Directory UMM :Data Elmu:jurnal:A:Agricultural Water Management:Vol44.Issue1-3.Apr2000:
Agricultural Water Management 44 (2000) 43±61
The Weiherbach data set:
An experimental data set for pesticide model
testing on the ®eld scale
I. Schierholza, D. SchaÈferb,*, O. Kollec
a
State Agricultural Research Station of Baden-WuÈrttemberg (LUFA Augustenberg), Neûlerstr. 23,
D-76227 Karlsruhe, Germany
b
Hoechst Schering AgrEvo GmbH, Hoechst Works, G 836, D-65916 Frankfurt am Main, Germany
c
Institute for Meteorology and Climatic Research, University of Karlsruhe, Max-Planck-Institut fuÈr
Biogeochemie, Tatzendpromenade 1a, D-76128 Karlsruhe, Germany
Abstract
There are few consistent and comprehensive data sets for the calibration and veri®cation of
computer models of pesticide fate in agro-ecosystems. To partly close this gap the data base of the
multidisciplinary Weiherbach research project was used to form a data set that is well suited for that
purpose. It has been successfully used during the COST Action 66 model comparison.
The Weiherbach research area is a small, intensively cultivated catchment in south-western
Germany. The soils of the region are developed from loess and are strongly in¯uenced by erosion.
An important feature is the abundance of large macropores that cause preferential ¯ow events. Field
dissipation and ®eld lysimeter studies with the herbicide isoproturon and the tracer KBr were
conducted in a typical Calcaric Regosol for a late autumn as well as for a spring application
scenario. For the lysimeter studies 10 undisturbed soil monoliths (0.45 m long, 0.3 m in diameter)
from the same ®eld were used to allow for an estimate of the spatial variability of solute transport.
During the spring experiment, one half of the ®eld plot and selected lysimeters were irrigated to
simulate wet conditions with higher leaching potential.
The Weiherbach data set comprehensively characterises the hydrological, agricultural and soil
properties of the experimental sites (including site-speci®c degradation and sorption data for
isoproturon) as well as the meteorological conditions during the experiments. In the ®eld studies,
depth pro®les of isoproturon and tracer were measured at several dates whereas in the lysimeter
studies the percolate was regularly analysed. A detailed description of the experimental results and
the whole data set as it was used for the comparison of pesticide transport models within COST
Action 66 will be given by Schierholz (1999).
*
Corresponding author.
E-mail addresses: dieter.schaefer@agrevo.com (D. SchaÈfer), olaf.kolle@bgcjena.mpg.de (O. Kolle).
0378-3774/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 3 7 7 4 ( 9 9 ) 0 0 0 8 3 - 9
44
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
In the experiments both matrix and macropore ¯ow occurred and the kind and amount of solute
transport clearly depended on the precipitation (irrigation) conditions. The autumn application was
followed by an unusually wet winter and represents a `worst case' scenario with deep leaching of
isoproturon. After the spring application there were about average meteorological conditions, but
the irrigated variants again represent a `worst case'. # 2000 Elsevier Science B.V. All rights
reserved.
Keywords: Data set; Multidisciplinary measuring; Pesticide leaching; Solute transport in soil
1. Introduction
There are numerous computer models for the simulation of the behaviour of agrochemicals in the soil environment but few comprehensive, consistent and freely available
data sets that can be used to test these models. A data set of that kind would be helpful for
both model developers and model users. It should contain information on the pesticide
distribution in the soil as well as measured values of the necessary model input
parameters. It is also essential that the parameter values are determined at the same site
(and preferably the same time) the pesticide measurements take place.
From 1989 to 1997 the multidisciplinary Weiherbach project was performed by several
institutes of the University of Karlsruhe, by the State Agricultural Research Station of
Baden WuÈrttemberg and by the University of Heidelberg, the University of Bayreuth and
the Technical University Cottbus with one institute each. The aim of the project was the
development of a physically based numerical model capable of describing all relevant
water and solute fluxes within and out of a small rural catchment (Plate, 1998). Over the
course of the project extensive data regarding the hydrological, agricultural and soil
properties of the research area were collected. Combining these data with results from
several field and lysimeter studies on pesticide behaviour in soil the so-called Weiherbach
data set was formed that more or less meets the criteria outlined above.
This text gives a short overview of the experiments and of the data as far as they were
used for modelling purposes in the framework of COST Action 66. A complete
description and documentation of the Weiherbach data set can be found in Schierholz
(1999).
2. Research area and experimental sites
The research area of the Weiherbach Project is an intensively cultivated rural
catchment of ca. 6 km2 in the Kraichgau region, 30 km north-east of Karlsruhe (SouthWest Germany). Its main characteristic is a homogeneous loess layer up to 30 m thick
that covers most of the area. The catchment has an undulating surface with moderate to
steep (5±15%) slopes and is strongly influenced by erosion. This leads to typical hill
slope catenaries with eroded soils low in clay and organic carbon at the hilltop and soils
developed from accumulated fine material rich in clay and organic carbon at the bottom.
In general, the unsaturated zone stretches over several meters with the groundwater more
45
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Table 1
Soil characteristics of ®eld plot VIII and of sites I and III
C/N
pH
CEC
(mval 100 gÿ1)
MWHCa
(ml 100 gÿ1)
0.9
1.1
0.8
7
8
9.7
7.3
7.3
7.3
10.7
15.7
11.9
38.8
38.0
n.m.b
10.9
14
0.2
0.5
n.m.
n.m.
n.m.
7.4
n.m.
11.0
n.m.
n.m.
11.4
14
13.8
17.0
0.1
0.3
(0.2)
(0.1)
n.m.
n.m.
n.m.
n.m.
n.m.
7.1
n.m.
n.m.
n.m.
9.4
n.m.
n.m.
n.m.
n.m.
35.0
35.4
Depth
(m)
Site
Sand
(%)
Silt
(%)
Clay
(%)
Corg
(%)
0±0.2/0.3
I
III
VIII
6.3
2.5
7
78.8
74.4
79
14.9
23.1
14
0.3±0.6
I
VIII
6.3
7
82.8
79
0.6±0.9
I
VIII
I
I
4.4
8
4.8
3.2
84.2
78
81.4
79.8
1.0±2.0
2.0±3.0
a
b
Maximum water holding capacity.
Not measured.
than 30 m below the surface at some hilltop sites. In the framework of COST Action 66
data from two field dissipation studies on the so-called field plot VIII and from two
experiments at a field lysimeter station were used for modelling purposes. Only details of
these four studies, all with the herbicide isoproturon, are provided here, together with all
model-relevant data from field site VIII and also from sites I and III. Soil samples from
the two latter sites were used in a number of laboratory studies, with site I exhibiting
properties very similar to field site VIII and site III being investigated for comparison.
Field plot VIII (7 3 m2) and the lysimeter station were located on a hilltop in the
centre of the Weiherbach area (co ordinates1 of field plot: E 34.81050/N 54.45925,
height: 203 m asl, slope: 18), near the main meteorological station of the Weiherbach
project. The lysimeter station consisted of 10 small, free draining lysimeters filled with
undisturbed soil monoliths (0.45 m long, 0.3 m in diameter) that were excavated adjacent
to field plot VIII. Sites I and III are located on the top and on the bottom, respectively, of
a hillslope, ca. 1 km away from field plot VIII.
Site I and field plot VIII both show the Ap-lC soil profile2 of an eroded Calcaric
Regosol as it is commonly found on the hilltops of the Weiherbach area while site III is a
Cumulic Anthrosol with an Ap-M profile. The soil properties of field plot VIII and of sites
I and III are summarised in Tables 1 and 2. In all soils there is a high density of large
macropores (earthworm holes) that provide preferential flow paths. They are clearly
separated from the silty soil matrix.
3. Herbicide and tracer transport studies
Field plot VIII had been covered with excavated soil material when the lysimeter
station was constructed in 1990 and remained bare until the start of the first field study in
1
Gauss-KruÈger-coordinates of one edge point of the plot, E Eastern/N Northern.
