multiple tracing of fast solute transport in a drained grassland soil

24
Multiple tracing of fast solute transport in a drained grassland soil Christian Stamm a, * , Raphael Sermet a,1 , Jo ¨rg Leuenberger a , Hans Wunderli a , Hannes Wydler a , Hannes Flu ¨hler a , Mathias Gehre b a Institute of Terrestrial Ecology, Soil Physics, Grabenstrasse 3, CH-8952 Schlieren, ETHZ, Switzerland b UFZ-Umweltforschungszentrum Leipzig-Halle, Sektion Analytik, Permoserstr. 15, 04318 Leipzig, Germany Received 4 July 2001; received in revised form 20 February 2002; accepted 17 May 2002 Abstract Fast transport of fertilizers and other agrochemicals into subsurface drainage systems has been recognized as a serious threat to surface waters. We report on a tracer experiment carried out on a 7.3 20 m 2 plot on a loamy grassland soil to determine the flow paths to a tile drain at 1 m depth. The experiment consisted of a series of consecutive tracer applications including seven solutes and liquid manure that were applied either on the entire plot or on limited bands. Based on the discharge behavior under natural conditions, we estimated the effective hydraulic conductivity of the subsoil to be in the order of 8–29 cm day 1 . Under experimental conditions, the soil transmitted 120 mm day 1 into the subsurface drain and two vertical profiles without producing surface runoff. Only part of the soil water, corresponding to 6 –27 mm of the soil depth, contributed to the fast hydrological response. The transport of the tracers was very fast. Within 7– 16 h after application of the conservative Br , Cl and HDO and the slightly sorbing substances brilliant blue (BB) and amino-G-acid (AG), these tracers reached relative concentrations in the outflow between 19% and 35% of the input concentrations. From the mass balance for water and solutes, it follows that the tracers were quickly transported over lateral distances of several meters. The manure constituents dissolved reactive P (DRP), NH 4 + and Cl , applied as liquid manure on the surface on a 1 m wide band above the tile drain, reached the drain within 5 min after application. After the early peak of DRP and NH 4 + , their concentration in the drain decreased quickly to background levels, whereas Cl exhibited a second peak. Despite the fast transport and the small soil volume conducting water and solutes, the interaction between irrigation water and soil matrix 0016-7061/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0016-7061(02)00178-7 * Corresponding author. Fax: +41-1-633-11-23. E-mail addresses: [email protected] (C. Stamm), [email protected] (R. Sermet), [email protected] (J. Leuenberger), [email protected] (H. Wunderli), [email protected] (H. Wydler), [email protected] (H. Flu ¨hler), [email protected] (M. Gehre). 1 Present address: ProCert Safety AG, Thunstr. 17, 3000 Bern 6, Switzerland. www.elsevier.com/locate/geoderma Geoderma 109 (2002) 245 – 268

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Page 1: Multiple tracing of fast solute transport in a drained grassland soil

Multiple tracing of fast solute transport in a

drained grassland soil

Christian Stamm a,*, Raphael Sermet a,1, Jorg Leuenberger a,Hans Wunderli a, Hannes Wydler a, Hannes Fluhler a, Mathias Gehre b

aInstitute of Terrestrial Ecology, Soil Physics, Grabenstrasse 3, CH-8952 Schlieren, ETHZ, SwitzerlandbUFZ-Umweltforschungszentrum Leipzig-Halle, Sektion Analytik, Permoserstr. 15, 04318 Leipzig, Germany

Received 4 July 2001; received in revised form 20 February 2002; accepted 17 May 2002

Abstract

Fast transport of fertilizers and other agrochemicals into subsurface drainage systems has been

recognized as a serious threat to surface waters. We report on a tracer experiment carried out on a

7.3� 20 m2 plot on a loamy grassland soil to determine the flow paths to a tile drain at 1 m depth.

The experiment consisted of a series of consecutive tracer applications including seven solutes and

liquid manure that were applied either on the entire plot or on limited bands. Based on the

discharge behavior under natural conditions, we estimated the effective hydraulic conductivity of the

subsoil to be in the order of 8–29 cm day � 1. Under experimental conditions, the soil

transmitted 120 mm day� 1 into the subsurface drain and two vertical profiles without producing

surface runoff. Only part of the soil water, corresponding to 6–27 mm of the soil depth,

contributed to the fast hydrological response. The transport of the tracers was very fast. Within 7–

16 h after application of the conservative Br � , Cl� and HDO and the slightly sorbing substances

brilliant blue (BB) and amino-G-acid (AG), these tracers reached relative concentrations in the

outflow between 19% and 35% of the input concentrations. From the mass balance for water and

solutes, it follows that the tracers were quickly transported over lateral distances of several meters.

The manure constituents dissolved reactive P (DRP), NH4+ and Cl� , applied as liquid manure on the

surface on a 1 m wide band above the tile drain, reached the drain within 5 min after application.

After the early peak of DRP and NH4+ , their concentration in the drain decreased quickly to

background levels, whereas Cl � exhibited a second peak. Despite the fast transport and the small

soil volume conducting water and solutes, the interaction between irrigation water and soil matrix

0016-7061/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0016 -7061 (02 )00178 -7

* Corresponding author. Fax: +41-1-633-11-23.

E-mail addresses: [email protected] (C. Stamm), [email protected] (R. Sermet),

[email protected] (J. Leuenberger), [email protected] (H. Wunderli),

[email protected] (H. Wydler), [email protected] (H. Fluhler), [email protected] (M. Gehre).1 Present address: ProCert Safety AG, Thunstr. 17, 3000 Bern 6, Switzerland.

www.elsevier.com/locate/geoderma

Geoderma 109 (2002) 245–268

Page 2: Multiple tracing of fast solute transport in a drained grassland soil

was intimate enough to retain the two sorbing tracers. From the stained flow paths, the hydrologic

behavior of the field under natural conditions and the hydrometric data during the experiment, it

follows that the fast lateral tracer transport occurred mainly close to soil surface and not through the

subsoil. Only in the immediate vicinity of the tile drain and of two lateral pits at the edge of the

experimental plot water was redirected downwards and discharged from the tile drain and the bottom

parts of the profiles, respectively. Hence, effluent from tile drains may not be representative for water

reaching the subsoil or shallow ground water in undisturbed soils.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Tile drains; Agriculture; Preferential flow; Surface runoff; Sprinkling experiment; Dye tracers

1. Introduction

The flow patterns occurring in natural and managed soils are in many, if not most cases,

highly irregular. This irregularity is often called preferential flow, which means that part of

the water and solutes move rapidly through a small portion of the soil without much

exchange with the surrounding soil matrix over a substantial length of the flow domain

(Fluhler et al., 1996). The flow velocities are large and even strongly sorbing solutes like

pesticides (Flury, 1996; Flury et al., 1995; Kladivko et al., 2001), phosphorus (Addiscott et

al., 2000; Heckrath et al., 1995; Hergert et al., 1981a,b; Sims et al., 1998; Stamm et al.,

1998) or radionuclides (Bundt et al., 2000) may reach groundwater or subsurface drainage

systems before being sorbed or biologically degraded.

