762

DOI: 10.1002/jpln.200621992

J. Plant Nutr. Soil Sci. 2006, 169, 762–774

Measurement and modeling of soil-water dynamics and evapotranspirationof drained peatland soils° nek2, Martinus Th. van Genuchten3, and Gerd Wessolek4
Kai Schwärzel1*, Jirka Šimu1 Institute of Soil Science and Site Ecology, Dresden University of Technology, Pienner Str. 19, D-01735 Tharandt, Germany
2 Department of Environmental Sciences, University of California Riverside, Riverside, CA 92521, USA
3 George E. Brown Jr. Salinity Laboratory, USDA-ARS, 450 W. Big Springs Road, Riverside, CA 92507, USA
4 Institute of Ecology, Department of Soil Sciences and Soil Protection, Berlin University of Technology, Franklinstr. 29,

D-10587 Berlin, Germany
Dedicated in memory of Prof. Dr. Rienk R. van der Ploeg
Accepted July 23, 2006

PNSS P199/2B

Summary
Natural peat soils serve as important sinks for nutrients,organic components, and water. Peat soils can pose major
environmental problems when they are drained for agricultural production, which may change their role in the landscape from a sink to a source. To successfully restore andconserve peat soils, it is important to understand the soilmoisture dynamics and water demand of drained peat soils
for different climate and groundwater conditions. For this purpose, we conducted a series of lysimeter experiments withpeat soils subject to different groundwater levels. Evapotranspiration (ET) rates and upward capillary fluxes in peat soils
under grass were measured, while TDR probes and tensiometers were used to monitor the soil-water dynamics in thelysimeter during the growing season. The lysimeter data
were simulated using an extended version of HYDRUS-1D toenable ET calculations using the Penman-Monteith equation.
A physically based approach was tested to predict the

1 Introduction
About 10% of NE Germany is covered by fens. Many fens inGermany in the past suffered from inadequate reclamation,overexploitation, and overall degradation. As a result of intensive agricultural use, the ecological function of fens has
slowly changed over the years from being a sink of various
nutrients and organic components to becoming a source.
This transformation of the function of peat soils has becomea major environmental concern. Many studies suggest thatgroundwater-level regulation can significantly influence therelease of C (Walter et al., 2001; Renger et al., 2002; Lafleur
et al., 2005). Restoration and well-balanced management ofpeat soils requires a thorough understanding of the hydrological processes involved (Joris and Feyen, 2003), particularly
the evaporative fluxes from the wetlands, in order to achieve
the best use of available water resources for ecological andenvironmental purposes (Gavin and Agnew, 2004).
Evapotranspiration is a major component of the water budgetof both natural and managed bogs and fens (Drexler et al.,2005). Although several studies have focused on the influence of groundwater levels and plant communities on evapo* Correspondence: Dr. K. Schwärzel;
e-mail: Kai.Schwaerzel@Forst.TU-Dresden.decanopy resistance as a function of the average pressure
head of the soil root zone. The numerical simulations closely
followed the observed soil-moisture dynamics in the lysimeter
and were consistent with measured differences in ET rates
for different groundwater levels. Besides average climate
conditions, the effects of extreme dry and wet weather conditions on ET and groundwater recharge during the growing
season were evaluated using the calibrated numerical modelfor different groundwater levels. Evapotranspiration rates during dry years depended very much on upward capillary flowfrom the water table and hence on the soil hydraulic properties. During wet years, however, ET was controlled mostly by
the evaporative demand of the atmosphere, and much lessby the soil hydraulic properties.

Key words: modeling / soil-water dynamics / evapotranspiration /peatland soils

transpiration (ET) from wetlands (e.g., Spieksma et al., 1997;
Lott and Hunt, 2001; Acreman et al., 2003; Lafleur et al.,2005), there is still a general lack of knowledge about thehydrology of fens. Especially little is known about dynamic
interactions between the depth of the water table, soil-moisture conditions in the root zone, ET, and climate conditions(Kellner, 2001). Furthermore, the importance of transient soilmoisture conditions in the unsaturated zone of peat soils andtheir effect on the water budget is often underestimated and/or neglected in hydrological studies (Bradley and Gilvar,
2000; Baird and Gaffney, 2000). The lack of comprehensive
physically based hydrologic models may well have hampered
the development and implementation of more effective conservation and restoration management practices of wetlands(Kellner and Halldin, 2002).
The most common methods for measuring ET in wetlands
are the Bowen ratio energy balance and eddy covariancemethods (cf. review by Drexler et al., 2005). Evapotranspiration may also be measured from the diurnal fluctuations of
the shallow water table in wetlands (e.g., Lott and Hunt,
2001) or by the use of the lysimeter technique but their installation is often very expensive. In contrast to conventional lysimeters, we developed a lysimeter that can be installed moreeasily and with minimal technical and financial effort (Schwär- 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1436-8730/06/0612-762

J. Plant Nutr. Soil Sci. 2006, 169, 762–774

Soil-water dynamics and evapotranspiration of drained peatland soils 763

zel and Bohl, 2003). This type of field lysimeter was used tomeasure the soil-water dynamics of peat soils at different
groundwater levels.
The aims of this study were (1) to apply a physically based
approach to predict the canopy resistance as a function of
the pressure head of the soil root zone and (2) to incorporate
and use the approach in the HYDRUS-1D numerical software(Šimunek et al., 1998a) to predict root water uptake and ET
for various groundwater and climate conditions. Experimental
data from field lysimeters were used to study interactions between ET, groundwater recharge, soil hydraulic properties,
and the groundwater level. The calibrated numerical model
was subsequently used to examine the influence of groundwater level and climate on the water consumption of peatsoils. The numerical results should provide more reliable
estimates of the water demand of peat soils for more effective
and environmentally friendly land use.

2 Material and methods
2.1 Study area
The lysimeter experiments were conducted in Rhinluch, a fen
area of about 87,000 ha ca. 60 km NW of Berlin, Germany, inthe Havelland basin (52°85’ N, 12°8’ E). Peat soils in the area
were first drained more than 250 y ago. Drainage was
expanded considerably in the 1970s by means of a dense
network of open ditches and subsurface perforated pipes.
From the 1970s until the collapse of the communist regime in
E Germany, agricultural use of fen areas became much more
intensive. To enable better access to agricultural machinery,
the water tables were lowered during the growing season to adepth of 100 cm below the soil surface. Land use in the early
1990s changed again when hay crops became very popularin the area.
Peat in the Rhinluch area formed under the influence of paludification (swamp formation). The average thickness of thepeat is ca. 120 cm. Glacifluvial sands (mostly fine sand) andlimnic sediments such as detritus or calcareous mud underlay the peat. The upper peat layers are strongly decomposed
and were altered pedogenically by the intensive land use andTable 1: Surrounding water table and target groundwater depths inthree lysimeters during three growing seasons.
GrowingseasonSurrounding

Lysimeter 1 Lysimeter 2 Lysimeter 3— Target groundwater depth [cm] —

1995

40–60

60–80

–

–

1996

30–40

-

30–40

<30
<30

<30

1997

30–50

60–80

drainage. Deeper layers are dominated by sedge (Carex sp.),reed peats (Phragmites sp.), or often a mixture of both typesof peat. The field site was vegetated predominantly with reedcanary grass (Phalaris arundinacea).
The climate of the region is characterized by an averageannual temperature of 8.1°C, an average annual precipitation
rate of 526 mm, and an average annual crop reference evaporation of ca. 600 mm. The Rhinluch area has one of thelowest precipitation rates in Germany. The climatic water balance (precipitation minus crop reference ET) is highly negative during the summer months from April to October. We
calculated an average value of –170 mm between 1993 and1998.

