltman, B.,
Verhoeven, J.T.A., Whigham, D.F. (Eds.), Wetlands: Functioning, Biodiversity
Conservation, and Restoration. Springer Series: Ecological Studies, vol. 191, pp.
285–310.
Wendt, T.A., Haddink, E., 2003. Flooding in the water board region De Stichtse
Rijnlanden. H2O J. Water Manage. 36 (9), 12–16 (in Dutch).
Wösten, J.H.M., Bouma, J., Stoffelsen, G.H., 1985. Use of soil survey data for regional
soil water simulation models. J. Soil Sci. 49, 1238–1244.
Further reading
Armentano, T.V., Menges, E.S., 1986. Patterns of change in the carbon balance of
organic soil wetlands of the temperate zone. J. Ecol. 74, 755–774.
Eggelsman, R., 1976. Peat consumption under influence of climate, soil condition
and utilization. International peat society. In: Proc. of the 5th Int. Peat Congress.
Poznan, Poland, pp. 233–247.
Kasimir-Klemedtsson, A., Klemedtsson, L., Berglund, K., Martikainen, P., Silvola, J.,
Oenema, O., 1997. Greenhouse gas emissions from farmed organic soils: a
review. Soil Use Manage. 13, 245–250.