Due to the large sampling volume, subsurface drainage systems are sometimes taken

as ideal experimental systems for transport phenomena in soils (Czapar and Kanwar,

1991; Flury, 1996; Gish et al., 2000; Lennartz et al., 1999). Such studies rely on the

assumption that sampling large volumes averages out the small-scale spatial hetero-

geneities. Classical drainage theory assumes a vertical transport through the unsaturated

and a predominantly lateral flow through the saturated zone towards the drain. If this

concept was true and preferential transport into the drains observed an interconnected

system of vertical and lateral preferred flow paths should exist. This has been shown,

e.g., for some heavy clay soils (Inoue, 1993; Ruland et al., 1991) or for forest soils

(Luxmoore et al., 1990).

We have found preferential P transport into subsurface drains in weakly structured

loamy soils (Stamm et al., 1998). Based on infiltration experiments, we concluded that

vertical worm burrows are the main macropore structures in our study region. In one case,

we observed lateral transport of a dye tracer and we could attribute this to a dense network

of well-preserved ancient root channels in the saturated zone. Such structures are rather

exceptional features. Therefore, it is not evident what the lateral preferred flow paths

towards a tile drain are in weakly structured soils. The purpose of this paper is to

investigate the fast transport into a subsurface drain in such a soil and to study how the

lateral distance to the drains affects the fast solute transport. We carried out a sprinkling

experiment on a drained grassland plot. The flow paths were assessed based on the

breakthrough behavior of spatially separated tracers, hydrometric data and infiltration

patterns in the soil.

C. Stamm et al. / Geoderma 109 (2002) 245–268246

Page 3: Multiple tracing of fast solute transport in a drained grassland soil

2. Study site and methods

2.1. Site description

The study site is situated in a subcatchment (‘‘Kleine Aa’’) of Lake Sempach in the

central Swiss Plateau at an altitude of 585 m a.s.l. (Schweizerische Landestopographie,

coordinates 659,100/221,000). The mean air temperature is about 7.5 jC and the mean

annual precipitation amounts to 1200 mm. The soil has developed from glacial till (Wurm

glaciation) and is classified as a loamy, frigid to mesic Oxyaquic Eutrochrept (Soil Survey

Staff, 1992), characterized by periodic waterlogging as well as strong dryout of the profile.

Some of the soil properties are summarized in Table 1.

The tile drains of the field were installed in the 1940s to a depth of 100–155 cm.

Natural soil material was used as backfill. Even after the 50 years of settlement, the bulk

densities at 5–15 cm depth were different for samples from the backfill material

(1.24F 0.08 mg m� 3) compared to undisturbed soil (1.36F 0.03 mg m � 3). Below that

depth, no differences in bulk densities could be detected. The water characteristic

exhibited slight differences between undisturbed soil and backfill material down to a

depth of 35 cm and disappeared below (Fig. 1).

The experimental plot was part of a 1.02 ha field used as permanent grassland for

many years (Fig. 2). The usual practice in this region is to cut the grass five to seven

times during the growing season (April–October). After each mowing, liquid manure is

applied. For some weeks each year, the parcel is used as a pasture for cows. The average

slope of the experimental plot was 2.5% parallel to the tile drain and 5% perpendicular

to it.

2.2. Methods

The experiment was carried out on a plot of 146 m2 (Fig. 3). From January to March

1996, we monitored the temporal and spatial variability of the groundwater table within

the entire 1 ha field. In June, we compared the hydrological response of the experimental

plot under natural conditions with that of the entire parcel. From August 12 to 19, we

conducted the multitracer experiment on the experimental plot. During the final period

Table 1

Soil properties of the field

Horizon Depth

(cm)

Bulk density

(mg m� 3)

Clay

(%)

Sand

(%)

Organic

mattera (%)

pHb

A (A) 0–10 1.25 – – – 5.6

B (B) 10–30 1.50 28 29 2.7 5.5

Bw (B-Sw) 30–80 1.55 27 27 0.6 6.4

C (Sd-C) > 80 1.75 17 40 0.6 7.9

The nomenclature refers to the US soil taxonomy (Soil Survey Staff, 1992). The symbols corresponding to the

German nomenclature are added in parentheses (Arbeitsgrupped Bodenkunde, 1982).a Determined from weight loss after digestion with H2O2.b Measured in 0.01 M CaCl2 solution.

C. Stamm et al. / Geoderma 109 (2002) 245–268 247

Page 4: Multiple tracing of fast solute transport in a drained grassland soil

Fig. 1. Soil water characteristics of the undisturbed soil (six samples per depth from one profile) and the backfill

material (10–15 samples from two profiles) for three depths.

Fig. 2. Experimental area with tile drains, contours, location of piezometers and location of the experimental plot.

C. Stamm et al. / Geoderma 109 (2002) 245–268248

Page 5: Multiple tracing of fast solute transport in a drained grassland soil

(August 20–September 3), we excavated 17 soil profiles to study the stained infiltration

patterns of the dye tracers.

2.2.1. Discharge and water table measurement in the entire field

We installed 29 piezometers arranged in three downslope rows over the entire parcel

(Fig. 2). The locations of the piezometers were chosen such that half of them were in

immediate vicinity to the drains whereas the others were in-between two neighboring

drains. From January 10 to March 11, we determined the level of the water table by weekly

manual measurements. Daily precipitation values for this winter period were obtained

from a weather station 2 km away.