2.2 Field and laboratory methods
Three groundwater-table lysimeters (ca. 1 m × 1 m × 1 m)
were equipped with fully automated tensiometers(176PC28Honeywell) and manually operated TDR probes(Easy Test, 1993), both installed horizontally in two replicatesat depths of 10, 20, 30, 40, 50, 60, 70, and 80 cm (Fig. 1).
TDR readings were translated into water-content values
using the third-order polynomial
h = 3.93 · 10–2 + 2.84 · 10–2 × e– 4.19 · 10–4 · e2 + 2.77 · 10–6 · e3,(1)which was fitted to observed data of the dielectric constant e[-] and the volumetric water content h [cm3 cm–3] for peat

Figure 1: Experimental setup of the groundwater
lysimeters.

 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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764

Schwärzel, Šimu°nek, van Genuchten, Wessolek

J. Plant Nutr. Soil Sci. 2006, 169, 762–774

soils at our site. Equation (1) had a standard error of calibration of 0.022 [cm3 cm–3].
Groundwater levels in the lysimeters and the surrounding soil
were monitored using slotted PVC pipes connected via flexible tubes to a differential pressure transducer having a precision of 1 cm. Following procedures outlined by Feddes(1971), the water levels in the lysimeters were either automatically adjusted to the surrounding groundwater level (a reference level) or fixed at a certain depth. Surrounding watertable and target groundwater depths in three lysimeters during three monitored seasons are summarized in Tab. 2.
Because of the shallow surrounding water table we assumed
that so-called oasis effects were minimal during our lysimeter
experiments. In some periods, we observed higher deviations
from the target water table. Such measurement periods were
not used for quantification of ET.
Pressure differences recorded by a control unit either triggered operation of a bilge pump in the lysimeter pipe orreleased a valve in a supply-water container to increase thewater level in the lysimeter. The volume of water added or
removed was measured by recording water levels in the containers. Hysteresis in the imposed groundwater level was at
most 2 cm.
The average actual ET rate (ETact) between times t1 and t2
was calculated as follows
t2

t2

ETact dt 
t1

L t2

P  Q dt
t1

0 t1

∂h
dtdz
∂t

(2)

where P is the precipitation rate [mm d–1], Q represents thenet inflow of water [mm d–1] into the lysimeter needed tomaintain the water table inside the lysimeter equal to that of
the surrounding groundwater, L is the depth of the lysimeter[cm], and z is the vertical coordinate [cm]. More information
about the groundwater lysimeter and its operation is given in
Schwärzel and Bohl (2003).
At the site, continuous measurements of air temperature,humidity, wind speed, and global radiation were made at theheight of 2 m above the ground. The precipitation rates were
measured using a Hellmann rain gauge.

Evaporation experiments on undisturbed soil samples wereconducted in the laboratory to determine the unsaturated
hydraulic conductivity (Schwärzel et al., 2006). The unsaturated soil hydraulic properties were described with the van
Genuchten-Mualem (VGM) model (van Genuchten, 1980;
Mualem, 1976):
Se 

h
hS

hR
1

m h≤0,hR
1  ah n (3)Se  1 h > 0(4)

hK Se   KS Se0:5 1
m1


1

1=m

Se

m i2

(5)

1=n n > 1

(6)where Se is effective saturation [-], h is the volumetric watercontent [cm3 cm–3], hR is the residual water content[cm3 cm–3], hS is the saturated water content [cm3 cm–3], h is
the pressure head [cm], K is the hydraulic conductivity[cm d–1], KS is the saturated hydraulic conductivity [cm d–1],
and a [cm–1], n [-], and m [-] are empirical parameters. Parameter optimization (inverse) methods (Šimunek et al., 1998b)
were used to fit hR, hS, a, n, and Ks to the laboratory evaporation data. The final set of soil-hydraulic parameters used inthis study is given in Tab. 2.

2.3 Numerical model
2.3.1 Governing equations
Variably saturated flow in the lysimeters was simulated using
HYDRUS-1D (Šimunek et al., 1998a) which numericallysolves the Richards equation:


∂h
∂
∂h

K
K
S,
(7)
∂t ∂z
∂z
where t is time [d], z is the vertical coordinate (positive
upward) [cm], S represents the volume of water taken up by
roots per unit bulk volume of the soil in unit time [cm3 cm–3d–1], and where the other variables are as defined before.
The solution of Eq. 7 requires a set of initial and boundary
conditions. The initial condition in our study was given interms of the measured pressure head, hi:
h z; t   hi z 

t 0

(8)

Table 2: Soil-physical and soil-hydraulic parameters for various horizons of the studied peat soil (Schwärzel et al., 2006). hs is the saturated
water content, KS is the saturated hydraulic conductivity, and [cm–1] and n are empirical parameters in the van Genuchten (1980) function.
Depth
[cm]

Peat typeBD[g cm–3]

X[%]hs[cm3 cm–3]a[cm–1]nKs[cm d–1]

0–20

very humified peat

0.312

77

0.797

0.020

1.23

14.0

20–25

very humified peat, compacted

0.371

76

0.730

0.012

1.12

6.69

25–30

humified peat, compacted

0.386

77

0.741

0.005

1.15

0.92

>30

weakly humified, reed-sedge peat

0.194

84

0.891

0.003

1.16

104

For all cases, hR = 0.0 and k = 0.5
BD = bulk density
X = ignition loss

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J. Plant Nutr. Soil Sci. 2006, 169, 762–774

Soil-water dynamics and evapotranspiration of drained peatland soils 765

The following atmospheric boundary condition was specified
at the soil surface:
K

∂h
∂z

K ≤E

at z  0

The potential root water-uptake function Sp in Eq. 11 corresponds to the potential transpiration rate Tp [cm d–1] of a
growing canopy as follows(9)
Sp dx Tp ,

and

(15)

Lr

hA ≤ h ≤ hS

at z  0,(10)

in which Tp depends on weather conditions, plant height, andcanopy resistance against water loss. The actual root water
uptake rate S as a function of depth is obtained by substituting Eq. 13 into Eq. 11, while the actual transpiration rate Ta[cm d–1] is calculated by integrating S over the root zone:

where E is the maximum potential rate of infiltration or evaporation under the current atmospheric conditions [cm d–1],
and hA and hS are, respectively, the minimum and maximum
permitted pressure heads at the soil surface [cm]. FollowingŠimunek et al. (1998a), the values of hA and hS wereassumed to be –106 and 0 cm, respectively. A time-variable
pressure-head boundary condition consistent with our measurements was specified at the lower boundary.

which shows that Ta equals Tp only when a = 1 over the entire
root zone (i.e., no water stress in any part of the root zone).