The soil horizons are denoted according to the German soil classi®cation system, i.e. Ap plough layer,
M subsoil horizon developed from eroded soil material, IC parent material (loess).
2
46
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Table 2
Depth pro®les of the bulk density (kg dmÿ3) of ®eld plot VIII at the beginning and at the end of the spring
application study in 1995
Date
15 May 1995
16 June 1995
Depth (cm)
0±5
5±10
10±15
15±20
20±25
25±35
35±45
45±55
55±75
1.099
1.343
1.100
1.357
1.106
1.335
1.101
1.315
1.131
1.219
1.241
1.236
1.363
1.349
1.385
1.388
1.442
1.464
autumn 1993. To raise soil fertility to normal agricultural field level, 10 kg mÿ2 of
compost were added in October 1993 and the plot was ploughed to a depth of 0.25 m.
Sowing of the crop (winter wheat) for the first field study was done on 28 October 1993
with a dibbling machine. The seed-bed was prepared with a hand tool.
For the second field study in 1995 the plot was ploughed 10 days before sowing. Due to
heavy rainfalls the soil silted up in the following days and the plot had to be dug over by
spade again on 7 April 1995, immediately before seed-bed preparation and sowing of
summer barley. For that spring application study one half of plot VIII (denoted VIII/2,
plot area: 3.5 3 m2) was irrigated discontinuously three times a week for about four
weeks using a sprinkler irrigation device (rain simulator) installed 1 m above the soil
surface. The irrigation rate was kept below 4 mm hÿ1 to avoid ponding; the total
irrigation volume was 260 mm. To separate field plot VIII/2 from not irrigated field plot
VIII/1 a metal plate that ended 0.1 m above the surface was inserted into the soil to a
depth of 0.5 m.
Both lysimeter studies ran parallel to the field studies in winter 1993/1994 and in
spring 1995. Throughout the experiments the lysimeters were covered by the same plants
as site VIII and were manually cultivated. During the 1995 study two pairs of lysimeters
(Ll/L2 and L3/L4) were irrigated with 30 and 60 mm of water per week, respectively,
combining to a total irrigation of 140 and 280 mm, respectively. The other lysimeters
received only the natural precipitation.
Isoproturon (IPU), a phenyl urea herbicide that is commonly used in the Weiherbach
area for weed control in summer and winter cereals, was chosen as test substance. IPU
exhibits a relatively high water solubility and a weak sorption compared to other
pesticides with widespread application in the Kraichgau region. That seemed suitable for
the field and lysimeter studies as the Kraichgau region is characterised by an almost
neutral climatic water balance (cf. Table 5) and therefore a relatively poor leaching
potential for pesticides.
IPU was applied on the field plot at twice the normal rate3 (i.e. at a nominal rate of
3.0 kg haÿ1 of active ingredient) using its commercial formulation Arelon fluÈssig. A
saline tracer (KBr; 150 kg bromide haÿ1) was added simultaneously. IPU and KBr were
applied with a knapsack spray device during the three-leaf-stage of the cereals on 6
3
The doubled application rate was chosen to follow IPU dissipation under ®eld conditions for a longer period
of time, considering the analytical limit of detection. It does not signi®cantly change the degradation kinetics.
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
47
December 1993 (first study) and on 11 May 1995 (second study). The actual application
rates of IPU, calculated from the concentrations measured in the top 0.20 m of soil on 10
December 1993 and on 15 May 1995 were 2.83 kg haÿ1 in 1993 and 3.25 kg haÿ1 in 1995
of active ingredient, respectively. The lysimeters were manually supplied with 21.2 mg
IPU and 1060 mg KBr each.
During the 1993/1994 study the top 0.25 m of soil were sampled with a short, wide soil
corer (internal diameter 0.035 m) at least monthly. A PuÈrckhauer soil corer (internal
diameter 0.018 m) was then driven into the bore hole to obtain further soil samples from
0.25 to 0.95 m depth. The sampling sites were chosen randomly, avoiding the spots of
former insertions as far as possible, considering the restricted area of the field plot. The
soil cores were divided into sub-samples corresponding to 10 soil layers of thickness
0.05±0.20 m and sub-samples of the same layer from four insertions were mixed to form
the final bulk sample. In contrary, during the 1995 experiment the PuÈrckhauer soil cores
were taken only close to the wider bore holes to improve the integrity of the original soil
layering. The top 0.25 m of these PuÈrckhauer soil cores were discarded. All soil samples
were transported to the laboratory in cooling bags and stored in a freezer at ÿ188C if
necessary.
The water content of the soil samples was determined gravimetrically. IPU residues
were extracted from the soil samples with methanol. After 10 min of equilibration,
30 min of stirring and subsequent 30 min of sedimentation the supernatant was decanted
and filtrated. The IPU content was then determined by HPLC using a gradient elution
technique, the limit of detection being 50 mg of active ingredient kgÿ1 dry soil. The
bromide tracer was measured by an ion selective electrode (limit of detection 0.2±0.4 mg
bromide kgÿ1 dry soil). At the lysimeter station, the percolates from the soil cores were
regularly sampled and their IPU and tracer content analysed as described above.
Additionally, soil samples were taken from selected lysimeters at the end of the studies,
extracted and checked for their residual tracer content. Some basic results of the field and
lysimeter studies are given in Tables 3 and 4.
4. Measurement of model input parameters
The most important input parameters for pesticide transport models were determined
either in the field (meteorological parameters) or in the laboratory (soil and pesticide
parameters). For technical reasons, the laboratory studies mainly used samples from site I
that show properties very similar to those of field plot VIII (cf. Table 1). In the
Weiherbach area, the soil properties are systematically linked to the topographical
position of a site due to the homogeneous parent material and the dominant influence of
erosion (Plate, 1998).
4.1. Meteorological parameters
The meteorological parameters were measured at a micrometeorological station close
to field site VIII (co-ordinates: E 34.81050/N 54.44887, height: 205 m asl). Among the
measured data were precipitation, air temperature, air humidity, wind velocity, and global
48
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Table 3
Total amounts of bromide and IPU extracted from soil samples from ®eld plot VIII up to a depth of 0.95 m for
the 1993/1994 (late autumn application) and 1995 (spring application) studiesa
Bromide (kg haÿ1)
Not irrigated
1993/1994 autumn study
10 December 1993
16 December 1993
23 December 1993
17 January 1994
31 January 1994
28 February 1994
14 March1994
28 March1994
26 April 1994
n.m.
143
136
96
93
61
87
78
56
1995 spring study
15 May 1995
19 May 1995
26 May 1995
29 May 1995
2 June 1995
9 June 1995
16 June 1995
n.m.
146
140
114
96
41
22
IPU (kg haÿ1)
Irrigated
Not irrigated
Irrigated
2.83
2.29
1.67
2.06
0.91
0.65
0.40
0.22
0.08
n.m.
143
142
119
105
35
12
3.26
3.12
2.11
1.38
0.55
0.21
0.00
3.40
2.67
2.22
1.82
1.49
0.46
0.00
a
Values calculated from the measured concentrations assuming a bulk density of 1.3 kg dmÿ3 in the plough
layer and 1.5 kg dmÿ3 in the subsoil in 1993 and bulk densities derived from measurements at the ®rst and last
sampling date (see Table 2) by linear interpolation in 1995.
radiation. Additionally, the soil moisture was determined at five depths with a special
dielectric system similar to a TDR probe. This allows the determination of infiltration
rates and water uptake by plants under natural conditions. The usual temporal resolution
of all data was 10 min.
Table 5 shows the yearly mean values or yearly sums of some important meteorological
parameters of the Weiherbach area. The daily sums of potential evapotranspiration were
calculated according to the Penman-Monteith equation (SchroÈdter, 1985). The
precipitation measurements were corrected with regards to wetting losses assuming a
mean wetting loss of 0.1 mm per precipitation event and calculating the drying time of
the gauge from the potential evaporation. The method of Allerup and Madsen (1980) was
applied to correct the underestimation due to aerodynamic effects that amounted to an
overall average of approximately 11%. The daily mean precipitation intensity was
determined by weighting the precipitation intensity of each 10-min-interval with its
precipitation amount and then calculating the arithmetical mean value.