In June, discharge from the main drain leaving the field was measured with an inductive

flow meter (DISCOMAG TDMI 6731, Endress & Hauser, Reinach, Switzerland). Rainfall

intensity was recorded on-site by a rain gauge at 15 min intervals. Samples of the drainage

water were taken by an automatic sampling device (ISCO 2900, Lincoln, NEB) whenever

a predefined discharge level was exceeded. Samples were filtered within 24 h using 0.45

Am filters (Schleicher & Schuell, FP 030/20, Keene, NH). Sampling and discharge

measuring from the experimental plot are described below.

2.2.2. Sprinkling experiment

The experimental plot had an area of 20� 7.3 m2 covered by a tent during the

experiment in August. Parallel to the long, lower side of the plot, there was a tile drain

installed 50 years ago (see above) at 100 cm depth (Fig. 3) at a lateral distance of 0.5 m

from the lower edge of the irrigated area (inside the plot). Two pits, at the upper (PU) and

the lower (PL) ends, each 1.15 m deep, were excavated over a distance of 2.5 m at the

Fig. 3. Set-up of the experimental plot.

C. Stamm et al. / Geoderma 109 (2002) 245–268 249

Page 6: Multiple tracing of fast solute transport in a drained grassland soil

short sides of the plot to install instruments (see below). The profile walls were protected

and stabilized by planks. The rest of the boundary was undisturbed soil.

The plot was sprinkled by a moving spray bar that was oriented perpendicular to the tile

drain. Sprinkling was stopped at 0.25 m from the profile walls. We irrigated the plot at a

rate of approximately 5 mm h� 1 (Fig. 5). The sprinkler was running on two aluminum

tracks and consisted of 24 nozzles (TeeJet 80010LP, Spraying Systems, Wheaton, IL)

installed 30 cm apart and 30 cm above ground. The sprinkling solution was applied at a

pressure of 2.2 bar inside the nozzles.

Before applying the tracers, we irrigated the plot with tap water for 2 days. Afterwards,

we sequentially applied different tracer combinations for periods of 7–16 h (Table 2). In

order to detect lateral transport towards the tile drain, we simultaneously applied different

tracers on two bands parallel to the drain (Fig. 2): the lower band (LB) covered the surface

directly above the tile drain, while the upper band (UB) was about 2.5 m from the drain. The

first tracer combination consisted of the three conservative tracers Br� , Cl � and HDO

(Table 2). With the second combination, slightly sorbing fluorescent dyes were applied

[amino-G-acid (AG; 7-amino-1,3-naphthalene disulfonic acid, Aldrich, Buchs, Switzer-

land), brilliant sulfaflavine (SF; Sigma, St. Louis, MO) and sulforhodamine B (SB; C.I.

45100, Siegfried, Zofingen, Switzerland)]. Next followed 20 l of liquidmanure spread with a

watering can on the area of LB just above the tile drain. Finally, we applied two dye tracers

[acid red 1 (AR; C.I. 18050, Sigma) and brilliant blue FCF (BB; C.I. 42090, Hoechst,

Frankfurt, Germany)] to stain the infiltration pathways. Immediately afterwards, the

irrigation was terminated. Between the different tracer combinations, tap water was applied.

The tracer concentrations in the sprinkling solution and in the manure are given in Table 3.

Discharge from the tile drain was measured by a tipping bucket. Surface runoff was

collected right above the tile drain being the lowest location of the soil surface inside the

plot and measured by a tipping bucket. To prevent tracer transport at the soil surface from

area UB towards the drainage area, a PVC panel was inserted 5 cm into the topsoil at the

lower side of the area UB. The seepage rate from the upper profile PU into the upper pit

Table 2

Experimental schedule of the tracer applications

Time (days) Tracer

Lower tracer band (LB) Upper tracer band (UB) Main area (MA)

0.00–2.00 Water Water Water

2.00–2.67 HDO, NH4Br NaCl, NH4Br NH4Br

2.67–2.92 Water Water Water

2.92–3.63 No application No application No application

3.63–4.13 Water Water Water

4.13–4.80 Amino-G-acid and

sulforhodamine B

Brilliant sulfaflavine Water

4.80–5.13 Water Water Water

5.13 Liquid manure Water Water

5.13–6.00 Water Water Water

6.00–6.29 Brilliant blue Acid red 1a Water

Start of the experiment (t= 0) was on August 12 at 4 PM. For the experimental design, see Fig. 3.a Applied only over a distance of 2 m from the lower end. The rest of UB was not irrigated during this period.

C. Stamm et al. / Geoderma 109 (2002) 245–268250

Page 7: Multiple tracing of fast solute transport in a drained grassland soil

was determined by the rate of water table increase in the deepest part of the pit. This rate

was measured occasionally using a stopwatch and yardstick installed in the pit. Seepage

from the lower profile PL was measured with bucket and stopwatch at one location in the

pit, where the water was channeled and flowing into the deepest part.

Samples from the drainage effluent were taken by automatic samplers (ISCO 2700 and

2900). The sampling interval varied from 3 min (for the conservative tracers) or 5 min (all

other tracers) immediately following the changes in tracer composition to 60 min later on.

Samples were taken manually from the seepage water at the locations PU and PL. The

samples were filtered within few hours (0.45 Am; Schleicher & Schuell, FP 030/20) and

stored at 5 jC under dark conditions. Samples for HDO analysis were stored in sealed

vials to prevent gas exchange. Samples for the fluorescent dyes were stored in brown snap-

cap vials.

Two-rod TDR probes of 15 cm length were installed horizontally at four depths (15, 30,

50 and 80 cm) in the two profiles PU and PL. The center of each rod was placed at 50 cm

horizontal distance from the profile faces. In both profiles, two columns of TDR probes

were installed in duplicates, one row adjacent to the tile drain and the second at 2.5 m

uphill. Tensiometers were inserted into the profiles PU and PL corresponding to the layout

of the TDR probes. The center of the cup (High Flow Porous Ceramic Cub 653� 1B1M3

1bar, Soil Moisture Equipment, Goleta, CA) was placed at 50 cm from the profile. Within

the plot, additional tensiometers were installed vertically at seven locations. At each

location, one or two tensiometers were installed at the same depths as in the profiles. The

matric potential was measured every 5 min by pressure transducers (26 PCCFA3D,

Honeywell, Minneapolis, MN).