2.3.2 Root water uptake

2.3.3 Daily evapotranspiration

The sink term, S, in Eq. 3 represents the volume of water
removed from a unit volume of soil per unit time due to theroot water uptake. Feddes et al. (1978) defined S as
S h  a h Sp ,(11)

where a (h) is a prescribed dimensionless function (0 ≤ a ≤ 1)
accounting for the effects of water stress on root water
uptake, and Sp is the potential root water-uptake distribution[d–1]. When the potential root water-uptake rate is uniformly
distributed over the root zone, Sp is given by
Sp 

1
T ,
Lr p

(12)

where Lr is the depth [cm] of the root zone and Tp is the potential
transpiration rate [cm d–1]. Equation 12 may be generalized byintroducing a nonuniform distribution of the potential root wateruptake rate over a root zone of arbitrary shape as follows:Sp  b z Tp ,(13)

where b(z) is the normalized root water-uptake distribution[cm–1]. To ensure that b(z) integrates to unity over the root
zone, any measured or prescribed root distribution function,b′(z) over the root zone may be normalized into b(z) using
the equationb z  b ′ z
.
b ′ z dz

(14)

Lr

A number of soil pits were dug to estimate the mean root
depth depending on groundwater conditions. In the mostcases, a well-defined transition between the very densely
rooted topsoil and the weakly rooted subsoil was found.
Therefore, we assumed a uniform root distribution in ourmodeling studies. Based on our field observations, we chosefor groundwater levels ≤30 cm a root depth of 10 cm, forgroundwater levels between 40 and 60 cm a root depth of
30 cm, for groundwater levels between 60 and 80 cm a root
depth of 35 cm, and for groundwater levels >90 cm a rootzone depth of 40 cm.
 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Ta  Tpa h b z dz,

(16)

Lr

Evapotranspiration ETI [cm d–1] is equal to the sum of transpiration by plants T [cm d–1], evaporation from the soil surface E [cm d–1], and evaporation of water intercepted by the
canopy surface I [cm d–1], further referred to as interception.
The potential transpiration (maximum possible water uptake)rate can be calculated as follows
Tp  ETI

I

E.

(17)

In this study, we assume that evaporation from the soil surface is negligible because of the presence of an essentiallycomplete plant cover.
The daily ET rate [cm d–1] from a dry canopy was estimatedusing the Penman-Monteith equation (Monteith, 1965) as follows:hiqcp
1 s Rn G   ra ea ed 
h

i
,
(18)
ETPM 
rksc 1 crawhere k is the latent heat of vaporization [J g–1], Rn is the net
radiation flux at the canopy surface [J m–2 d–1], G is the soil
heat flux [J m–2 d–1], q is the atmospheric density [g cm–3], cp
is the specific heat of humid air [J g–1 °C–1], (ea – ed) is thevapor pressure deficit [kPa], rc is the crop canopy resistance[s m–1], ra is the aerodynamic resistance [s m–1], s is theslope of the vapor-pressure curve [kPa °C–1], and c is thepsychrometric constant [kPa °C–1]. The FAO (Allen et al.,2000) suggested a procedure for evaluating the PenmanMonteith equation using daily values of air temperature[°C–1], air humidity [kPa], wind speed [m s–1], and solar radiation [J m–2 d–1]. This procedure was modified as described
below and implemented into HYDRUS-1D.
The daily evapotranspiration of a wet canopy (with rc = 0)
was calculated as follows (Monteith, 1965; Rijtema, 1965):
h
i
qcp
s
R
G

e
e

n
a
d
1
ra
.
(19)
ETwet kscwww.plant-soil.com

766

Schwärzel, Šimu°nek, van Genuchten, Wessolek

The transpiration rate from a wet canopy is generally larger
than the corresponding value from a dry canopy. This reduction in the potential transpiration rate from a dry as comparedwith a wet canopy is given by the ratio of Eqs. 18 and 19 (seealso Feddes, 1971):
ETPM
sc

.

ETWet s  c 1  rc

(20)

J. Plant Nutr. Soil Sci. 2006, 169, 762–774

3.2 Soil water regime during the summer
Figure 2 shows daily values of precipitation, depth of thegroundwater table, and measured values of the soil watercontent and pressure head at various depths in one of thelysimeters covered by plants during the growing season. In
addition, calculated daily values of the crop reference ET(Allen et al., 2000) are also presented.

raDuring periods with precipitation, part of the incoming energy
is used for the evaporation of intercepted water, which leaves
less energy for potential transpiration relative to dry periods
under the same conditions. The potential transpiration rate
may then be calculated as (cf., Rijtema, 1965; Feddes,
1971):Tp s  c
 ETwetrsc 1 c

I .

(21)

In early July, the groundwater level was located between 70
and 75 cm depth. An extremely intense rain storm (25 mm in4 h) in the middle of July caused a sudden rise of the groundwater table to a depth of ca. 20 cm (Fig. 2b). The water table
during the next several days dropped to the target groundwater level of 70 cm depth. Except for a period with technicalproblems in the beginning and the middle of August, this target groundwater level was maintained until the end of September.

ra

The interception (I) of precipitation (P) was calculated according to Hoyningen-Hüne (1983), Braden (1985), and van Dam
et al. (1997)
0 as follows:1
I  aLAI @1

1
1

bP

A,

(22)

aLAI

where a is an empirical coefficient [cm], LAI is the leaf areaindex [-], and b is the soil cover fraction [-]. We assumed avalue of 0.5 for a and 1.0 for b. The leaf-area index (LAI) for
grass was calculated from the vegetation height hVeg [cm]using a modification of an equation proposed by Menzel(1997):LAI  0:7862  0:4123hVeg

2
5:49 × 10 3 hVeg.