4.2. Soil hydraulic parameters
The soil hydraulic properties of the experimental sites are described here according to
the model of Mualem and van Genuchten (van Genuchten, 1980). Undisturbed soil cores
49
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Table 4
Sums of percolate and irrigation and cumulative loads of bromide and IPU (as masses and as percentages of the
applied amount) in the percolate of ®eld lysimeters L1 to L10 over the course of the 1993/1994 autumn and 1995
spring studiesa
L1
L2
1993/1994 autumn study
Sum of percolate (mm)
47
144
Load of bromide (g)
(%)
0.16
15
0.70
66
0.20
19
0.22
21
0.35
35
0.89
84
0.53
50
0.91
86
0.0
0.0
0.0
0.0
0.0
0.0
49
0.23
1.1
0.01
0.0
0.0
4.5
0.02
182
0.86
Load of IPU (mg)
(%)
1995 spring study
Sum of irrigation (mm)
Sum of percolate (mm)
L3
88
L4
L5
L8
L9
Ll0
124
106
197
112
139
L1
L2
L3
L4
140
124
140
102
280
239
280
201
Load of bromide (g)
(%)
0.52
49
0.73
69
0.61
58
0.87
82
Load of IPU (mg)
(%)
74
0.35
57
0.27
1570
7.41
301
1.42
L7
±
4.2
L8
L9
±
13
0.001
0.05
0.001
0.09
0.0
0.0
0.0
0.0
L10
±
9.9
±
4.2
0.021
2.0
0.001
0.11
101
0.47
3.2
0.01
a
Lysimeters L6 and L7 were not used in 1993/1994 and there was no percolate from lysimeters L5 and L6 in
1995.
(volume 100 cm3) were saturated to a water suction of 0.01 bar and drained by a stepwise increased overpressure of up to 1 bar. The multi-step outflow characteristics of the
samples were then used for an inverse identification of their hydraulic parameters (van
Dam et al., 1994). Additionally, the saturated hydraulic conductivity and saturated water
content of the samples were determined by standard methods. Table 6 summarises the
results for sites I and III. At field plot VIII no hydraulic properties were determined, but
as sites I and VIII are very similar regarding texture, organic carbon content and other soil
properties it can be safely assumed they also have similar hydraulic properties
(Montenegro et al., 1998).
In view of the experimental conditions and the restrictions of the parametric model the
results only characterise the hydraulic properties of the soil matrix but not of the large
Table 5
Yearly averages and yearly sums of meteorological parameters measured at the Weiherbach station (potential
evapotranspiration calculated according to Penman-Monteith)
Parameter
1991
1992
1993
1994
1995
Air temperature (8C)
Wind velocity (m sÿ1)
Net radiation (W mÿ2)
Precipitation (mm)
Potential evapotranspiration (mm)
9.61
3.32
56.0
512.9
784.1
10.36
3.26
59.6
786.0
784.4
9.99
3.54
59.4
699.7
806.0
11.18
3.53
59.1
828.9
745.8
10.15
3.52
55.6
1099.9
747.0
50
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Table 6
Soil hydraulic parameters for sites I and III (saturated hydraulic conductivity ksvalid for the soil matrix only)
Site
Depth (m)
a (cmÿ1)
n
yr (cm3 cmÿ3) ys (cm3 cmÿ3) ks (cm hÿ1)
Water content at
pF 1.8
pF 2
pF 4.2
I
0.0±0.3
0.3±1.0
0.015
0.005
1.30
2.25
0.03
0.08
0.46
0.45
0.5
0.3
0.400
0.436
0.372 0.115
0.413 0.082
III
0.0±0.3
0.3±1.0
0.040
0.020
1.20
1.25
0.10
0.08
0.43
0.41
0.4
0.3
0.362
0.359
0.343 0.192
0.339 0.159
macropores. As the macropores are mainly worm holes there abundance depends on the
biological activity of the soil. Typically, about five times more macropores are found in
the Cumulic Anthrosols that are relatively wet and rich in clay than in the eroded
Regosols at the hill tops (Zehe, 1997). That explains why on average the directly
measured saturated hydraulic conductivity (including macropores) is five times higher
than the one derived from the inverse approach in one of the Regosols and ten times
higher in a Cumulic Anthrosol (Montenegro et al., 1998).
The bulk densities of the loess soils range from 1.2 to 1.6 kg dmÿ3. It was proposed to
use an average bulk density of 1.3 kg dmÿ3 in the plough layer and 1.5 kg dmÿ3 in the
subsoil for modelling purposes. These values were also used for the calculation of
substance-per-area values from measured concentrations in all tables. An exception was
made for those data concerning the field studies in 1995, when the bulk densities of
different soil layers were measured at the first and last sampling date. In this case, data for
the intermediate dates were obtained by linear interpolation (Table 2).
4.3. Plant parameters
Plant parameters raised for different crops in the Weiherbach area include the plant
height, the leaf area index, the crop cover, the root length density and the yield of grain
and straw (Ritz, 1996). As these data are not available for field site VIII, rough estimates
based on measured data from similar topographical positions are given in Table 7. It must
be noted, though, that in 1995 the summer barley was sowed later (7 April 1995) on the
field plot than on all comparable sites of the Weiherbach area. The cropping data for that
year should therefore be used with care. Unfortunately, an indication of the root
distribution is impossible for both years for the same reason.
4.4. Sorption and degradation studies with IPU in the laboratory
The sorption coefficients (Kd-values) for IPU were determined in batch experiments
using a soil-to-water ratio of 1 : 2.5 and an equilibration time of 24 h (Mokry, 1996a per.
com.). The soil material originated from site I, with soil characteristics very similar to
that of field plot VIII.
Three laboratory incubation experiments were carried out to study the influence of
different environmental factors on the degradation of IPU. MuÈller (1995) investigated the
51
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Table 7
Cropping data for ®eld plot VIII: growth stages (BBCH code; Lancashire, 1991), plant height, leave area index
(LAI) and crop cover (all values estimated)
Crop
Date
Julian
day
Growth stage
(BBCH)
Plant height
(cm)
LAI
(m2 mÿ2)
Crop cover
(%)
Winter
wheat
28 October 1993
14 November 1993
6 December 1993
1 April 1994
1 February 1994
1 March 1994
1 April 1994
301
318
340
1
32
60
91
sowing
10
13
21
21
23
25
0
1
8
10
10
12
20
0
0
0.2
0.5
0.5
0.6
1
0
0
8
12
12
15
20
Summer
barley
7 April 1995
17 April 1995
1 May 1995
1 June 1995
1 July 1995
1 August 1995
97
107
121
152
182
201
sowing
10
21
40
75
91
0
1
15
60
85
80
0
0
1
5
4.5
4
0
0
20
70
80
75
degradation of IPU under three different temperature (5, 15, and 258C) and up to eight
different moisture regimes (5±100% of maximum water holding capacity, MWHC4) in
soils from sites I and III (sampling depth: 0.0±0.2 m, sampling date: 13 January 1995).
Before the incubation at 5, 15, and 258C the soil samples had been stored for 18, 10, and
6 weeks, respectively, in loosely covered plastic boxes and kept moist. The microbial biomass of the samples was determined by the substrate induced respiration method (Thun
and Herrmann, 1949). The soil samples were then incubated under aerobic conditions in
the dark and the residual amount of herbicide was determined at six dates over 31 days (at
258C), 57 days (at 158C) and 97 days (at 58C) after application. The change of sorption
over time and the corresponding desorption coefficients Kdes were examined by water
extraction of selected samples (parallel aliquots) incubated at 5 and 158C and at 40% of
MWHC, in addition to the extraction of total residues with methanol. The adsorbed mass
was calculated by the difference of the fraction extracted with methanol and the water
soluble fraction.
