The Weiherbach data set:
An experimental data set for pesticide model
testing on the ®eld scale
I. Schierholz
a, D. SchaÈfer
b,*, O. Kolle
caState Agricultural Research Station of Baden-WuÈrttemberg (LUFA Augustenberg), Neûlerstr. 23,
D-76227 Karlsruhe, Germany
bHoechst Schering AgrEvo GmbH, Hoechst Works, G 836, D-65916 Frankfurt am Main, Germany cInstitute 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).
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 agro-chemicals 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
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 (73 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
Table 1
Soil characteristics of ®eld plot VIII and of sites I and III Depth
1 Gauss-KruÈger-coordinates of one edge point of the plot, EEastern/NNorthern. 2The soil horizons are denoted according to the German soil classi®cation system, i.e. A
pplough layer,
Msubsoil horizon developed from eroded soil material, ICparent material (loess).
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: 3.53 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
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 Depth (cm)
0±5 5±10 10±15 15±20 20±25 25±35 35±45 45±55 55±75 15 May 1995 1.099 1.100 1.106 1.101 1.131 1.241 1.363 1.385 1.442 16 June 1995 1.343 1.357 1.335 1.315 1.219 1.236 1.349 1.388 1.464
3The doubled application rate was chosen to follow IPU dissipation under ®eld conditions for a longer period
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 thePuÈrckhauersoil 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 50mg 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
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
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) IPU (kg haÿ1)
Not irrigated Irrigated Not irrigated Irrigated 1993/1994 autumn study
19 May 1995 146 143 3.12 2.67
26 May 1995 140 142 2.11 2.22
29 May 1995 114 119 1.38 1.82
2 June 1995 96 105 0.55 1.49
9 June 1995 41 35 0.21 0.46
16 June 1995 22 12 0.00 0.00
aValues calculated from the measured concentrations assuming a bulk density of 1.3 kg dmÿ3in the plough
layer and 1.5 kg dmÿ3in the subsoil in 1993 and bulk densities derived from measurements at the ®rst and last
(volume 100 cm3) were saturated to a water suction of 0.01 bar and drained by a step-wise 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 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 L3 L4 L5 L8 L9 Ll0
1993/1994 autumn study
Sum of percolate (mm) 47 144 88 124 106 197 112 139 Load of bromide (g) 0.16 0.70 0.20 0.22 0.35 0.89 0.53 0.91
(%) 15 66 19 21 35 84 50 86 Sum of percolate (mm) 124 102 239 201 4.2 13 9.9 4.2 Load of bromide (g) 0.52 0.73 0.61 0.87 0.001 0.001 0.021 0.001
(%) 49 69 58 82 0.05 0.09 2.0 0.11
Load of IPU (mg) 74 57 1570 301 0.0 0.0 101 3.2 (%) 0.35 0.27 7.41 1.42 0.0 0.0 0.47 0.01
aLysimeters L6 and L7 were not used in 1993/1994 and there was no percolate from lysimeters L5 and L6 in
1995.
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) 9.61 10.36 9.99 11.18 10.15 Wind velocity (m sÿ1) 3.32 3.26 3.54 3.53 3.52
Net radiation (W mÿ2) 56.0 59.6 59.4 59.1 55.6
Precipitation (mm) 512.9 786.0 699.7 828.9 1099.9 Potential evapotranspiration (mm) 784.1 784.4 806.0 745.8 747.0
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
Table 6
Soil hydraulic parameters for sites I and III (saturated hydraulic conductivityksvalid for the soil matrix only)
Site Depth (m) a(cmÿ1) n yr(cm3cmÿ3) ys(cm3cmÿ3) ks(cm hÿ1) Water content at
pF 1.8 pF 2 pF 4.2 I 0.0±0.3 0.015 1.30 0.03 0.46 0.5 0.400 0.372 0.115
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 bio-mass 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 coefficientsKdes 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.).