2.2.2.1. Infiltration patterns. We prepared and photographed 17 soil profiles of 1�1 m2

stained by the dye tracers to analyze the infiltration patterns according to Forrer et al.

Table 3

Tracer concentrations in the sprinkling solutions

Tracer Mean concentration (mg l� 1) Coefficient of variation (%)a

Conservative tracers

Br 60.8 1.3

Cl 1670 1.9

HDO 2244 5.5

Fluorescent dyes

Amino-G-acid 17.2 8.3

Sulforhodamine B 17.9 1.7

Sulfaflavine 12.3 4.5

Manure

Cl 1830 < 0.1

NH4 218.8 b

DRP 57.3 11.1

Nonfluorescent dyes

Acid red 1 12,000 b

Brilliant blue 4100 13.7

a Measured in the different cups sampling the sprinkling water on the plot.b Only one sample was measured.

C. Stamm et al. / Geoderma 109 (2002) 245–268 251

Page 8: Multiple tracing of fast solute transport in a drained grassland soil

(2000). After preparing a profile face as smooth as possible, the pit was covered with a

light tent to produce diffuse illumination. A uniform gray frame was mounted around the

profile to correct the pictures afterwards for inhomogeneous illumination. We used

Ektachrom Elite (100 ASA) and Ektachrom Panther (200 ASA) film for color slides.

On five profiles, the dye patterns were categorized according to structure types (e.g., worm

burrows or plant roots) that had caused the preferential dye transport.

2.2.2.2. Laboratory analysis. Phosphorus was determined as soluble-reactive P accord-

ing to Vogler (1965) and NH4+ was measured according to DIN 38,406 (1993). We

measured brilliant blue concentrations with a spectrophotometer at 630 nm (PU 8620,

Philips Scientific, Cambridge, UK) and the fluorescent dyes with a spectrofluorimeter

(Jasco FP 821, Tokyo, Japan). Amino-G-acid (excited at 355 nm) was determined from the

emission difference between 400 and 420 nm, sulforhodamine B (excited at 565 nm) from

the difference between 575 and 580 nm and brilliant sulfaflavine (excited at 420 nm) from

the difference between 505 and 525 nm. We measured the anionic tracers (Cl� and Br� )

by an ion chromatograph (DX-100, Dionex, Sunnyvale, CA). The HDO concentrations

were measured by mass spectroscopy according to Gehre et al. (1996a,b).

3. Results

3.1. Natural conditions

3.1.1. Spatial and temporal variation of the water table in the field

As expected, the level of the water table was substantially higher between two drains

than in the immediate vicinity of the drains (Fig. 4). The drainage system was functional

despite its age of about 50 years. During February 1996, snow (62.5 mm) fell on frozen

ground and melted at the end of the month. The melting water raised the water table from

February 6 to 28 close to the surface followed by a pronounced decrease during the dry

period from February 28 to March 11. The dynamics of the water table position (Fig. 4)

allows estimating the effective hydraulic conductivity of the subsoil in that field. This

estimation is based on the equation for the water table dynamics in the half-space between

two drains developed by Youngs (1999):

Hm ¼ H0mð1þ tKeff

S ða � 1ÞðH0mÞ

a�1S�1D�aÞa�1 ð1Þ

where Hm is level of the water table between two drains relative to the depth of the drain

expressed as a function of time t, Hm0 is the value of Hm for a given time zero, a is the ratio

between the depth Zb of an assumed impermeable layer below the drain and D the half of

the distance between the two drains, Kseff is the effective saturated hydraulic conductivity

and S is the specific yield, which is the change of the water storage in the profile given a

change of the level the water table of a given unit.

Assuming a constant value for S over the profile and over time, we can estimate an

upper limit for S based on the amount of precipitation and the change in the water table

from the dry condition on February 6 to the wet condition after snow melt on February 28.

C. Stamm et al. / Geoderma 109 (2002) 245–268252

Page 9: Multiple tracing of fast solute transport in a drained grassland soil

The values range from 0.091 to 0.112 mm mm� 1 for eight pairs of piezometers. Solving

Eq. (1) for K seff, using the obtained values for S and varying Zb over a range from 0.01 to 3

m allows estimating of the hydraulic conductivity. Based on the data for the eight pairs of

neighboring drains with the required time series, we obtained for the draining period

(February 28–March 5) the mean values of 1.3F 0.3 and 0.6F 0.1 cm day � 1 for the two

extreme values of Zb, respectively. For the prolonged draining period until March 11, the

corresponding results were of a similar order of magnitude, being 0.6F 0.1 and

0.36F 0.09 cm day � 1, respectively.

These values correspond to the behavior of the integrated subsoil drainage system. In

order to estimate the hydraulic properties of the soil itself, one has to consider the nonideal

behavior of real drains (Dierickx, 1999) and account for the influence of the limited

openings for the water to enter real drain tubes. Based on the analysis of Youngs (1974),

the saturated conductivity of the soil Kssoil can be expressed as a product of the

conductivity of the soil drainage system and a factor accounting for the flow geometry

and the entry resistance b:

KsoilS ¼ Keff

S

2pb þ logð DpR0

Þlogð D

pR0Þ ð2Þ

Fig. 4. Spatial and temporal variability of the water table along the second transect comprising piezometers 11–

20. The date of February 6 corresponds to a dry period, February 28 to the end of the melting period and March 5

to the first measuring date of the dry draining period.

C. Stamm et al. / Geoderma 109 (2002) 245–268 253

Page 10: Multiple tracing of fast solute transport in a drained grassland soil

with R0 being the radius of the tile drain (5 cm). Assuming a large value for the entry

resistance of the tiles of five (Youngs, 1974), the estimated hydraulic conductivity of the

subsoil ranges between 8 and 29 cm day � 1. These values correspond well with data

reported from the literature (Leij et al., 1996) for loamy soils.

3.1.2. Natural rainfall event

A single natural rainfall event occurred during the observation period of June 1996

(June 10). It was a short but intensive thunderstorm. Prior to the storm, there was no

discharge from the experimental plot at 1 m depth. In contrast, the main drain at the field

outlet was discharging, probably caused by the larger depth (1.55 m) of the drain as

compared to that of the lateral plot drain and caused by the uphill position of the

experimental plot (Fig. 2).