In a second incubation study, Mokry (1996b per. com.) used soil samples from
the plough layers (0.0±0.3 m) of field sites I and III taken in April, July, and October
1990 to determine the seasonal variability of IPU degradation. A third incubation study
was conducted to characterise the influence of the microbial activity on the dissipation of
IPU, where top soil samples from sites I and III (sampled in December 1992) were
incubated under sterile and non-sterile conditions. This study also included subsoil
samples from two different depth ranges from site I (1±2 and 2±3 m; Mokry, 1996c
per. com.).
4
According to (Thun and Herrmann, 1949) sieved soil samples were ®lled into ceramic ®lters and saturated
from below using clay cylinders standing in water. After 24 h, the MWHC of the samples was determined by
differential weighing.
52
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Table 8
Kd-va1ues (ml gÿ1) for isoproturon as a function of equilibration time and pro®le depth (soil samples from site I)
Equilibration time (min)
10
1440
2880
Profile depth (m)
0.0±0.3
0.3±0.6
0.6±0.9
1.95
2.06
2.00
1.72
1.84
1.53
1.64
1.68
1.53
5. Results of the laboratory studies
The results of the sorption studies are given in Table 8. IPU mainly sorbs to the clay
fraction of the soil while organic carbon has less influence. As the Calcaric Regosols of
the Weiherbach area show a more or less uniform clay content there was only a small
decrease in sorption with increasing soil depth. The Kd-values determined in this study
(1.7±2.1) seem rather high compared to literature data for similar soils (1.0±1.3). As an
alternative the initial Kdes-values from the incubation study described in the next
paragraph (1.0±1.1 at 158C) could be used for modelling purposes.
In the incubation study with three temperature and eight soil moisture regimes the soil
samples from hilltop (site I) and bottom (site III) showed a similar degradation behaviour
(Fig. 1). This can be explained by opposite influences of different soil properties
cancelling each other out, such as clay content and organic carbon content that positively
influence both sorption and biological activity (degradation). At 258C, the calculated
half-life times ranged from 690 days at 5% of MWHC to nine days at 60% MWHC which
Fig. 1. Degradation rates of IPU (determined by linear regression after log-transformation of the degradation
data assuming ®rst-order kinetics) in topsoil samples from sites I and III incubated at different soil water
contents and different temperatures.
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
53
Fig. 2. Time-dependency of the Kdes-values for IPU in soil samples from sites I and III.
was the optimum water content for degradation. At higher water contents microbial
activity was restricted by oxygen deficiency. An increase in temperature (within the
examined range) led to a higher degradation rate, the increase from 5 to 158C being
smaller than that from 15 to 258C. Regarding the long-term sorption behaviour of IPU an
increase of the Kdes-values up to seven times the initial values after 60 days was found
when the sorption apparently had reached an equilibrium (Fig. 2).
The second incubation study showed a significant influence of the sampling date on the
degradation rates. Soil samples taken in July showed the fastest degradation followed by
samples from October, while the decrease of IPU in the April-soil was even slower. The
measured microbial bio-mass of the soils turned out to be almost constant over the
seasons, however. This is due to the fact, that the substrate induced respiration method
only measures the potential respiration activity, which is a relatively constant soil
property.
In the third incubation study with top and subsoil samples and different pre-treatment
of soil, the degradation of IPU was fastest in the non-sterile top soil with a half-life time
of around 20 days (Fig. 3). In the subsoil samples only 40% of the IPU had disappeared at
the end of the study after 100 days. The degradation due to abiotic processes amounted to
only 20% of IPU after 100 days.
6. Results of the ®eld studies
The first study at field plot VIII (KruÈger, 1994) was conducted from December 1993 to
April 1994 under very wet conditions. The cumulative precipitation was 365 mm, which
is considerably higher than the long-term average during that period (260 mm). The
cumulative volume of leachate (measured in the parallel lysimeter study) summed up to
54
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Fig. 3. Degradation of IPU in the laboratory in samples from different depths of site I and in a sterilised top soil
sample.
228 mm. Soil temperatures dropped below 08C only for one week, with a minimum
temperature of ÿ2.58C at the 0.02 m depth.
The maximum depth where IPU was retrieved was 0.65 m in mid-January (Fig. 4),
while the conservative tracer bromide was leached below the sampling depth of 0.95 m
till April 1994. So this 1993/1994 field experiment qualifies as a `worst case' scenario
with unusually deep leaching of the herbicide. The average retardation coefficient of IPU
in relation to bromide was 4.0.
During the second field experiment starting in May 1995 (Zimmermann, 1996)
precipitation and evapotranspiration were almost equal (Fig. 5). These are slightly wetter
than normal conditions as in the long-term average the evapotranspiration dominates
precipitation in May and June. For the last 10 days before the application of the herbicide
the sum of rainfall was only 6 mm compared to 42 mm of evapotranspiration. The
herbicide was applied on relatively dry soil, the initial moisture content of plots VIII/1
and VIII/2 being 59 and 66% of the field capacity, respectively.
IPU was found in depths of up to 0.5 m in the non-irrigated field plot VIII/1 with
leaching mainly occurring during the first 14 days after application (Fig. 6). Due
to the irrigation that started 10 days after the application of IPU the percolation in plot
VIII/2 was sevenfold higher and the herbicide reached a depth of up to 0.7 m (Fig. 7).
Most of the leaching again occurred during the first days after application and
was caused by natural rainfall, though. In relation to bromide, retardation coefficients
of 1.3±2.0 could be calculated. These very low Rd-values indicate that there probably
was preferential transport of the herbicide in macropores. The overall half-life time
of IPU was ca. 9 days under natural rainfall conditions and ca. 11 days in the irrigated
field plot. This slower dissipation can be attributed to the smaller degradation rates in
deeper soil layers that more of the herbicide was leached to in shorter time due to the
irrigation.
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Fig. 4. IPU pro®les at selected sampling dates during the 1993/1994 ®eld study (late autumn application).
55
56
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Fig. 5. Cumulative precipitation, evapotranspiration (calculated according to Penman-Monteith), irrigation and
percolation (calculated from water balance) during the 1995 ®eld study (spring application).
Regarding the lysimeter studies it must be noted that ponding and subsequent overflow
of some lysimeters seem to have occurred during the 1993/1994 experiment. The sums of
percolate as well as the mass balance of the tracer (checked by sampling some of the soil
columns at the end of the experiment) indicate a loss of water and tracer. A corresponding
loss of IPU can be assumed. This in part explains the great differences in percolate
volume between the lysimeters (Table 4), although these differences also reflect the
natural spatial variability of the field soil. The average cumulative IPU load in the
percolates in this study was 0.14% of the applied amount. Yet significant amounts of IPU
were only found in the early percolates of lysimeters L4 and L10, with concentrations of
up to 500 mg lÿ1. This points to possible preferential transport of IPU, associated with the
high volumes of percolate (ca. 70 mm per lysimeter) during the first three weeks of the
experiment.