(23)

The mean vegetation height of the site was measured weekly
from 1993 until 1998 during the growing season.
Knowledge of the canopy resistance rc is needed in order tocalculate the actual ET rate of a growing crop using the Penman-Monteith equation. When ET is evaluated using Eq. (2),the canopy resistance rc can be derived from weather data
and the known plant height. This approach assumes that forgiven conditions evaporation from a fully covered soil can beneglected. The canopy resistance rc for precipitation-free periods can then be obtained assuming that the measured ET
rate is equal to the actual transpiration rate as follows (cf.,also Henning, 1992):



s
ETwet
1 .
(24)
rc  ra 1 
c ETmeasured 

3 Results and discussion
3.1 Soil-water dynamics

Figures 2c and 2d show that drying of the soil during themonths of June and July was mostly limited to the upper
10 cm in spite of the high evaporative demand of the atmosphere (Fig. 2a). The relatively high level of the water table
and the comparatively frequent precipitation events prevented more substantial changes in the water content of the
subsoil. Until early July, observed pressure heads were very
similar at all depths (Fig. 2d). Afterwards, due to the high evaporative demand, pressure heads decreased faster at a
depth of 10 cm than at the other measurement depths. Withina few days, the measurement limit (ca. –700 cm) of the tensiometer located at the shallowest depth was reached. The
rapid decrease in the pressure head at a depth of 10 cm wascaused by drastic reductions in the unsaturated hydraulicconductivity as the soil dried out, with a simultaneous
decrease in upward flow from the water table through the capillary fringe. At the end of the dry period in the middle ofsummer, the soil water content at a depth of 10 cm reached
the wilting point (pF 4.2). Figure 2c shows that the water content was reduced by nearly 50% within 2 months.
The soil profile rewetted slowly following several precipitation
events at the end of the dry period, with a delay of several
days. The rain water was found to infiltrate very fast without
fully wetting the peat, thus causing a rapid rise of the watertable. The pressure head during this time period increased
quickly at all depths. However, the water contents increased
only very slowly because of peat water repellency (Figs. 2c,d). Our findings agree well with those of Kellner and Halldin(2002) who for a Swedish Sphagnum mire found that rainwater moved rapidly to layers just above the water table without any significant storage in the upper horizons. The slow
increase in water content is related to enhanced hysteresis in
the soil water-retention curve (cf., also Kellner, 2001). As
further described in detail by Schwärzel et al. (2002), differences in water content of up to 30% were observed between
the draining and wetting branches of the retention curve at
the same pressure head.

In this section, we discuss our lysimeter data describing thesoil water regime of a drained fen during the summer of 1995.
Interactions between weather conditions, the groundwaterhydrograph, and soil-moisture dynamics are presented andanalyzed.
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J. Plant Nutr. Soil Sci. 2006, 169, 762–774

Soil-water dynamics and evapotranspiration of drained peatland soils 767ETGras [mm]840Jun/22

A

Jul/12

Aug/01

Aug/21

Sep/10GWL below surface[cm]

Precipitation

B

100

Groundwater level (GWL)

50

0Jun/22

Jul/12

Aug/01

Aug/21

Sep/10

Aug/01

Aug/21

Sep/10

Water content [m3 m-3]

0.9

0.6
10 cm
20 cm
30 cm
40 cm
0.3
Jun/22

Jul/12Pressure heads [cm]

C
0
-300
-600

-900
Jun/22
D

10 cm
20 cm
30 cm
Jul/12

Aug/01

Aug/21

Sep/103.3 High-temporal resolution soil-plantatmosphere studies
Our understanding of the transient nature of soil-water
dynamics in peat soils can be improved with high-resolution
values of pressure head, precipitation, and evapotranspiration vs. time. Figure 3 shows such data, where precipitation
was measured hourly, while ET was calculated hourly according to Penman (1948) as modified by Wendling (1991). Daily
fluctuations of ca. 100 cm in the pressure head were recorded
at a depth of 20 cm in mid August. These daily fluctuations
were a result of root water uptake during the day and more
continuous, stable capillary rise of water from the water table.
The decrease in the pressure head during daytime followed
the ET peaks with a time delay of a few hours (Fig. 3). At
 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 2: Plots showing (a) calculated daily values of thepotential evapotranspiration rate (ETgrass), measured daily
values of (b) the precipitation rate and the groundwatertable, (c) the soil water content, and (d) observed pressure
heads in one of the lysimeters (from June 22 throughSeptember 21, 1995).

noon, when the evaporative demand of the atmosphere washighest, the pressure head started to decrease at a depth of
20 cm. At this time, the upward flux from deeper soil zones
was clearly smaller than the water uptake rate by plant roots.
Water continued to be taken up by the roots until the evenings (although at a diminishing rate), at which time the lowest pressure head values were reached (Fig. 3b).
The increase in pressure head overnight is an indication thatwater absorbed by plants is being replenished by capillaryrise and other redistribution processes. Yet, it is also striking
that daily amplitudes of the pressure head decreased notwithstanding the relatively high evaporative demand of theatmosphere (Fig. 3). This may have been caused by theshape of the water-retention function, with the soil waterwww.plant-soil.com

Schwärzel, Šimu°nek, van Genuchten, Wessolek

768

ETPenman [mm h-1]

A

0.75

J. Plant Nutr. Soil Sci. 2006, 169, 762–774

Aug/17 -Sep/2

0.5

0.25

0
Aug/17B

Aug/21

Aug/23

Aug/25

Aug/27

Aug/29

Aug/31
6.0

Precipitation
Pressure head at 20 cm

Pressure head at 40 cm
Pressure head at 60 cm
Precipitation [mm h-1]

0
Pressure head [cm]Aug/19

-100

3.0

-200

-300
Aug/170.0
Aug/19Aug/21

Aug/23

Aug/25

Aug/27

Aug/29capacity (dh/dh) increasing with decreasing pressure heads.
At the same time, the maximum pressure heads gradually
decreased (Fig. 3b). During this time period, the daily wateruptake rate by plant roots exceeded the daily capillary supplyof water from the groundwater table. This is due to a reduction in the hydraulic conductivity caused by the lower watercontents, thus limiting upward capillary flow from the watertable. Only when the evaporative demand of the atmospheredeclined markedly at the end of August, upward capillary flow
provided more water than was taken up the roots. As a result,the daily maximum values of the pressure head at that time
gradually increased again (Fig. 3b). The rapid increase in thepressure head at a depth of 20 cm corresponded with the firstprecipitation events after a long dry period (August 26 and
27). Within a few hours, the large amounts of precipitation on
August 31/September 1 caused a substantial increase in thepressure heads at all depths (Figs. 2d, 3b), occasionallycausing positive pressures. The groundwater level then also
increased substantially (Fig. 2b).
Our interpretation of the high temporal pressure-head measurements during the precipitation free period, and of theobserved minimum values in the evening, confirm results of 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Aug/31Figure 3: Plots showing (a) calculated
hourly values of the potential evapotranspiration rate and (b) measured
hourly values of the precipitation rate
and observed pressure heads at threedepths in one of the lysimeters.