The drainage outflow responded quickly to rainfall. Peak discharge was measured 15

min after peak precipitation intensity. Discharge from the plot drain decreased rapidly after

precipitation had ceased. Flow from the whole field decreased more slowly and continued

for days at rates higher than those prior to the event. The discharge/rainfall ratio was much

larger for the whole field (13.7% for the period of 3 h during which discharge occurred

from the plot) than for the plot (1.3%). The about 10-fold larger discharge/rainfall ratio for

the whole field suggests that substantial lateral flow towards the tile drain was induced in

those parts of the field where the water table was above the tile drain.

As found in a previous study (Stamm et al., 1998), the dissolved reactive P (DRP)

concentration increased with discharge. The values from the experimental plot (maximum

value 680 Ag l� 1) were about 2.5 times higher than those from the entire field. This

indicates that at the larger scale the outflow of subsurface water low in P was contributing

relatively more than at the plot scale.

3.2. Sprinkling experiment

The sprinkling period lasted for 6.25 days with one major interruption of 18.5 h after

2.9 days due to technical problems with the sprinkler (Fig. 5). Additional short

interruptions of 2–5 min were due to switching tracer tanks and a electrical power failure.

The sprinkling rate varied between 4.44 and 5.50 mm h� 1. The spatial variability of the

sprinkling rate perpendicular to the tile drain had an average coefficient of variation of

8.9%. During the entire experiment, 95,300F 3600 l of water and tracer solutions were

applied, which is equivalent to 652F 25 mm. This corresponded to about 1.5 pore

volumes of the soil down to the depth of the tile drain. The large amount of irrigation was

necessary to apply several tracers sequentially under conditions as close as possible to

steady state.

Between days 2.33 and 2.90, the length of the sprinkled area was reduced from 20 to

14.2 m. The nonirrigated area was adjacent to the upper profile PU. Irrigation was

completely stopped between days 2.90 and 3.69. Thereafter, the period prior to this

interruption is called the first stage of the experiment and the period after the interruption

until the definite end of irrigation is called the second stage.

Immediately before the start of irrigation (t= 0), the matric potential at 80 cm depth was

about � 4.5 kPa at both profiles and about � 3.7 kPa in the center of the plot. After the

C. Stamm et al. / Geoderma 109 (2002) 245–268254

Page 11: Multiple tracing of fast solute transport in a drained grassland soil

start of sprinkling, a water table developed quickly. In the middle of the plot, the water

table was close to soil surface at a depth of about 15 cm. It declined towards the two

profiles PU and PL as expected. The influence of the tile drain was much less than that of

the two profiles. Even at 0.8 m distance from the drain, the level of the water table was not

affected by the drain such as to be measurable. After about 12 h, the hydraulic head

remained practically constant for the rest of the experiment, except during the mentioned

interruption period (Fig. 5).

3.2.1. Discharge behavior

Discharge occurred from the tile drain and from the two profiles PU and PL. Based on

the hydraulic conductivity estimated from the piezometer data (see above), one can

calculate the outflow from the two profiles and the tile drain according to Hooghoudt’s

(1937) equation for an elliptical water table. The expected outflow is in the order of 2–7

mm day� 1 or only few percent of the applied amount of irrigation. Despite this limited

discharge capacity, we did not observe any surface runoff at the lowest point of the plot

during the entire experiment. The outflow from the two profiles as well as from the tile

drain was much larger than expected based on the estimated hydraulic conductivity.

Discharge from the profile PL contributed the major portion of the total outflow volume.

For the last day of the experiment, we have data of all three outflow components to

Fig. 5. Sprinkling intensity, flow rate from the tile drain and level of the water table in the center of the plot during

the experiment of August 1996. The discharge data values indicate moving averages over five measurements.

C. Stamm et al. / Geoderma 109 (2002) 245–268 255

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compare. At 16:00 h of that day, 5.7 l min� 1 (or 56 mm day � 1) were measured from

profile PL, discharge from the tile drain was 1.9 l min� 1 (or 18.7 mm day � 1) and from

PU only 1.5 l min� 1 (or 14.7 mm day � 1). With the sprinkling rate being 12.2 l min � 1

(or 120 mm day � 1), this results in a discharge ratio of 75% for the entire plot.

Total discharge from the plot during the whole experiment was 392 mm. The increase in

water content measured by TDR accounted for 44F 11 mm and evapotranspiration (ET) for

less than 10 mm. There remained 207F 28 mm or 32F 4% of the input that was not

accounted for. The loss includes deep seepage, lateral flux out of the plot and nondetected

seepage draining from profile PL into the sinkhole of the pit where we could not measure

discharge. Influx from outside the plot could be excluded because the weather was dry.

According to the three discharge outlets, the whole plot can be divided into three

subcatchments CatchPU, CatchPL and CatchD drained by the outlets upper profile (PU),

lower profile (PL) and the tile drain (D), respectively. The relative sizes of these

subcatchments can be estimated from the discharge volumes and were therefore 16%

for CatchPU, 63% for CatchPL and 21% for CatchD. These figures are obtained by

assuming that the 32% of the water not accounted in the water balance (see above) affected

the three subcatchments proportionally to their relative size. Hence, the contributing area

of the lower profile PL CatchPL was very large and almost four times the size of CatchPU.

Considering the moderate slope of only 2.5% between the two profiles, such a large

discrepancy was surprising. The average width of the contributing area for the tile drain

was 1.5 m.

Discharge was very sensitive to any changes in the irrigation rate. It decreased within

minutes after interruptions of sprinkling (Fig. 5). From the total outflow after stopping

irrigation, one can calculate the volume Vcontr of the water stored in the soil that was

contributing to discharge. Dividing this outflow volume through the area of the subcatch-

ment results in the average thickness of Vcontr. For the tile drain, this contributing volume

was extremely small: after sprinkling was stopped at the end of Stage 1, only 57 l were

discharged until outflow ceased completely. This corresponds to only 2.7 mm of water or

the amount irrigated within about 30 min (at reduced sprinkling distance of 14.2 m). At the

end of the experiment, a total discharge of 102 l was measured, which was equivalent to 3

mm of water over the entire plot length of 20 m. From the lower profile PL, 1236 l of

discharge was measured at the end of the experiment until the flow rate dropped below 0.1

l min � 1 or less than 5% of the maximum flow rate. This volume is equivalent to 13.4 mm

water or a soil layer of about 27 mm thickness.