In the second experiment the amount of percolate clearly depended on the irrigation
regime of the lysimeters: from the non-irrigated variants no more than 15 mm of
percolate were sampled compared to the 10±20-fold amount from the irrigated variants
(Table 4). As can be seen in Fig. 8 (a) and (b), there are clear indications of preferential
flow in the heavily irrigated lysimeters L3 and L4 (280 mm of irrigation). An early
breakthrough with very high concentrations of up to 150 mg lÿ1 of bromide and up to
300 mg lÿ1 of IPU due to the transport in macropores is followed by a much broader peak
due to transport in the soil matrix. The lighter irrigation (140 mm) of lysimeters L1 and
L2 on the other hand was not enough to induce macropore flow although it enhanced the
leaching of bromide (Fig. 8a). Because of its relatively rapid degradation there is no
corresponding IPU peak. The average cumulative load was 4.4% of the applied IPU for
lysimeters L3 and L4, 0.3% for LI and L2, and
The Weiherbach data set:
An experimental data set for pesticide model
testing on the ®eld scale
I. Schierholza, D. SchaÈferb,*, O. Kollec
a
State Agricultural Research Station of Baden-WuÈrttemberg (LUFA Augustenberg), Neûlerstr. 23,
D-76227 Karlsruhe, Germany
b
Hoechst Schering AgrEvo GmbH, Hoechst Works, G 836, D-65916 Frankfurt am Main, Germany
c
Institute for Meteorology and Climatic Research, University of Karlsruhe, Max-Planck-Institut fuÈr
Biogeochemie, Tatzendpromenade 1a, D-76128 Karlsruhe, Germany
Abstract
There are few consistent and comprehensive data sets for the calibration and veri®cation of
computer models of pesticide fate in agro-ecosystems. To partly close this gap the data base of the
multidisciplinary Weiherbach research project was used to form a data set that is well suited for that
purpose. It has been successfully used during the COST Action 66 model comparison.
The Weiherbach research area is a small, intensively cultivated catchment in south-western
Germany. The soils of the region are developed from loess and are strongly in¯uenced by erosion.
An important feature is the abundance of large macropores that cause preferential ¯ow events. Field
dissipation and ®eld lysimeter studies with the herbicide isoproturon and the tracer KBr were
conducted in a typical Calcaric Regosol for a late autumn as well as for a spring application
scenario. For the lysimeter studies 10 undisturbed soil monoliths (0.45 m long, 0.3 m in diameter)
from the same ®eld were used to allow for an estimate of the spatial variability of solute transport.
During the spring experiment, one half of the ®eld plot and selected lysimeters were irrigated to
simulate wet conditions with higher leaching potential.
The Weiherbach data set comprehensively characterises the hydrological, agricultural and soil
properties of the experimental sites (including site-speci®c degradation and sorption data for
isoproturon) as well as the meteorological conditions during the experiments. In the ®eld studies,
depth pro®les of isoproturon and tracer were measured at several dates whereas in the lysimeter
studies the percolate was regularly analysed. A detailed description of the experimental results and
the whole data set as it was used for the comparison of pesticide transport models within COST
Action 66 will be given by Schierholz (1999).
*
Corresponding author.
E-mail addresses: dieter.schaefer@agrevo.com (D. SchaÈfer), olaf.kolle@bgcjena.mpg.de (O. Kolle).
0378-3774/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 3 7 7 4 ( 9 9 ) 0 0 0 8 3 - 9
44
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
In the experiments both matrix and macropore ¯ow occurred and the kind and amount of solute
transport clearly depended on the precipitation (irrigation) conditions. The autumn application was
followed by an unusually wet winter and represents a `worst case' scenario with deep leaching of
isoproturon. After the spring application there were about average meteorological conditions, but
the irrigated variants again represent a `worst case'. # 2000 Elsevier Science B.V. All rights
reserved.
Keywords: Data set; Multidisciplinary measuring; Pesticide leaching; Solute transport in soil
1. Introduction
There are numerous computer models for the simulation of the behaviour of agrochemicals in the soil environment but few comprehensive, consistent and freely available
data sets that can be used to test these models. A data set of that kind would be helpful for
both model developers and model users. It should contain information on the pesticide
distribution in the soil as well as measured values of the necessary model input
parameters. It is also essential that the parameter values are determined at the same site
(and preferably the same time) the pesticide measurements take place.
From 1989 to 1997 the multidisciplinary Weiherbach project was performed by several
institutes of the University of Karlsruhe, by the State Agricultural Research Station of
Baden WuÈrttemberg and by the University of Heidelberg, the University of Bayreuth and
the Technical University Cottbus with one institute each. The aim of the project was the
development of a physically based numerical model capable of describing all relevant
water and solute fluxes within and out of a small rural catchment (Plate, 1998). Over the
course of the project extensive data regarding the hydrological, agricultural and soil
properties of the research area were collected. Combining these data with results from
several field and lysimeter studies on pesticide behaviour in soil the so-called Weiherbach
data set was formed that more or less meets the criteria outlined above.
This text gives a short overview of the experiments and of the data as far as they were
used for modelling purposes in the framework of COST Action 66. A complete
description and documentation of the Weiherbach data set can be found in Schierholz
(1999).
2. Research area and experimental sites
The research area of the Weiherbach Project is an intensively cultivated rural
catchment of ca. 6 km2 in the Kraichgau region, 30 km north-east of Karlsruhe (SouthWest Germany). Its main characteristic is a homogeneous loess layer up to 30 m thick
that covers most of the area. The catchment has an undulating surface with moderate to
steep (5±15%) slopes and is strongly influenced by erosion. This leads to typical hill
slope catenaries with eroded soils low in clay and organic carbon at the hilltop and soils
developed from accumulated fine material rich in clay and organic carbon at the bottom.
In general, the unsaturated zone stretches over several meters with the groundwater more
45
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Table 1
Soil characteristics of ®eld plot VIII and of sites I and III
C/N
pH
CEC
(mval 100 gÿ1)
MWHCa
(ml 100 gÿ1)
0.9
1.1
0.8
7
8
9.7
7.3
7.3
7.3
10.7
15.7
11.9
38.8
38.0
n.m.b
10.9
14
0.2
0.5
n.m.
n.m.
n.m.
7.4
n.m.
11.0
n.m.
n.m.
11.4
14
13.8
17.0
0.1
0.3
(0.2)
(0.1)
n.m.
n.m.
n.m.
n.m.
n.m.
7.1
n.m.
n.m.
n.m.
9.4
n.m.
n.m.
n.m.
n.m.
35.0
35.4
Depth
(m)
Site
Sand
(%)
Silt
(%)
Clay
(%)
Corg
(%)
0±0.2/0.3
I
III
VIII
6.3
2.5
7
78.8
74.4
79
14.9
23.1
14
0.3±0.6
I
VIII
6.3
7
82.8
79
0.6±0.9
I
VIII
I
I
4.4
8
4.8
3.2
84.2
78
81.4
79.8
1.0±2.0
2.0±3.0
a
b
Maximum water holding capacity.
Not measured.
than 30 m below the surface at some hilltop sites. In the framework of COST Action 66
data from two field dissipation studies on the so-called field plot VIII and from two
experiments at a field lysimeter station were used for modelling purposes. Only details of
these four studies, all with the herbicide isoproturon, are provided here, together with all
model-relevant data from field site VIII and also from sites I and III. Soil samples from
the two latter sites were used in a number of laboratory studies, with site I exhibiting
properties very similar to field site VIII and site III being investigated for comparison.
Field plot VIII (7 3 m2) and the lysimeter station were located on a hilltop in the
centre of the Weiherbach area (co ordinates1 of field plot: E 34.81050/N 54.45925,
height: 203 m asl, slope: 18), near the main meteorological station of the Weiherbach
project. The lysimeter station consisted of 10 small, free draining lysimeters filled with
undisturbed soil monoliths (0.45 m long, 0.3 m in diameter) that were excavated adjacent
to field plot VIII. Sites I and III are located on the top and on the bottom, respectively, of
a hillslope, ca. 1 km away from field plot VIII.
Site I and field plot VIII both show the Ap-lC soil profile2 of an eroded Calcaric
Regosol as it is commonly found on the hilltops of the Weiherbach area while site III is a
Cumulic Anthrosol with an Ap-M profile. The soil properties of field plot VIII and of sites
I and III are summarised in Tables 1 and 2. In all soils there is a high density of large
macropores (earthworm holes) that provide preferential flow paths. They are clearly
separated from the silty soil matrix.
3. Herbicide and tracer transport studies
Field plot VIII had been covered with excavated soil material when the lysimeter
station was constructed in 1990 and remained bare until the start of the first field study in
1
Gauss-KruÈger-coordinates of one edge point of the plot, E Eastern/N Northern.