Table 3: Values of the canopy resistances of grass as compiled fromthe literature.
Type of Grassrc [s m–1] Reference

Reed canary grass

22This study

Mesotrophic grassland

50–100

Acreman et al. (2003)

Grass

75–90

Zenker (2003)

Japanese moor grass

111

Takagi et al. (1998)

Reed canary grass-nettle reeds 15

Böhm and Quast (1998)

Purple moor grass

22

Spieksma et al. (1997)

Grass

40–85

Menzel (1996)

Grass, 12 cm

70

Allen et al. (1994)

Field weeds

74

Henning (1992)

Tall feccue

1–11

Feldhake and Boyer(1986)

Meadow knawel

11–29

Feldhake and Boyer(1986)

Creeping finger grass

20–48

Feldhake and Boyer(1986)

Grass, cut

26

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J. Plant Nutr. Soil Sci. 2006, 169, 762–774

Soil-water dynamics and evapotranspiration of drained peatland soils 769

Kellner (2001) who derived hourly values of the canopy resistance in Eq. 18 from energy-budget measurements. In thatstudy, the canopy resistance often had its diurnal maximum
in late afternoon or the evening when the vapor deficit had
passed its daily maximum. Kellner (2001) suggested that withincreasing evaporative forcing, the capillary-flow capacity
was too low to deliver water to the canopy.

Numerical simulations of the water balance were carried out
using HYDRUS-1D and the experimentally determined
canopy resistance of 22 s m–1. Figure 4 compares modeling
results with values collected in lysimeter 1 during the summerof 1996. Measured and calculated cumulative ET rates
agreed well, with the cumulative ET rate being underestimated by <5% over a period of 70 d. The dynamics of soil
water contents was also predicted satisfactorily by thenumerical model.

3.4 Canopy resistance and water budget for themeasurement periods

Evapotranspiration rates were next calculated for various
time periods with groundwater levels set to maintain a canopy
resistance, rc, of 22 s m–1. In Fig. 5, the measured and calculated ET rates are plotted against the averaged pressure
heads in the root zone. In Fig. 6, ETIoptimal is the ET rate calculated for conditions of unlimited supply of water using theestimated canopy resistance of 22 s m–1. The hand-fitted
curve through the data closely resembles the piece-wise linear water-stress response function a(h) initially proposed by
Feddes et al. (1978). Near saturation, the measured ET gradually declined compared to the calculated ET rate as indicated by the a(h) function. Despite the scatter, the dataclearly show that water uptake by the reed canary grass cropis reduced when pressure heads in the root zone increase toca. –15 cm or higher. Notice that the measured evapotranspiration rate was equal to the calculated ET only in a comparatively small range of pressure heads. The ratio of measured and calculated ET rates started to decline alreadybelow pressure heads of ca. –70 cm.

Evapotranspiration rates of lysimeter 2, measured in thesummer of 1996 when the groundwater level was <30 cm
below the soil surface, were used to calculate the minimal
canopy resistance because the water supply can beassumed to be unlimited under these conditions. From thesecalculations, using Eq. 24 we obtained an average value of
22 s m–1 for the minimal canopy resistance rc. The minimal
canopy resistance of a crop can be interpreted as the maximum conductivity of the canopy to deliver water at unlimitedsoil-water availability. A compilation of canopy resistances
published for various grasses is presented in Tab. 3, with ourvalue being in the middle of the range of reported values. The
value of 22 s m–1 is also close to the value of 15 s m–1 foundby Böhm and Quast (1998) for very similar reed canary
grass–nettle reeds vegetation.
Table 4: Simulated and measured water-balance components for different groundwater levels (July 29 through September 16, 1997).
Average daily values are given in parentheses (in mm d–1).
PeriodGroundwater
depth [cm]measured [mm] simulated [mm]ETI

Qcapillar ETI

Qcapillar

225
(4.6)

199
(4.1)

201
(4.1)

163
(3.3)

7/29– 9/16/97 30–50

226
(4.6)

159
(3.2)

197
(4.0)

138
(2.8)

60–80

174
(3.6)

116
(2.4)

166
(3.4)

106
(2.2)

<30

Qcapillar = water flow into the lysimeter to keep constant groundwaterlevel

Our finding that the ET rate depends strongly on the soilwater pressure head has, to our knowledge, not been foundin other wetland studies. Several previous studies (e.g., Mundel, 1982; Ingram, 1983) suggest that the depth of the watertable to a large extent controls peatland ET, while more
recently, Gavin and Agnew (2004) observed a considerable
effect of peat surface moisture on ET of a temperate wet
grassland. In contrast, Lafleur et al. (2005) showed for a
shrub-covered bog in Canada that the ET rate appeared
unaffected by the water table until it dropped below 65 cm.
For groundwater tables <65 cm, the moisture supply to thevascular plants became limiting and transpiration decreased.
Key to understanding these different results appears to be
the amount of water available for root water uptake. Whenevaluating the role of groundwater in the supply of water tovegetation in peat soils, one must consider the upward capil-

ETIactual [mm]

300

200

100

0
Jul/16 Aug/05 Aug/25 Sep/14

Water content [cm3 cm-3]

0.9
measuredsimulatedmeasuredsimulated

0.7

0.5
Jul/16 Aug/05 Aug/25 Sep/14

 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 4: Simulated and measured cumulative
evapotranspiration rates (left) and water contents inthe top horizon (right) during the 1996 water-balance
period. The average depth of the water table during
this time interval was 25 cm.

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Schwärzel, Šimu°nek, van Genuchten, Wessolek

ETI measured/ ETI optimal

770

J. Plant Nutr. Soil Sci. 2006, 169, 762–774
where the conversion factor d is equal to 10–1 m–1, rc,minimal is
22 s m–1, and hcrit is –70 cm. This equation is a modificationof a similar expression proposed by Sambale (1998). Our
field measurements (Fig. 5) indicate that Eq. 25 is valid forpressure heads >–120 cm. Figure 6 further shows that when
the pressure head in the root zone becomes >–15 cm, root
water uptake is reduced linearly because of poor aeration,
similarly as in the approach of Feddes et al. (1978).

1.0

0.5

0.0
0

-50

-100

-150

Pressure head in the root zone [cm]Figure 5: Plot of the ratio of measured and calculated evapotranspiration rates as a function of the average root-zone pressure head.
The curve was hand-fitted through the data.