Given the contributing water volume Vcontr, the expected mean travel time of an inert

tracer is given as the ratio between Vcontr and irrigation rate I. With I of 5 mm h� 1 and the

volumes as given above, the mean travel times into the tile drain and towards the lower

profile PL, respectively, are expected to be very short. For the drain, this expected value

was less than 1 h and about 3 h for PL. These estimates will be compared to the measured

data in Section 3.2.2.1.

3.2.2.1. Solute transport. The tracer applications started on day 2 with the conservative

tracers Br � , Cl � and HDO (Table 2). Bromide was applied onto the whole plot, HDO

onto the lower tracer band (LB) above the tile drain and Cl � onto the upper tracer band

(UB). The comparison of the three tracers should therefore indicate whether lateral

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transport towards the tile drain occurred. In absence of a fast lateral transport, only HDO

and Br are expected to exhibit a fast breakthrough in the tile drain. It was further expected

that the two tracers should reach the same relative concentrations.

The shape of the breakthrough curves for Br and HDO were indeed very similar (Fig.

6). The concentrations started to increase after about 1 h and peaked at the end of the tracer

application with a slight increase of HDO after the end of application. After the end of the

tracer application, the concentrations decreased quickly. Chloride was only measured at

low concentrations never exceeding 1.5% of the input concentration. After 16 h or about

0.18 pore volumes being applied, the maximum Br� concentration was as high as 32% of

the input concentration, underlining the quantitative importance of the fast solute transport.

The lower value of 19% for HDO indicated that Br� was rapidly transported laterally

towards the tile drain from areas where no HDO was applied. Although the tracer

concentration in the effluent increased rapidly to large values, the measured travel times

were much larger than those expected based on the contributing water volume Vcontr (see

above). This is only possible if the volume relevant for the solute transport is larger than

Vcontr. Hence, despite the fast transport and the very wet conditions, there was significant

mixing between the irrigation water and the soil solution. However, only part of this

mixing volume contributed to the fast hydrological response. The plot behaved like a dual-

porosity system.

Fig. 6. Breakthrough of the conservative tracers Br� , Cl� and HDO in the drainage effluent.

C. Stamm et al. / Geoderma 109 (2002) 245–268 257

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Two distinct concentration peaks occurred for both tracers during their application (Fig.

6). Such peaks are completely unexpected during a constant tracer input. These peaks

occurred immediately after the two short periods when the sprinkler was mechanically

blocked and stood still for a few minutes. During these periods, sprinkling continued but

the tracer application was concentrated at one location. This suggests that not only the

flow rate but also the tracer concentrations in the effluent responded very sensitively to

changes of the irrigation conditions. The two peaks may well have been caused by local

ponding due to the high local irrigation intensity during the stop of the spray bar.

In the effluent from profile PL, the conservative tracers were detected at high

concentrations as well. In Fig. 7, the breakthrough of Br � in the tile drain and in the

effluent from profile PL is shown. The concentrations in the two different effluents were in

phase, although the maximum concentration in the effluent from profile PL was about

25% higher than in the outflow from the tile drain. Chloride was also measured at high

concentrations (up to 8% of the input concentration) in the effluent from PL. These high

tracer concentrations show that large amounts of the applied substances were transported

quickly over lateral distances of several meters.

The behavior of the fluorescent dyes (AG, SB and SF) applied on day 4 (Table 2)

confirmed the results of the conservative tracers. The two substances AG and SB were

Fig. 7. Breakthrough of Br� in the drainage water and the effluent from the lower profile PL.

C. Stamm et al. / Geoderma 109 (2002) 245–268258

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applied to the band LB above the tile drain, while SF was sprinkled on UB. Despite

Kd values determined in batch experiments ranging from 3.1 (AG) to 23.7 l kg� 1 (SB),

the breakthrough of the tracers was very fast, and the relative concentrations reached

values of 31% and 9% for AG and SB in the tile drain and 9% for SF in the effluent

of PL: The very low concentrations of AG and SB in the water from PL ( < 1x)

and of SF in the drain (2%) show that the two tracer bands belonged to separated sub-

catchments.

At the end of the experiment, we applied two dye tracers to stain the infiltration

pathways. BB solution was sprinkled onto the lower tracer band (LB). Fifteen minutes

after the beginning of the application, its concentrations in the drainage effluent started

to increase slightly. After 1 h, an almost linear increase set in for about 5 h. The relative

BB concentrations of 21% reached at the end of application after 7 h were higher than

that for HDO after 16 h (19%) and similar to that of AG applied earlier on the same

tracer band. Accordingly, the dye export was also quite high, amounting to 15% of the

applied mass during the time of application (Table 4). Acid red 1 was applied on a

length of 2 m on band UB. It was visually detected in the outflow from PL but not

analyzed in detail.

Corresponding to the fast increase of the concentrations of the tracer after their

application, these values decreased rapidly after the irrigation solution had been changed

to tap water. Basically, the decrease was the inverse of the breakthrough curve. The

increasing tracer concentrations C(t)pred can be expressed as a function of the decreasing

values C(t + Tend)meas after stopping application at time Tend:

CðtÞpred ¼ CðTendÞ � Cðt þ TendÞmeas; for 0VtVTend ð3Þ

In Fig. 8, we have shown the comparison between the measured and the predicted

values for Br, AG and SB in the drainage effluent. Generally, the agreement is good with

Table 4

Tracer export from the plot and the lower profile (PL)

Tracer Export from plot (percent of applied mass)

During

entire

experiment

Duration of

sampling period

after application (h)

During tracer

application

Duration of

application (h)

HDOa 43 74 12 16

Brb 29 74 14 16

Bra 36 74 24 16

Cl (manure)a 19 32 – pulse

Amino-G-acida 36 40 20 16

Sulforhodamine Ba 13 40 12 16

Sulfaflavineb 41 40 4 16

Brilliant bluea 19 4 15 7

DRPa 6 32 – pulse

NH4 (manure)a 12 32 – pulse

a Export through the tile drain.b Export through the tile drain and exfiltrating from the profile PL.