The soil horizons are denoted according to the German soil classi®cation system, i.e. Ap plough layer,
M subsoil horizon developed from eroded soil material, IC parent material (loess).
2
46
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Table 2
Depth pro®les of the bulk density (kg dmÿ3) of ®eld plot VIII at the beginning and at the end of the spring
application study in 1995
Date
15 May 1995
16 June 1995
Depth (cm)
0±5
5±10
10±15
15±20
20±25
25±35
35±45
45±55
55±75
1.099
1.343
1.100
1.357
1.106
1.335
1.101
1.315
1.131
1.219
1.241
1.236
1.363
1.349
1.385
1.388
1.442
1.464
autumn 1993. To raise soil fertility to normal agricultural field level, 10 kg mÿ2 of
compost were added in October 1993 and the plot was ploughed to a depth of 0.25 m.
Sowing of the crop (winter wheat) for the first field study was done on 28 October 1993
with a dibbling machine. The seed-bed was prepared with a hand tool.
For the second field study in 1995 the plot was ploughed 10 days before sowing. Due to
heavy rainfalls the soil silted up in the following days and the plot had to be dug over by
spade again on 7 April 1995, immediately before seed-bed preparation and sowing of
summer barley. For that spring application study one half of plot VIII (denoted VIII/2,
plot area: 3.5 3 m2) was irrigated discontinuously three times a week for about four
weeks using a sprinkler irrigation device (rain simulator) installed 1 m above the soil
surface. The irrigation rate was kept below 4 mm hÿ1 to avoid ponding; the total
irrigation volume was 260 mm. To separate field plot VIII/2 from not irrigated field plot
VIII/1 a metal plate that ended 0.1 m above the surface was inserted into the soil to a
depth of 0.5 m.
Both lysimeter studies ran parallel to the field studies in winter 1993/1994 and in
spring 1995. Throughout the experiments the lysimeters were covered by the same plants
as site VIII and were manually cultivated. During the 1995 study two pairs of lysimeters
(Ll/L2 and L3/L4) were irrigated with 30 and 60 mm of water per week, respectively,
combining to a total irrigation of 140 and 280 mm, respectively. The other lysimeters
received only the natural precipitation.
Isoproturon (IPU), a phenyl urea herbicide that is commonly used in the Weiherbach
area for weed control in summer and winter cereals, was chosen as test substance. IPU
exhibits a relatively high water solubility and a weak sorption compared to other
pesticides with widespread application in the Kraichgau region. That seemed suitable for
the field and lysimeter studies as the Kraichgau region is characterised by an almost
neutral climatic water balance (cf. Table 5) and therefore a relatively poor leaching
potential for pesticides.
IPU was applied on the field plot at twice the normal rate3 (i.e. at a nominal rate of
3.0 kg haÿ1 of active ingredient) using its commercial formulation Arelon fluÈssig. A
saline tracer (KBr; 150 kg bromide haÿ1) was added simultaneously. IPU and KBr were
applied with a knapsack spray device during the three-leaf-stage of the cereals on 6
3
The doubled application rate was chosen to follow IPU dissipation under ®eld conditions for a longer period
of time, considering the analytical limit of detection. It does not signi®cantly change the degradation kinetics.
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
47
December 1993 (first study) and on 11 May 1995 (second study). The actual application
rates of IPU, calculated from the concentrations measured in the top 0.20 m of soil on 10
December 1993 and on 15 May 1995 were 2.83 kg haÿ1 in 1993 and 3.25 kg haÿ1 in 1995
of active ingredient, respectively. The lysimeters were manually supplied with 21.2 mg
IPU and 1060 mg KBr each.
During the 1993/1994 study the top 0.25 m of soil were sampled with a short, wide soil
corer (internal diameter 0.035 m) at least monthly. A PuÈrckhauer soil corer (internal
diameter 0.018 m) was then driven into the bore hole to obtain further soil samples from
0.25 to 0.95 m depth. The sampling sites were chosen randomly, avoiding the spots of
former insertions as far as possible, considering the restricted area of the field plot. The
soil cores were divided into sub-samples corresponding to 10 soil layers of thickness
0.05±0.20 m and sub-samples of the same layer from four insertions were mixed to form
the final bulk sample. In contrary, during the 1995 experiment the PuÈrckhauer soil cores
were taken only close to the wider bore holes to improve the integrity of the original soil
layering. The top 0.25 m of these PuÈrckhauer soil cores were discarded. All soil samples
were transported to the laboratory in cooling bags and stored in a freezer at ÿ188C if
necessary.
The water content of the soil samples was determined gravimetrically. IPU residues
were extracted from the soil samples with methanol. After 10 min of equilibration,
30 min of stirring and subsequent 30 min of sedimentation the supernatant was decanted
and filtrated. The IPU content was then determined by HPLC using a gradient elution
technique, the limit of detection being 50 mg of active ingredient kgÿ1 dry soil. The
bromide tracer was measured by an ion selective electrode (limit of detection 0.2±0.4 mg
bromide kgÿ1 dry soil). At the lysimeter station, the percolates from the soil cores were
regularly sampled and their IPU and tracer content analysed as described above.
Additionally, soil samples were taken from selected lysimeters at the end of the studies,
extracted and checked for their residual tracer content. Some basic results of the field and
lysimeter studies are given in Tables 3 and 4.
4. Measurement of model input parameters
The most important input parameters for pesticide transport models were determined
either in the field (meteorological parameters) or in the laboratory (soil and pesticide
parameters). For technical reasons, the laboratory studies mainly used samples from site I
that show properties very similar to those of field plot VIII (cf. Table 1). In the
Weiherbach area, the soil properties are systematically linked to the topographical
position of a site due to the homogeneous parent material and the dominant influence of
erosion (Plate, 1998).
4.1. Meteorological parameters
The meteorological parameters were measured at a micrometeorological station close
to field site VIII (co-ordinates: E 34.81050/N 54.44887, height: 205 m asl). Among the
measured data were precipitation, air temperature, air humidity, wind velocity, and global
48
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Table 3
Total amounts of bromide and IPU extracted from soil samples from ®eld plot VIII up to a depth of 0.95 m for
the 1993/1994 (late autumn application) and 1995 (spring application) studiesa
Bromide (kg haÿ1)
Not irrigated
1993/1994 autumn study
10 December 1993
16 December 1993
23 December 1993
17 January 1994
31 January 1994
28 February 1994
14 March1994
28 March1994
26 April 1994
n.m.
143
136
96
93
61
87
78
56
1995 spring study
15 May 1995
19 May 1995
26 May 1995
29 May 1995
2 June 1995
9 June 1995
16 June 1995
n.m.
146
140
114
96
41
22
IPU (kg haÿ1)
Irrigated
Not irrigated
Irrigated
2.83
2.29
1.67
2.06
0.91
0.65
0.40
0.22
0.08
n.m.
143
142
119
105
35
12
3.26
3.12
2.11
1.38
0.55
0.21
0.00
3.40
2.67
2.22
1.82
1.49
0.46
0.00
a
Values calculated from the measured concentrations assuming a bulk density of 1.3 kg dmÿ3 in the plough
layer and 1.5 kg dmÿ3 in the subsoil in 1993 and bulk densities derived from measurements at the ®rst and last
sampling date (see Table 2) by linear interpolation in 1995.
radiation. Additionally, the soil moisture was determined at five depths with a special
dielectric system similar to a TDR probe. This allows the determination of infiltration
rates and water uptake by plants under natural conditions. The usual temporal resolution
of all data was 10 min.
Table 5 shows the yearly mean values or yearly sums of some important meteorological
parameters of the Weiherbach area. The daily sums of potential evapotranspiration were
calculated according to the Penman-Monteith equation (SchroÈdter, 1985). The
precipitation measurements were corrected with regards to wetting losses assuming a
mean wetting loss of 0.1 mm per precipitation event and calculating the drying time of
the gauge from the potential evaporation. The method of Allerup and Madsen (1980) was
applied to correct the underestimation due to aerodynamic effects that amounted to an
overall average of approximately 11%. The daily mean precipitation intensity was
determined by weighting the precipitation intensity of each 10-min-interval with its
precipitation amount and then calculating the arithmetical mean value.