lary flow of water from the water table into the root zone. This
upward flow depends on the unsaturated hydraulic conductivity of the peat soil and the hydraulic gradient, which are controlled by both the weather conditions and depth to groundwater. In general, the larger the vertical distance between
water table and the bottom of the root zone, the less capillaryrise should be expected.
In previous papers (Wessolek et al., 2002; Schwärzel andBohl, 2003), we showed that for an average groundwaterlevel of 40 cm, the highest capillary delivery rates occurred
during periods with high evaporative demand. In someinstances, the amount of capillary rise was independent of
the evaporative demand of the atmosphere. The capillary
delivery from groundwater was delayed in time when a periodof high evaporative demand was followed by a period of low
evaporation and vice versa. Capillary rise is negligible during
and after rainfall events. The plant water demand is thendirectly satisfied by precipitation water infiltrating the rootzone. These transient flow conditions explain why in somestudies, no significant relation was found between the depthof the water table and the daily ET rate (e.g., Lafleur et al.,2005), while in other studies (e.g., Lapen et al., 2000), the ET
rate depended on soil water, as we found here. Still, Kellner(2001) observed no correlation between soil moisture, as
measured by TDR, and the rate of ET. However, our results
show that the average pressure head of the root zone is amore sensitive indicator for the relation between evaporative
demand of the atmosphere and the water consumption offens and bogs; small changes in the water table and soil
moisture may lead to a relatively large and easily detectablechange in the pressure head.
To consider the observed reduction in ET (Fig. 5) in numericalmodels, we suggest the introduction of a critical pressure
head, hcrit, of the root zone. This parameter should indicate
when soil water conditions in the root zone are no longer optimal for the standing vegetation. Plants respond to suboptimal
conditions by increasing their minimal canopy resistance towater loss. When the actual pressure head in the root zone,
hact, reaches values <hcrit (i.e., more negative), then theactual canopy resistance rc,act of reed canary grass is calculated usingrc;act  rc;minimal 1  d jhact jjhcrit j,(25)

 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

To test the reduction approach, simulations were carried out
for a period of 7 weeks in 1997 for lysimeters in which thegroundwater levels were kept at average depths of 20, 45,
and 65 cm. Table 4 compares calculated and observed valuesof the ET rate and the water flux into the lysimeters to maintain constant groundwater levels. Differences in ET between
the three lysimeters were predicted well with the model.
Deviations between measured and calculated cumulative ET
rates were <10% during the 49 d period. The largest deviations between measured and calculated values occurred forthe scenario with the shallowest depth of the groundwaterlevel after the high-precipitation events (not shown). Furthermore, the model underestimated capillary rise for the same
scenario by ca. 15%, probably due to underestimation of thehydraulic conductivity near saturation as obtained with thelaboratory tests. Measured and calculated values for upward
flow rates (capillary rise) agreed well when the water tables
were deeper. Results listed in Tab. 4 show that for shallow
groundwater conditions ca. 90% of the evaporated water aresupplied by capillary rise from groundwater. When the
groundwater level is deeper, 65% of root water uptake stillcomes from groundwater.
Simulated and measured water contents and pressure heads
at depths of 10 and 20 cm for conditions where the averagewater table was kept at about 45 cm below the soil surface
are presented in Fig. 6. Pressure heads at a depth of 10 and
20 cm were underestimated with the model, relative to themeasurements at early times. Rewetting of the peat after precipitation late in the summer caused a rapid increase in pressure-head values, a tendency which was captured well withthe model. In contrast, the soil only gradually wetted as aresult of air entrapment and the water-repellency problems.

3.5 Water-budget simulations dependent on
groundwater and climate conditions
3.5.1 Water balance
The tested model was next used to investigate the influenceof various groundwater and weather conditions on ET. For
this purpose, we used meteorological data from years 1993/
94 and 1997/98 to represent relatively wet and dry years, respectively, and assumed that the water table remained constant at specific depths. Figure 7 shows the dependence of
the annual ET and groundwater recharge rates on the
groundwater level for the dry and wet years. The ET rate was
generally higher in a dry year than in a wet year due to higher
solar radiation and higher temperatures. While during the dry
year, 80 mm less water evaporated when groundwater was at
a depth of 70 cm compared to a depth of 30 cm, the difference was only 50 mm for the wet year.
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J. Plant Nutr. Soil Sci. 2006, 169, 762–774

Soil-water dynamics and evapotranspiration of drained peatland soils 771
0measuredsimulated
Pressure heads [cm]Water content [cm3 cm-3]

0.9

0.7

-50

-100

-150
simulatedmeasured

Measurement depth: 10 cm

0.5
Jul/28 Aug/11 Aug/25 Sep/08-200
Jul/28 Aug/11 Aug/25 Sep/08

0measuredsimulated
Pressure heads [cm]Water content [cm3 cm-3]

0.9

0.7

-50

-100

-150
simulatedmeasured

Measurement depth: 20 cm

0.5
Jul/28 Aug/11 Aug/25 Sep/08-200
Jul/28 Aug/11 Aug/25 Sep/08

The water balance of drained fens during a dry year can be
extremely negative (Fig. 7). Considerable capillary rise canthen occur, even with groundwater table at 120 cm. In contrast, we calculated groundwater recharge of up to 240 mm
y–1 in the wet years with much more precipitation and low
actual ET rates. From Fig. 8, we conclude that in dry years
the water supply of plants (and hence ET) depends much
more on capillary rise from groundwater than in wet years.
The legend of Fig. 7b shows also calculated values of thecrop reference evapotranspiration ET0 (Allen et al., 2000).
These annual values underestimate the simulated data at
water tables ≤60 cm below the soil surface. This is not sur-

750

Figure 6: Simulated and measured soil water contents
and pressure heads at two depths in one of thelysimeters with the average depth of the water table
maintained at 45 cm during the 1997 water-balance
period.

prising because the approach of Allen et al. (2000) is a climatic parameter and does not consider the plant characteristic and the soil factors.
3.5.2 Water demand of drained peat soils
Since peat mineralization accelerates when the water table isdeep and the soil surface dry, raising the groundwater table
and rewetting the soil profile are measures that protect thepeat (Renger et al., 2002). Simulations over many years may
be used to obtain estimates of the average water demandneeded to successfully implement such measures. Figure 8
presents the results of such simulations, showing the water

Wet year: 570 mm ETo-1ETactual [mm y ]-1

Groundwater recharge [mm y ]

500

550

Dry year: P = 380 mm
Wet year: P = 740 mm

350
20

50

80

110

Grundwater depth [cm]Dry year: 620 mm ETo

250

0

-250

-500
20

50
80
110
Groundwater depth [cm] 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 7: Simulated components of the soil
water balance of a peat soil for relatively dry andwet years. ET0 is the crop reference evapotranspiration (Allen et al., 2000).

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Schwärzel, Šimu°nek, van Genuchten, Wessolek

772

demand needed to establish a given groundwater level forvarious climate conditions. As an indicator of the evaporative
demand of the atmosphere (or “climate stress”) during thegrowing season, we used the climate water balance (CWB),
which is defined here as the difference between precipitationand crop reference ET (ET0). Assuming a mean groundwater
depth of 1 m, Fig. 8 shows that 210 mm water must be supplied for a CWB of –330 mm during the summer months toreach a groundwater level of 30 cm. This is two times higher
compared to a slightly negative CWB value of –50 mm. To
maintain the water table at only 70 cm, the water demanddecreases to 130 mm and 70 mm for CWB values of –330
and –50 cm, respectively.

