Preferential flow in sandy loam soils as affected by irrigation intensity

SOIL TECHNOLOGY ELSEWIER

Soil Technology ll(1997)

139-152

Preferential flow in sandy loam soils as affected by irrigation intensity B. Gjettermann, K.L. Nielsen, C.T. Petersen, H.E. Jensen * , S. Hansen The Royal Agrohydrology

Veterinary and Agricultural University, and Bioclimatology, Thorvaldsensvej

Department of Agricultural 40, DK-1871 Fredetiksberg

Sciences, Laboratory of C, Copenhagen, Denmark

Accepted 6 January 1997

Abstract Dye-tracer studies in the field using Brilliant Blue FCF as tracer were performed to investigate the effect of irrigation intensity and soil heterogeneity on preferential flow. In two fields, both level and newly tilled in terms of seed bed preparation, to plots of 1.6 X 1.6 m were applied 50 mm of dye solution at rates of 10 and 50 mm h-‘. In the second year level, plots of grass of similar size were applied with 25 mm dye solution at a rate of 3.1, 6.2, 12.5, and 25 mm h-t. For all plots the stained patterns were examined one or two days after application of dye solution by the excavation of 11 vertical cross sections of 100 X 100 cm and 10 cm apart from each other. Flow patterns were digitized and depth functions for the degree of dye coverage and the number of activated flow channels were calculated. Furthermore, the structural features of each cross section were examined visually. The results show that deep penetration of water into the soil profile took place as preferential flow through macropores, mainly earthworm channels, with much of the water thus bypassing the soil matrix. In the top O-25 cm layer, the degree of dye coverage tended to be larger for the lower irrigation intensities indicating that water flow in the top soil took place through a relatively great proportion of the pores in the soil matrix. In the 35- 100 cm subsoil layer the number of stained macropores tended to be larger for the higher irrigation intensities. Thus, at higher irrigation intensity a positive pressure potential apparently developed more extensively in the topsoil initiating preferential flow through a greater number of macropores in the subsoil. In the newly tilled soil, water flow took place through a relatively great part of the topsoil matrix. Deeply penetrating stained earthworm channels originated, predominantly, in the well defined transition zone between topsoil and subsoil. In the soil left untilled and grass covered for about one year the continuity of macropores was more pronounced, and stained

* Corresponding author. Tel.: +45-35283385;

fax: +45-3528-3384; e-mail: [email protected].

00933-3630/97/$17.00 Copyright 0 1997 Elsevier Science B.V. AU rights reserved. PII SO933-3630(97)00001-9

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channels could frequently be traced from the subsoil all the way to the soil surface, in particular at low irrigation intensity. Keywords:

Macropore

flow; Irrigation

intensity;

Soil structure;

Earthworm

channels

1. Introduction

The success of any management measure to prevent losses of chemicals from agroecosystems to the aquatic environment depends on the understanding of the mechanisms of transport of water and solutes in soil. The phenomenon of preferential flow in soil has been known for many years and studied in detail more recently (Ehlers, 1975; Beven and Germann, 1982). It is realized that preferential flow plays a significant role in transport of water and solutes through the root zone of most soils (Ritsema and Dekker, 1994; Flury et al., 1994). Studies on the influence of initial and boundary conditions on preferential flow in soil have received particular interest. Such studies have included the effect of irrigation or simulated rain intensity on the extent of preferential flow in small blocks of soil and soil colurm~ (Edwards et al., 1992; Trojan and Linden, 1992; Bicki and Guo, 1991; Edwards et al., 1989; Germann et al., 19841, as well as in field soils (Ghodrati and Jury, 1990 and Ghodrati and Jury, 1992; Homberger et al., 1990). In general it is found that preferential flow is most pronounced at high irrigation intensity. However, in most studies the amount of irrigation and the irrigation intensity has been relatively high as compared with normal rain events. Thus, less information is available regarding effects at low and moderate irrigation intensity. Furthermore, small blocks or columns of soil does not necessarily represent the heterogeneity of the soil in the field. To obtain a better understanding of preferential flow mechanisms in the field, it appears beneficial to carry out in-situ experiments at a more representative scale. This paper presents results of field experiments on the effect of irrigation intensity on preferential flow appearing in the root zone of a newly tilled field as well as in a second-year grass field.