4.2. Soil hydraulic parameters
The soil hydraulic properties of the experimental sites are described here according to
the model of Mualem and van Genuchten (van Genuchten, 1980). Undisturbed soil cores
49
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Table 4
Sums of percolate and irrigation and cumulative loads of bromide and IPU (as masses and as percentages of the
applied amount) in the percolate of ®eld lysimeters L1 to L10 over the course of the 1993/1994 autumn and 1995
spring studiesa
L1
L2
1993/1994 autumn study
Sum of percolate (mm)
47
144
Load of bromide (g)
(%)
0.16
15
0.70
66
0.20
19
0.22
21
0.35
35
0.89
84
0.53
50
0.91
86
0.0
0.0
0.0
0.0
0.0
0.0
49
0.23
1.1
0.01
0.0
0.0
4.5
0.02
182
0.86
Load of IPU (mg)
(%)
1995 spring study
Sum of irrigation (mm)
Sum of percolate (mm)
L3
88
L4
L5
L8
L9
Ll0
124
106
197
112
139
L1
L2
L3
L4
140
124
140
102
280
239
280
201
Load of bromide (g)
(%)
0.52
49
0.73
69
0.61
58
0.87
82
Load of IPU (mg)
(%)
74
0.35
57
0.27
1570
7.41
301
1.42
L7
±
4.2
L8
L9
±
13
0.001
0.05
0.001
0.09
0.0
0.0
0.0
0.0
L10
±
9.9
±
4.2
0.021
2.0
0.001
0.11
101
0.47
3.2
0.01
a
Lysimeters L6 and L7 were not used in 1993/1994 and there was no percolate from lysimeters L5 and L6 in
1995.
(volume 100 cm3) were saturated to a water suction of 0.01 bar and drained by a stepwise increased overpressure of up to 1 bar. The multi-step outflow characteristics of the
samples were then used for an inverse identification of their hydraulic parameters (van
Dam et al., 1994). Additionally, the saturated hydraulic conductivity and saturated water
content of the samples were determined by standard methods. Table 6 summarises the
results for sites I and III. At field plot VIII no hydraulic properties were determined, but
as sites I and VIII are very similar regarding texture, organic carbon content and other soil
properties it can be safely assumed they also have similar hydraulic properties
(Montenegro et al., 1998).
In view of the experimental conditions and the restrictions of the parametric model the
results only characterise the hydraulic properties of the soil matrix but not of the large
Table 5
Yearly averages and yearly sums of meteorological parameters measured at the Weiherbach station (potential
evapotranspiration calculated according to Penman-Monteith)
Parameter
1991
1992
1993
1994
1995
Air temperature (8C)
Wind velocity (m sÿ1)
Net radiation (W mÿ2)
Precipitation (mm)
Potential evapotranspiration (mm)
9.61
3.32
56.0
512.9
784.1
10.36
3.26
59.6
786.0
784.4
9.99
3.54
59.4
699.7
806.0
11.18
3.53
59.1
828.9
745.8
10.15
3.52
55.6
1099.9
747.0
50
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Table 6
Soil hydraulic parameters for sites I and III (saturated hydraulic conductivity ksvalid for the soil matrix only)
Site
Depth (m)
a (cmÿ1)
n
yr (cm3 cmÿ3) ys (cm3 cmÿ3) ks (cm hÿ1)
Water content at
pF 1.8
pF 2
pF 4.2
I
0.0±0.3
0.3±1.0
0.015
0.005
1.30
2.25
0.03
0.08
0.46
0.45
0.5
0.3
0.400
0.436
0.372 0.115
0.413 0.082
III
0.0±0.3
0.3±1.0
0.040
0.020
1.20
1.25
0.10
0.08
0.43
0.41
0.4
0.3
0.362
0.359
0.343 0.192
0.339 0.159
macropores. As the macropores are mainly worm holes there abundance depends on the
biological activity of the soil. Typically, about five times more macropores are found in
the Cumulic Anthrosols that are relatively wet and rich in clay than in the eroded
Regosols at the hill tops (Zehe, 1997). That explains why on average the directly
measured saturated hydraulic conductivity (including macropores) is five times higher
than the one derived from the inverse approach in one of the Regosols and ten times
higher in a Cumulic Anthrosol (Montenegro et al., 1998).
The bulk densities of the loess soils range from 1.2 to 1.6 kg dmÿ3. It was proposed to
use an average bulk density of 1.3 kg dmÿ3 in the plough layer and 1.5 kg dmÿ3 in the
subsoil for modelling purposes. These values were also used for the calculation of
substance-per-area values from measured concentrations in all tables. An exception was
made for those data concerning the field studies in 1995, when the bulk densities of
different soil layers were measured at the first and last sampling date. In this case, data for
the intermediate dates were obtained by linear interpolation (Table 2).
4.3. Plant parameters
Plant parameters raised for different crops in the Weiherbach area include the plant
height, the leaf area index, the crop cover, the root length density and the yield of grain
and straw (Ritz, 1996). As these data are not available for field site VIII, rough estimates
based on measured data from similar topographical positions are given in Table 7. It must
be noted, though, that in 1995 the summer barley was sowed later (7 April 1995) on the
field plot than on all comparable sites of the Weiherbach area. The cropping data for that
year should therefore be used with care. Unfortunately, an indication of the root
distribution is impossible for both years for the same reason.
4.4. Sorption and degradation studies with IPU in the laboratory
The sorption coefficients (Kd-values) for IPU were determined in batch experiments
using a soil-to-water ratio of 1 : 2.5 and an equilibration time of 24 h (Mokry, 1996a per.
com.). The soil material originated from site I, with soil characteristics very similar to
that of field plot VIII.
Three laboratory incubation experiments were carried out to study the influence of
different environmental factors on the degradation of IPU. MuÈller (1995) investigated the
51
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Table 7
Cropping data for ®eld plot VIII: growth stages (BBCH code; Lancashire, 1991), plant height, leave area index
(LAI) and crop cover (all values estimated)
Crop
Date
Julian
day
Growth stage
(BBCH)
Plant height
(cm)
LAI
(m2 mÿ2)
Crop cover
(%)
Winter
wheat
28 October 1993
14 November 1993
6 December 1993
1 April 1994
1 February 1994
1 March 1994
1 April 1994
301
318
340
1
32
60
91
sowing
10
13
21
21
23
25
0
1
8
10
10
12
20
0
0
0.2
0.5
0.5
0.6
1
0
0
8
12
12
15
20
Summer
barley
7 April 1995
17 April 1995
1 May 1995
1 June 1995
1 July 1995
1 August 1995
97
107
121
152
182
201
sowing
10
21
40
75
91
0
1
15
60
85
80
0
0
1
5
4.5
4
0
0
20
70
80
75
degradation of IPU under three different temperature (5, 15, and 258C) and up to eight
different moisture regimes (5±100% of maximum water holding capacity, MWHC4) in
soils from sites I and III (sampling depth: 0.0±0.2 m, sampling date: 13 January 1995).
Before the incubation at 5, 15, and 258C the soil samples had been stored for 18, 10, and
6 weeks, respectively, in loosely covered plastic boxes and kept moist. The microbial biomass of the samples was determined by the substrate induced respiration method (Thun
and Herrmann, 1949). The soil samples were then incubated under aerobic conditions in
the dark and the residual amount of herbicide was determined at six dates over 31 days (at
258C), 57 days (at 158C) and 97 days (at 58C) after application. The change of sorption
over time and the corresponding desorption coefficients Kdes were examined by water
extraction of selected samples (parallel aliquots) incubated at 5 and 158C and at 40% of
MWHC, in addition to the extraction of total residues with methanol. The adsorbed mass
was calculated by the difference of the fraction extracted with methanol and the water
soluble fraction.