C. Stamm et al. / Geoderma 109 (2002) 245–268 259

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certain deviations: AG exhibited strong tailing after the application, which was not

observed for the breakthrough. SB showed a shift between the observed and the predicted

concentrations due to a retarded concentration maximum after the end of the tracer

application. Nevertheless, the slopes of the increasing and decreasing branches were very

similar.

At the beginning of the irrigation, the DRP concentration increased with increasing

flow rate in the drainage effluent. The DRP peak concentration (280 Ag l � 1) was rather

small compared to the maximum value of about 680 Ag l � 1 reached during the natural

event of June 10, 1996. One possible explanation could be the effect of Ca2+ -rich tap

water used for the irrigation, which may have reduced the P mobility (Scharer et al., 2001).

Although discharge continued to increase, the DRP concentrations decreased after some

hours and reached a fairly constant value of about 100 Ag l� 1 until manure was applied on

day 5 of the experiment.

Immediately after the manure application, the DRP concentration increased dramati-

cally. In the first sample collected at the drain outlet 5 min after spreading liquid manure

on the soil surface of the lower tracer band LB, the DRP concentration had already

increased substantially. After 10 min, it reached a maximum value of 800 Ag l� 1.

Thereafter, the values rapidly decreased and reached background concentrations after

Fig. 8. Comparison of the observed concentrations of the tracers AG, Br� , and SB with the predicted values

according to Eq. (3).

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some 5–6 h (Fig. 9). About 6% of the DRP spread with the manure were lost to the tile

drain during the experiment (Table 4). We observed a similar breakthrough for NH4+ and

initially also for Cl� , which is abundant in manure (1830 mg l � 1 in our case). In

contrast to DRP and NH4+ , Cl � exhibited a second concentration peak, lasting much

longer than the first one. The relative amount of Cl � lost to the tile drain was larger

(19%) than that for NH4+ (12%) and DRP (6%) as expected in case of a conservative

solute.

3.2.2.2. Infiltration patterns. We applied two dye tracers to stain the infiltration

patterns. BB was intended to stain the flow paths in the immediate vicinity of the tile

drain, AR should reveal possible lateral flow paths towards the tile drain. However, it

turned out that AR lost its visibility after some days and could not be analyzed properly.

Therefore, we only present results obtained from the infiltration patterns from the lower

tracer band irrigated with BB solution.

The flow patterns were highly irregular with stained patches down to below 100 cm

depth (Fig. 10). On five profiles, we visually classified the stained areas according to the

type of soil structure that was the likely cause of preferential flow. Earthworm burrows

Fig. 9. Breakthrough of the manure constituents Cl� , NH4+ and DRP in the drainage effluent.

C. Stamm et al. / Geoderma 109 (2002) 245–268 261

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were the most abundant types of these structures. Second were aggregate surfaces (Table

5). The other categories were encountered much less frequently. Most of the stained

structures were concentrated in the upper parts of the profiles and their numbers declined

strongly with depth. Worm burrows were the only structures with sizable frequency at the

depth of the tile drains.

Table 5

Predominantly stained soil structures

Structure type Number Percentage of

stained structures

Earth worm burrows 182 46.7

Surfaces of aggregates 94 24.1

Root channels 31 7.9

Animal burrowsa 20 5.1

Stone surfaces 17 4.4

Undefined structures 46 11.8

Total 390 100

a Animals other than earthworms, taxa not identified.

Fig. 10. Bitmap of the infiltration pattern of brilliant blue on a vertical soil profile perpendicular to the tile drain.

The dark areas indicate the dye-stained areas.

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4. Discussion

In our experiment, solutes were quickly transported vertically as well as laterally over

considerable distances of several meters. This fast transport affected not only a small

percentage of the solutes but also up to 20–40% of the applied tracer mass. Because the

hydrometric data showed that the contributing water volume was small—in the order of

few millimeters to centimeters only—one may ask where this transport took place within

the soil.

In an earlier experiment at a smaller scale (2 m2), we observed brilliant blue to enter an

open drainage ditch at a lateral distance of 4.5 m from the lower end of the plot after only

75 min or 12.5 mm of irrigation (Stamm et al., 1998). Inspecting the soil profiles

afterwards showed that the dye infiltrated vertically into the initially dry soil until it

reached the saturated zone. At that depth, a well-preserved network of ancient root

channels seemed to act as conductors for the fast lateral transport downhill. In the present

study, no such soil structures were detected. The only structures related to the dye patches

at the depth of the tile drain were vertical worm burrows.

The hydrometric data obtained in the experiment also clearly contradict the possibility

of lateral flow in the subsoil being the major mechanism of rapid transport. Despite the

moderate slope of the terrain, the discharge rates from the upper and lower pits differed

almost by a factor of four. Using the SWMS-2D code (Simunek et al., 1994), we simulated

the steady-state discharge behavior through a cross-section from the upper to lower pit

given a constant infiltration rate of 5 mm h� 1. In order to simulate lateral preferred flow

paths, we introduced a layer of large hydraulic conductivity at a depth of 95–107 cm. By

varying the conductivity of this high-conductivity layer, we modified the level of the water

table and the outflow ratio between the upper and the lower profiles. As long as we

assumed the fast flow to occur in the subsoil, it was impossible to reconcile the disparity in

the flow rates with the high water table in the center of the plot (Fig. 11). Due to the slope

of only 2.5%, the flow rates from the two pits would have been similar for high levels of

the water table. Only by moving the high conductivity zone close to the soil surface could

a high water table be reconciled with a strong discrepancy in the outflow rates from the

upper and lower profiles.

The same conclusion has to be drawn by comparing the discharge behavior in the tile

drain and the movement of the water table. During the main sprinkling interruption (2.9–

3.67 days), discharge from the tile drain ceased very quickly. However, the water table in

the middle of the plot was still 70–75 cm above the level of the drain at a lateral distance

of only 0.8 m from the drain. The same result was observed at the end of the experiment.

Obviously, the drain discharge was not governed by the hydrostatic pressure in the soil

matrix below 20–25 cm depth. Hence, the critical soil layer causing the fast solute

transport into the tile drain was the topsoil and not the subsoil. The infiltration patterns also

indicate that the water and solutes were flowing in lateral directions only close to the

surface.