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 Winter 28 October 1993 301 sowing 0 0 0 wheat 14 November 1993 318 10 1 0 0 6 December 1993 340 13 8 0.2 8
1 April 1994 1 21 10 0.5 12
1 February 1994 32 21 10 0.5 12 1 March 1994 60 23 12 0.6 15
1 April 1994 91 25 20 1 20
Summer 7 April 1995 97 sowing 0 0 0 barley 17 April 1995 107 10 1 0 0
1 May 1995 121 21 15 1 20
1 June 1995 152 40 60 5 70
1 July 1995 182 75 85 4.5 80 1 August 1995 201 91 80 4 75
4According 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.
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. TheKd-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
Table 8
Kd-va1ues (ml g
ÿ1) for isoproturon as a function of equilibration time and pro®le depth (soil samples from site I)
Equilibration time (min) Profile depth (m)
0.0±0.3 0.3±0.6 0.6±0.9
10 1.95 1.72 1.64
1440 2.06 1.84 1.68
2880 2.00 1.53 1.53
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 theKdes-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
Fig. 2. Time-dependency of theKdes-values for IPU in soil samples from sites I and III.
228 mm. Soil temperatures dropped below 08C only for one week, with a minimum temperature ofÿ2.58C 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.
Fig. 4. IPU pro®les at selected sampling dates during the 1993/1994 ®eld study (late autumn application).
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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 500mg 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 300mg 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 <0.01% for the non-irrigated lysimeters (except L9).
Fig. 6. IPU pro®les at selected sampling dates during the 1995 ®eld study (spring application); plot VIII/1 without irrigation.
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Fig. 7. IPU pro®les at selected sampling dates during the 1995 ®eld study (spring application); plot VIII/2 with irrigation.
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7. Conclusions
The Weiherbach project offered the possibility to collect high quality data on pesticide behaviour in soil and to determine the most important meteorological, agricultural and soil parameters influencing that behaviour. The resulting data set is well suited for the testing of computer models of pesticide fate and was used for that purpose within COST
Fig. 8. (a) Breakthrough curves of bromide in the spring lysimeter study for the irrigated lysimeters L1/L2 (irrigation 60 mm per week) and L3/L4 (irrigation 30 mm per week). (b) Breakthrough curves of isoproturon in the spring lysimeter study for the irrigated lysimeters L1/L2 (irrigation 60 mm per week) and L3/L4 (irrigation 30 mm per week).
Action 66: The data set describes an agro-ecosystem in a loess region with intensive agriculture and erosion as it is commonly found in Middle Europe.
The different field and lysimeter studies were conducted under vastly different environmental conditions. Thus, the data set includes an about normal spring application scenario, a spring application scenario with heavy irrigation and an autumn application scenario with an unusually wet winter that qualifies as a natural `worst case' scenario for pesticide leaching. There were also different irrigation regimes in the lysimeter study. In addition, the data for up to eight parallel lysimeters give an estimate of the natural spatial variability of solute transport in a field.
Looking at processes, both matrix and preferential flow and transport of tracer and the test substance IPU were observed and there was a clear correlation of preferential flow events and the irrigation regime. It is also important to note the changing leaching behaviour of IPU over the course of the irrigated field experiment. Fast movement of IPU occurred only during the first days after application when most of the herbicide could still be preferentially transported. After diffusion into the soil matrix and the formation of strong sorptive interactions the amount of herbicide available for transport decreased and less leaching was observed despite significant percolation due to irrigation. This is a good example of the importance of soil conditions and precipitation patterns shortly after application for the assessment of pesticide leaching.
Acknowledgements
This work was funded by the Federal Ministry of Education and Research (BMBF) in the framework of the co-operative project `WEIHERBACH' and by the German Research Foundation (DFG). Special thanks to our numerous co-workers from the Weiherbach Project for their help and advice. Financial support by the COST 66 Action `Pesticides in the soil environment' of DGXII-EU allowed the presentation of the data set at several workshops and conferences.
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