Water demand [mm y -1]

225

150

75

CWB = - 50 mm
CWB = - 160 mm
CWB = - 330 mm

0
20

60

100

Mean groundwater level[cm below surface]

Figure 8: Water demand required to maintain the water table at adesired level for various climate conditions. CWB is the climate waterbalance during the growing season (precipitation minus cropreference evapotranspiration). The average depth of the natural
groundwater level was assumed to be 1 m.

4 Conclusions
We extended the HYDRUS-1D model to allow calculations ofsoil water-balance components (evaporation, groundwater
recharge, capillary rise) of drained fens from meteorological
data. Potential ET rates were calculated using the PenmanMonteith equation. Estimates of the canopy resistancerequired in this equation were derived from our lysimeter
experiments. Field measurements showed that the soil-water
dynamics depended on the depth of the groundwater level
and the evaporative demand of the atmosphere. We wereable to demonstrate that for Phalaris arundinacea, the ET
rate is controlled by the pressure head of the root zone andthe evaporative demand of the atmosphere. A physically
based approach was tested to relate the canopy resistanceof plants to the average pressure head in the root zone, thelatter being continuously adjusted in the model.
Model simulations described the observed lysimeter data relatively well. Some deviations between measured and calculated soil water contents occurred during wetting after rela 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

J. Plant Nutr. Soil Sci. 2006, 169, 762–774
tively long drying periods. These deviations were presumably
due to wettability problems of the water-repellent peat. These
problems of water repellency were not included in the present
version of HYDRUS-1D.
During dry years, a nearly linear relationship was found between the depth of the water table and the amount of ET. For
groundwater levels between 30 and 90 cm, the ET rate wascontrolled mostly by capillary rise of water from groundwater,
and thus on the soil-hydraulic properties. During wet years,
relatively small differences in ET rates existed for groundwater levels between 30 and 70 cm. Additional water supply
from groundwater was found to be far less important in wet
years than in dry years. In wet years, the amount of ET was
mainly controlled by the energy demand of the atmosphereand less by the soil hydraulic properties.
Lower soil water contents in the root zone of peat soils are
known to promote peat mineralization and the release of CO2
into the atmosphere (Renger et al., 2002). Shallow groundwater levels are essential for environmentally friendly land
use with minimal C release. This strategy, unfortunately, requires large amounts of water during the summer months.
This water may not be available for large peat areas such as
the Rhinluch fens in Germany. Hence, a common practice of
wetland management is to protect only parts of the peat soils
from mineralization. Deeper groundwater levels are thenallowed in the remaining areas. Our findings, however, showthat at least temporarily during drought periods, peat areas inCentral Europe will have a great demand for water. This is an
important finding that needs to be considered in the design
and implementation of sustainable restoration/conservation
practices for wetlands, particularly with regard to the predictions of climate change.

Acknowledgments
This study is dedicated to Rienk van der Ploeg who in the late1970s introduced in Germany the first numerical models
applicable to soil-hydrologic problems. Gerd Wessolek isthankful for the time at Georg August University in Göttingen
when Rienk taught a relatively small group how to solve problems in water and solute transport. Rienk’s vigorous supportof the use of advanced conceptual models and computations
tools has lasted through, among other things, his organization of yearly HYDRUS courses at the University of Hannover. Rien van Genuchten and Jirka Šimu° nek always verymuch enjoyed visiting Hannover and lecturing at thosecourses. Rienk van der Ploeg was a very giving, collaborative, and humorous colleague who always encouraged us to
use innovative methods to address soil and environmental
problems such as those described in this publication. Thanks,
Rienk, for the time that we could spend with you.

References
Acreman, M. C., Harding, R. J., Lloyd, C. R., McNeil, D. D. (2003):
Evaporation characteristics of wetlands: experience from a wet
grassland and a reedbed using eddy correlation measurements.
Hydrol. Earth Syst. Sci. 7, 11–21.

www.plant-soil.com

J. Plant Nutr. Soil Sci. 2006, 169, 762–774

Soil-water dynamics and evapotranspiration of drained peatland soils 773

Allen, R. G., Pereira, L. S., Raes, D., Smith, M. (2000): Crop evapotranspiration – Guidelines for computing crop water requirements.
FAO Irrigation and drainage paper 56, Rome.
Baird, A. J., Gaffney, W. (2000): Solute movement in fen peat: a field
tracer study in a Somerset (UK) wetland. Hydrol. Proc. 14,
2489–2503.
Böhm, G., Quast, J. (1998): Die Ermittlung der realen Evapotranspiration von Niedermoorgebieten unter den hydroklimatischen
Bedingungen Nordostdeutschlands am Beispiel des Rhinluchs(Determination of actual evapotranspiration of peatlands under thehydroclimatic conditions of Northeast Germany represented at theexample of Rhinluch). (in German). Forschunsgbericht ZALF,
Müncheberg.
Braden, H. (1985): Ein Energiehaushalts- und Verdunstungsmodell
für Wasser- und Stoffhaushaltsuntersuchungen landwirtschaftlich
genutzter Einzugsgebiete (An energy and evaporation model for
analysis of water and nutrient balance of catchments with agricultural land use). (in German) Mitteilgn. Dtsch. Bodenkundl.
Gesellsch. 42, 294–299.
Bradley, C., Gilvear, D. J. (2000): Saturated and unsaturated flow
dynamics in a flood plain wetland. Hydrol. Proc. 14, 2945–2958.
Drexler, J. Z., Snyder, R. L., Spano, D., Paw, K. T. (2005): A review ofmodels and micrometeorological methods used to estimate
wetland evapotranspiration. Hydrol. Proc. 18, 2071–2101.
Easy Test (1993): Product information Easy Test soil moisture/salinity
catalogue. Easy Test Ltd., Solarza 8b, 20–815 Lublin 56, PO Box
24, Poland.
Feddes, R. A. (1971): Water, heat and crop growth. Thesis Comm.
Agric. Univ. Wageningen 71–12.
Feddes, R. A., Kowalik, P. J., Zaradny, H. (1978): Simulation of fieldwater use and crop yield. Pudoc, Wageningen, the Netherlands.
Feldhake, C. M., Boyer, D. G. (1986): Effect of soil temperature on
evapotranspiration by C3 and C4 grasses. Agri. Forest Met. 37,
309–318.
Gavin, H., Agnew, C. A. (2004): Modelling actual, reference and equilibrium evaporation from a temperate wet grassland. Hydrol. Proc.
18, 229–246.
Henning, A. (1992): Die Ermittlung des Bulk-Stomata-Widerstandes
und der aktuellen Evapotranspiration aus Saugspannungsmessungen des Bodenwassers – dargestellt am Beispiel von AckerUnkrautgesellschaften auf Hochflutlehm (The determination of
bulk stomata resistance from tensiometric measurements of soilwater – represented at the example of field weed societies on highflood loam). (in German, with English summary). Z. Pflanzenernähr. Bodenkd. 155, 197–201.
Hoyningen-Hüne, J. V. (1983): Die Interzeption des Niederschlages
in landwirtschaftlichen Pflanzenbeständen (Interception of precipitation in agricultural plant stands). (in German). DVWK-Schriften
57, 1–53.
Ingram, H. A. P. (1983): Hydrology, in Gore, A. J. P. (ed.):Ecosystems of the world – Mires: Swamp, Bog, Fen and Moor.
Elsevier, Amsterdam.
Joris, I., Feyen, J. (2003): Modelling water flow and seasonal soil
moisture dynamics in an alluvial groundwater-fed wetland. Hydrol.
Earth Syst. Sci. 7, 57–66.
Kellner, E. (2001): Surface energy fluxes and control of evapotranspiration from a Swedish Sphagnum mire. Agri. Forest Met. 110,
101–123.
Kellner, E., Halldin, S. (2002): Water budget and surface-layer water
storage in a Sphagnum bog in central Sweden. Hydrol. Proc. 16,
87–103.