2. Materials and methods 2.1. Soils

The two soils in this study are situated 20 km west of Copenhagen on the experimental farms Hojbakkegaard and Snubbekorsgaard. Both belong to the Royal Veterinary and Agricultural University. The soils are developed on moraine deposits from the Weichsel Glacial Age. Soil classification and some profile characteristics are shown in Table 1. Both soils are well-structured sandy loam soils with several vertically oriented earthworm channels penetrating the B and the C horizons. The two soils differ, primarily, in clay content and in the development of structure in the E horizon. The

g 1OYR 3.5/2 1OYR 4/2 1OYR 5/2 5Y 5/2

Test site Snubbekorsgaard O-25 25-45 45-85 4 BC 85-120+

4.2 6.0 6.6 6.2

6.3 5.8 6.2 7.7

29.9 26.8 24.6 24.3

23.6 27.4 23.0 24.4

0.2-2

mm

(%)

(after

38.3 39.2 38.0 40.3

36.3 38.4 35.2 37.6

20-200

Petersen

j.rrn

18.6 20.4 19.0 18.8

23.3 18.4 18.6 20.4

2-20

et al., 1997)

pm

13.2 13.6 18.4 16.6

16.8 15.8 23.2 17.6

< 2 pm

1.44 (0.12) 1.55(0.10) 1.68 (0.09) 1.72 (0.07)

1.39 (0.10) 1.59 (0.08) 1.63 (0.08) -

density ’ g cme3

Bulk

MO VW MO WE

MO MO ST MO

Grade

d Type

ME FM FM

GR AB AB AB

MEGRP ME AB ME AB CO AB

Size

Soil sturcture

P/C C/P C C

C C C

Type

Voids

e

F/M M/F MC MC

F MC MC MC

Size

F/F C/F C F

F F C V

Abundance

a Determined in moist condition according to the Munsell Soil Colour Charts. b 0.01 M C&i,. ’ Determined in April 1994 from 15 100 cm3 soil samples taken approximately in the middle of the indicated soil layer. Standard deviation in (). * According to FAO (1990). Grade: VW = very weak, MO = moderate, ST = strong. Size: ME = medium, CO = coarse, FM = fine and medium. Type: GR = granular, AB = angular block. e According to FAO (1990). Type: C = channels (mainly vertically oriented, continuous earthworm channels), P = planes (mainly randomly oriented and discontinuous). Size: F = fine (0.5-2 mm), M = medium (2-5 mm), C = coarse (5-20 mm), MC = medium and coarse. Abundance: F = few, C = common, V = very few. f Soil class: Typic Agrudalf, sandy loam, mixed, mesic, calcareous. g Soil class: Typic Agrudalf, sandy loam, mixed, mesic.

A* E

3.5/2 4/4 4/4 6/6

f 1OYR 1OYR 1OYR 1oYR

Depth (cm)

pH b

Texture

characteristics

Colour

a

and some profile

Test site Hejbakkegaard o-25 A, 25-40 E 40-110 Bt 110+ C

Horizon

Table 1 Soil classification

2

2 I

2 I 8 2

ii 3. a c 3 t: 2 s 8 g

?J L.Q 3. ii: ii

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B. Gjettermann

Table 2 Mean and standard deviation layers at Hejbakkegaard Depth,

for log-transformed n

cm

o-5 10-15 About 20-25 55-60 a Samples

et al. /Soil

a

saturated

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conductivity,

K, ( pm

SC’) in 4 soil

ln( K,)

44 43 55 12

in this layer were centered

Technology

at the interface

Mean

Std.