This is plausible if one considers the actual local and instantaneous irrigation rates. For

the average sprinkling rate being 5 mm h� 1, it took the spray bar 24 runs an hour. The

nozzles irrigate a width of maximum 0.1 m at one given position. Therefore, the irrigation

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rate during each irrigation pulse of 0.75 s at a given location had an instantaneous intensity

of about 750 mm h� 1 followed by a period of an average duration of 120 s without

irrigation. This suggest that water was flowing intermittently at the soil surface during the

periods of the spray bar passing by.

The fact that we did not observe any surface runoff at the outlet of the plot indicates that

the lateral transport was redirected downwards in the drainage area and in the vicinity of

the open profiles. A few macropores connected to the tile drain or the open profiles were

sufficient to discharge the observed outflow volumes. Hence, we may summarize our

findings in the conceptual model depicted in Fig. 12.

Despite the fast transport and substantial tracer export, the mixing behavior between the

irrigation water and the soil solution was important. Most of the tracer mass was retained

in the soil (Table 4) and sorption effects were observed for SB, DRP and NH4+.

Similar results to ours, regarding the flow paths, have been presented for heavy clay

soils. Haria et al. (1994) concluded that in their arable field lateral transport occurred in the

A-horizon or occasionally by surface runoff towards the mole drains. Similar results are

reported by Spoor and Leeds-Harrison (1999). The work by Øygarden et al. (1997)

highlighted the importance of the disturbed soil above the drain itself for channeling water,

solutes and eroded soil particles into subsurface drains. For rice paddy fields, similar

effects are described by Iwata et al. (1995). That the drainage behavior reflects the

response of the coupled soil drainage system was also demonstrated by Turtola and

Fig. 11. Relationships between position of the water table and outflow ratio between discharge from the upper and

the lower profiles. The three curves indicate the relationship obtained by modeling a high-conductivity zone at

95–107 cm depth, at 35–50 cm depth or at the soil surface.

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Paajanen (1995). They have reported on the change in the percentage of surface runoff

compared to drainage discharge from different fields after reinstalling the tile tubes.

The connectivity between soil surface and a tile drain may be of major importance for

the fate of solutes. Shipitalo and Butt (1999) have shown that the connectivity of worm

burrows to tile drains was limited to a narrow band of only about 1 m. The hydraulic

conductivity of the connected burrows was larger than those farther away from the drain. If

the connectivity is not given, P losses may be small even with fast flow occurring. In a

recent study, we have found little P losses from free-draining lysimeters despite the

occurrence of preferential flow (Sinaj et al., 2002). It seemed that P lost from the topsoil

was retained in the strongly fixing subsoil because the preferred flow paths ended in the

capillary fringe above the free draining boundary.

It might be argued that our findings are mainly due to the experimental boundary

conditions and that the transport we have observed occurred only with the large amount of

applied water. However, measurements on similar sites in the study area show that the very

wet conditions as produced during the experiment also occur naturally (von Albertini et

Fig. 12. Conceptual model of the flow paths for the fast transport into a tile drain in a weakly structured loamy

soil as at our study site.

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Page 22: Multiple tracing of fast solute transport in a drained grassland soil

al., 1993) and that they are especially critical for surface runoff (von Albertini, 1990). In

the experiment, we reproduced these important conditions that in nature are mostly

transient states but extended its duration to make it accessible to experimental purposes

under steady-state conditions.

The two pits excavated for experimental purposes influenced of course the direction of

flow in the plot. However, they only affected the direction of the main lateral flow but not

the depths at which transport took place.

5. Conclusions

Often, surface runoff and subsurface flow are treated as separate processes in the sense

that water or solutes are thought to be transported to open waters by either one of the two.

Considering our findings, this clear distinction gets blurred. Instead of two parallel

processes, transport may be a sequence of two where water may (first) move laterally

as (near-) surface runoff that is intercepted by preferred flow paths in the vicinity of

subsurface drains.

This has also consequences for the interpretation of drainage water quality data. The

results of this experiment demonstrate that the assumption of subsurface drains being ideal

measuring devices for subsoil processes may be strongly violated not only for heavy clay

soils. The fast lateral transport in this loamy soil occurred in the topsoil and by-passed the

soil where the vertical macropores were connected to the tile drain or open profiles. Under

natural conditions, this could also be a river bank. Hence, the water quality in the drain as

well as in the profile effluent during storm flow was strongly influenced by the

composition of water coming from the topsoil. It is important to realize that the behavior

of solutes in the drainage effluent does not merely reflect the processes in the undisturbed

soil but is a combined response of the soil and the artificial drainage system. Hence, it may

well be that the large P losses reported in several drainage studies do not represent P losses

in the subsoil and shallow ground water but the losses in the immediate drainage area

where preferred flow paths directly connect to the tile drain below.

Therefore, we suggest that more attention should be given to the actual drain

configuration in order to complete the picture of the relevant factors determining solute

transport in drained soils. This is agreement with Richard and Steenhuis (1988) who

concluded that tile drains are useful for sampling over larger areas but suffer from the

possible problems due to soil disturbances or with Thomas and Barfield (1974) who

pointed out that nitrate concentrations in tile drains do not necessarily represent in a

reliable way the processes in the subsoil.

Acknowledgements

We would like to thank the many people who helped in the field or in the laboratory: F.

Denoth, H. Feyen, M. Fluhler, F. Funk, G. Gal, H. Hoffmann-Riehm, J. Hollinger, A.

Keller, B. Kulli, S. Lampert, H. Laser, P. Lehmann, A. Mares, R. Meuli, B. von Steiger, B.

Studer and V. Vouets. R. Hofling helped us by analyzing the deuterium samples. P.

Lazzarotto provided the precipitation data of the Sempach weather station. We want to

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thank also the farmer and landowner, Mr. Rindlisbacher, who agreed on the experiment

carried out on his land and who often supported us when we encountered practical

problems in the field. The people from the Cantonal Administration of Lucerne, especially

M. Achermann, J. Blum and P. Stadelmann, provided many helpful information

concerning the local situation. M.C. Jensen and an anonymous referee helped to improve

the manuscript by their comments. The project was funded by the Swiss National Science

Foundation (grant nos. 21-39314.93 and 20-49552.96).

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