 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Lafleur, P. M., Hember, R. A., Admiral, S. W., Nigel, N. T. (2005):
Annual and seasonal variability in evapotranspiration and watertable at a shrub-covered bog in southern Ontario, Canada. Hydrol.
Proc. 19, 3533–3550.
Lapen, D. R., Price, J. S., Gilbert, R. (2000): Soil water storage
dynamics in peatlands with shallow water tables. Can. J. Soil Sci.
80, 43–52.
Lott, R. B., Hunt, R. J. (2001): Estimating evapotranspiration innatural and constructed wetlands. Wetlands 21, 614–628.
Menzel, L. (1997): Modellierung der Evapotranspiration im System
Boden-Pflanze-Atmosphäre (Modeling of evapotranspiration in thesystem Soil-Plant-Atmosphere). (in German, with English
summary). PhD thesis, ETH-Zürich No. 11790.
Monteith, J. L. (1965): Evaporation and environment. Proc. Symp.
Soc. Exp. Biol. 19, 205–234.
Mualem, Y. (1976): A new model for predicting the hydraulic conductivity of unsaturated porous media. Water Resour. Res. 12,
513–522.
Mundel, G. (1982): Untersuchungen über die Evapotranspiration vonGrasland auf Grundwasserstandorten, 2. Mitteilung: Verlauf der
Verdunstung während der Vegetationszeit (Investigation into
evapotranspiration of grassland on groundwater soils. 2. Relations
between soil factors and evapotranspiration). (in German). Arch.
Acker- Pflanzenbau Bodenk. 26, 515–521.
Penman, H. L. (1948): Natural evaporation from open water, bare soil
and grass. Proc. Royal Soc. Ser. A 193, 120–145.
Renger, M., Wessolek, G., Schwärzel, K., Sauerbrey, R., Siewert, C.
(2002): Aspects of peat conserving and water management.
J. Plant Nutr. Soil Sci. 165, 487–493.
Rijtema, P. E. (1965): An analysis of actual evapotranspiration. Agricultural Research Reports, no 659, Wageningen University, theNetherlands.
Sambale, C. (1998): Experimentelle und modellgestützte Wasserhaushaltsuntersuchungen im System Boden-Pflanze-Atmosphäre(Experimental and model based analysis of the water balance inthe system Soil-Plant-Atmosphere). (in German). PhD thesis,International Graduate School Zittau, IHI -Schriften 8.
Schwärzel, K., Bohl, H. (2003): An easily installable groundwater lysimeter to determine water balance components and hydraulic properties of peat soils. Hydrol. Earth Syst. Sci. 7, 23–32.
Schwärzel, K., Renger, M., Sauerbrey, R., Wessolek, G. (2002): Soilphysical characteristics of peat soils. J. Plant Nutr. Soil Sci. 165,
479–486.
Schwärzel, K., Šimu°nek, J., Stoffregen, H., Wessolek, G., vanGenuchten, M. T. (2006): Estimation of the unsaturated hydraulicconductivity of peat soils: laboratory versus field data. Vadose
Zone J. 5, 628–640.
Šimu°nek, J., Šejna, M., van Genuchten, M. T. (1998a): HYDRUS-1D
software package for simulating the one-dimensional movement ofwater, heat and multiple solutes in variably saturated media.
Version 2.0, IGWMC-TPS-70, International Ground Water
Modeling Center, Colorado School of Mines, Golden, Colorado.
Šimu°nek, J., Wendroth, O., van Genuchten, M. T. (1998b): Parameter estimation analysis of the evaporation method for determing
soil hydraulic properties. Soil Sci. Soc. Am. J. 62, 894–905.
Spieksma, J. F. M., Moors, E. J., Dolman, A. J., Schouwenaars, J. M.
(1997): Modelling evaporation from adrained and rewetted
peatland. J. Hydrol. 199, 252–271.
Szeicz, G., Long, I. F. (1969): Surface resistance of crop canopies.
Water Resour. Res. 5, 622–633.
Takagi, K., Tsuboya, T., Takahashi, H. (1998): Diurnal hysteresis of
stomatal and bulk surface conductances in relation to vapor

www.plant-soil.com

774

Schwärzel, Šimu°nek, van Genuchten, Wessolekpressure deficit in a cool-temperate wetland. Agri. Forest Met. 91,
177–191.
van Dam, J. C., Huygen, J., Wesseling, J. G., Feddes, R. A., Kabat,P., van Walsum, P. E. V., Groenendijk, P., van Diepen, C. A.
(1997): Theory of SWAP. Version 2.0. Report 71, Dep. Water
Resources, Wageningen Agricultural University.
van Genuchten, M. T. (1980): A closed-form equation for predicting
the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am.
J. 44, 892–898.
Walter, B. P., Heimann, M., Matthews, E. (2001): Modeling modern
methane emissions from natural wetlands. J. Geophys. Res. D
106, 34189–34206.
Wendling, U. (1991): Schätzmethoden der Verdunstung landwirtschaftlicher Bestände nach den Ansätzen von Penman und

 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

J. Plant Nutr. Soil Sci. 2006, 169, 762–774
Turc (Estimating evaporation in crop stands according to Penman
and Turc formulas). (in German, with English summary). Arch.
Acker- Pflanzenbau Bodenkd. 35, 251–257.
Wessolek, G., Schwärzel, K., Renger, M., Sauerbrey, R., Siewert, C.
(2002): Soil hydrology and CO2 mineralization of peat soils. J. Plant
Nutr. Soil Sci. 165, 494–500.
Zenker, T. (2003): Verdunstungswiderstände und Gras-Referenzverdunstung – Lysimeteruntersuchungen zum Penman-MonteithAnsatz im Berliner Raum (Bulk stomata resistance and grass reference evapotranspiration. Lysimeter evaluations for the PenmanMonteith approach in the Berlin area). (in German, with English
summary). PhD thesis, Technical University Berlin, http://edocs.tuberlin.de/diss/2003/zenker_thomas.htm

www.plant-soil.com