In a second incubation study, Mokry (1996b per. com.) used soil samples from
the plough layers (0.0±0.3 m) of field sites I and III taken in April, July, and October
1990 to determine the seasonal variability of IPU degradation. A third incubation study
was conducted to characterise the influence of the microbial activity on the dissipation of
IPU, where top soil samples from sites I and III (sampled in December 1992) were
incubated under sterile and non-sterile conditions. This study also included subsoil
samples from two different depth ranges from site I (1±2 and 2±3 m; Mokry, 1996c
per. com.).
4
According to (Thun and Herrmann, 1949) sieved soil samples were ®lled into ceramic ®lters and saturated
from below using clay cylinders standing in water. After 24 h, the MWHC of the samples was determined by
differential weighing.
52
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Table 8
Kd-va1ues (ml gÿ1) for isoproturon as a function of equilibration time and pro®le depth (soil samples from site I)
Equilibration time (min)
10
1440
2880
Profile depth (m)
0.0±0.3
0.3±0.6
0.6±0.9
1.95
2.06
2.00
1.72
1.84
1.53
1.64
1.68
1.53
5. Results of the laboratory studies
The results of the sorption studies are given in Table 8. IPU mainly sorbs to the clay
fraction of the soil while organic carbon has less influence. As the Calcaric Regosols of
the Weiherbach area show a more or less uniform clay content there was only a small
decrease in sorption with increasing soil depth. The Kd-values determined in this study
(1.7±2.1) seem rather high compared to literature data for similar soils (1.0±1.3). As an
alternative the initial Kdes-values from the incubation study described in the next
paragraph (1.0±1.1 at 158C) could be used for modelling purposes.
In the incubation study with three temperature and eight soil moisture regimes the soil
samples from hilltop (site I) and bottom (site III) showed a similar degradation behaviour
(Fig. 1). This can be explained by opposite influences of different soil properties
cancelling each other out, such as clay content and organic carbon content that positively
influence both sorption and biological activity (degradation). At 258C, the calculated
half-life times ranged from 690 days at 5% of MWHC to nine days at 60% MWHC which
Fig. 1. Degradation rates of IPU (determined by linear regression after log-transformation of the degradation
data assuming ®rst-order kinetics) in topsoil samples from sites I and III incubated at different soil water
contents and different temperatures.
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
53
Fig. 2. Time-dependency of the Kdes-values for IPU in soil samples from sites I and III.
was the optimum water content for degradation. At higher water contents microbial
activity was restricted by oxygen deficiency. An increase in temperature (within the
examined range) led to a higher degradation rate, the increase from 5 to 158C being
smaller than that from 15 to 258C. Regarding the long-term sorption behaviour of IPU an
increase of the Kdes-values up to seven times the initial values after 60 days was found
when the sorption apparently had reached an equilibrium (Fig. 2).
The second incubation study showed a significant influence of the sampling date on the
degradation rates. Soil samples taken in July showed the fastest degradation followed by
samples from October, while the decrease of IPU in the April-soil was even slower. The
measured microbial bio-mass of the soils turned out to be almost constant over the
seasons, however. This is due to the fact, that the substrate induced respiration method
only measures the potential respiration activity, which is a relatively constant soil
property.
In the third incubation study with top and subsoil samples and different pre-treatment
of soil, the degradation of IPU was fastest in the non-sterile top soil with a half-life time
of around 20 days (Fig. 3). In the subsoil samples only 40% of the IPU had disappeared at
the end of the study after 100 days. The degradation due to abiotic processes amounted to
only 20% of IPU after 100 days.
6. Results of the ®eld studies
The first study at field plot VIII (KruÈger, 1994) was conducted from December 1993 to
April 1994 under very wet conditions. The cumulative precipitation was 365 mm, which
is considerably higher than the long-term average during that period (260 mm). The
cumulative volume of leachate (measured in the parallel lysimeter study) summed up to
54
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Fig. 3. Degradation of IPU in the laboratory in samples from different depths of site I and in a sterilised top soil
sample.
228 mm. Soil temperatures dropped below 08C only for one week, with a minimum
temperature of ÿ2.58C at the 0.02 m depth.
The maximum depth where IPU was retrieved was 0.65 m in mid-January (Fig. 4),
while the conservative tracer bromide was leached below the sampling depth of 0.95 m
till April 1994. So this 1993/1994 field experiment qualifies as a `worst case' scenario
with unusually deep leaching of the herbicide. The average retardation coefficient of IPU
in relation to bromide was 4.0.
During the second field experiment starting in May 1995 (Zimmermann, 1996)
precipitation and evapotranspiration were almost equal (Fig. 5). These are slightly wetter
than normal conditions as in the long-term average the evapotranspiration dominates
precipitation in May and June. For the last 10 days before the application of the herbicide
the sum of rainfall was only 6 mm compared to 42 mm of evapotranspiration. The
herbicide was applied on relatively dry soil, the initial moisture content of plots VIII/1
and VIII/2 being 59 and 66% of the field capacity, respectively.
IPU was found in depths of up to 0.5 m in the non-irrigated field plot VIII/1 with
leaching mainly occurring during the first 14 days after application (Fig. 6). Due
to the irrigation that started 10 days after the application of IPU the percolation in plot
VIII/2 was sevenfold higher and the herbicide reached a depth of up to 0.7 m (Fig. 7).
Most of the leaching again occurred during the first days after application and
was caused by natural rainfall, though. In relation to bromide, retardation coefficients
of 1.3±2.0 could be calculated. These very low Rd-values indicate that there probably
was preferential transport of the herbicide in macropores. The overall half-life time
of IPU was ca. 9 days under natural rainfall conditions and ca. 11 days in the irrigated
field plot. This slower dissipation can be attributed to the smaller degradation rates in
deeper soil layers that more of the herbicide was leached to in shorter time due to the
irrigation.
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Fig. 4. IPU pro®les at selected sampling dates during the 1993/1994 ®eld study (late autumn application).
55
56
I. Schierholz et al. / Agricultural Water Management 44 (2000) 43±61
Fig. 5. Cumulative precipitation, evapotranspiration (calculated according to Penman-Monteith), irrigation and
percolation (calculated from water balance) during the 1995 ®eld study (spring application).
Regarding the lysimeter studies it must be noted that ponding and subsequent overflow
of some lysimeters seem to have occurred during the 1993/1994 experiment. The sums of
percolate as well as the mass balance of the tracer (checked by sampling some of the soil
columns at the end of the experiment) indicate a loss of water and tracer. A corresponding
loss of IPU can be assumed. This in part explains the great differences in percolate
volume between the lysimeters (Table 4), although these differences also reflect the
natural spatial variability of the field soil. The average cumulative IPU load in the
percolates in this study was 0.14% of the applied amount. Yet significant amounts of IPU
were only found in the early percolates of lysimeters L4 and L10, with concentrations of
up to 500 mg lÿ1. This points to possible preferential transport of IPU, associated with the
high volumes of percolate (ca. 70 mm per lysimeter) during the first three weeks of the
experiment.
In the second experiment the amount of percolate clearly depended on the irrigation
regime of the lysimeters: from the non-irrigated variants no more than 15 mm of
percolate were sampled compared to the 10±20-fold amount from the irrigated variants
(Table 4). As can be seen in Fig. 8 (a) and (b), there are clear indications of preferential
flow in the heavily irrigated lysimeters L3 and L4 (280 mm of irrigation). An early
breakthrough with very high concentrations of up to 150 mg lÿ1 of bromide and up to
300 mg lÿ1 of IPU due to the transport in macropores is followed by a much broader peak
due to transport in the soil matrix. The lighter irrigation (140 mm) of lysimeters L1 and
L2 on the other hand was not enough to induce macropore flow although it enhanced the
leaching of bromide (Fig. 8a). Because of its relatively rapid degradation there is no
corresponding IPU peak. The average cumulative load was 4.4% of the applied IPU for
lysimeters L3 and L4, 0.3% for LI and L2, and