- 0.40 1.13 - 0.38 1.06

2.94 1.91 2.81 3.11

created during

the last ploughing.

number of vertically oriented earthworm channels in the subsoil is somewhat higher in the Snubbekorsgaard soil, especially in the E horizon. Saturated hydraulic conductivity, K, was measured using the constant head method &lute and Dirksen, 1986) on 100 cm3 samples taken in four depths in the Hojbakkegaard grass field (Table 2). The Shapiro-Wilk statistic (Shapiro and Wilk, 1965) revealed that K, was not normally distributed in any depth (P < 0.001). The same statistic for log-transformed values of K, (ln(K,>) did not lead to a rejection of the hypothesis of normality in any depth (P > 0.60). Consequently, the log-transformed K, data were used in the statistical analyses. Standard deviations and mean values for ln(K,) were compared in pairs. The standard deviation in lo-15 cm depth was significantly smaller than the standard deviations in any of the other layers (P < 0.011). There were no significant differences between standard deviations for the other layers. t-Tests revealed that mean values were significantly lower in O-5 cm depth and in the interface layer (about 20-25 cm depth) than in lo-15 cm depth and 55-60 cm depth (P < 0.014). Experiments were conducted in April 1994 on both soils and in April 1995 on the Hojbakkegaard soil only (Table 3). 2.2. Soil tillage

Both soils were ploughed in October 1993. Two seed bed harrowings to 5 cm depth were performed in each field at optimum soil moisture content early in April 1994. By the end of April 1994, the Hajbakkegaard field was planted with Italian rye-grass. The grass cover was removed just before dye application in April 1995 in order to avoid interference from the crop canopy. Thus, the first experiments in April 1994 were carried out on two newly harrowed fields, while the experiment in April 1995 on the Hojbakkegaard soil was carried out on bare soil in a grass field that had not been cultivated for a year (Table 3). 2.3. Dye tracer and dye application

The dye tracer used to stain the flow paths of water was Brilliant Blue FCF. Field experiments with dye do produce visual traces of the primary flow pathways, but they do not quantify the flow. Tracer characteristics of Brilliant Blue FCF has recently been

B. Gjeitennann Table 3 Dye application, Plot No.

initial

soil moisture

contents

Dye application Amount (mm)

Time (h)

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and moisture

content

at the time of excavation

Soil moisture

content

(cm3 cm- 3, ’

Initial(S-10 cm depth)

After dye application(5 cm depth)

- 10

After dye application(25-35 cm depth)

Newly 1 2

tilled soil in April 1994. Test site Hejbakkegaard 50 5 10 0.34 50 1 50 0.33

0.36 0.31

0.28 0.30

Newly 3 4

tilled soil in Apn’l 1994. Test sire Snubbekorsgaard 50 5 10 0.32 50 1 50 0.35

0.34 0.35

0.34 0.31

0.30 0.29 0.28 0.28

0.26 0.25 0.25 0.26

Second year grass field in Apn.11995. Test site Hajbakkegaard 5 25 8 3.1 0.29 6 25 4 6.3 0.29 7 25 2 12.5 0.29 8 25 1 25 0.29

a Average numbers, each based on about 10 100 cm3 samples per plot. Initial year grass field in 1995 is based on 9 joint samples.

moisture

content

for the second

given by Flury and Fliihler (1994, Flury and FlWer, 1995). The dye is anionic at pH larger than 5.83 and it is generally considered only to adsorb weakly on soils. The Brilliant Blue FCF dye tracer was applied in solution (4 g l- ’ > to the soil surface of 8 plots (1.6 X 1.6 m) using an automatized sprinkler apparatus similar in principle to the one described by Ghodrati et al. (1990) and Flury and Flier (1994). The applied amounts of dye solution and the rates of application are given in Table 3. It is noted that these rates are of the same size of magnitude as the geometric means of the saturated hydraulic conductivity (Table 2). Basically, the sprinkling device consists of a motor driven spray bar with nozzles (type 41 lo-10 from Hardi International A/S) aligned to apply the solution one-dimensionally directly under the bar, and a suction pump connected to tanks with the dye solution. The device was designed to ensure a controlled and spatially uniform distribution of dye tracer at the soil surface. The uniformity of application depends on nozzle distance above the soil surface and on nozzle pressure. The best combination of these parameters was found in laboratory tests by collecting irrigation water in 10 by 10 cm receptacles placed next to each other. For an inner area of approximately 110 by 110 cm of the plot it was possible to achieve a coefficient of variation for the amount of applied water of 5-6% in the direction perpendicular to the travel direction and 2-3% in the travel direction (Petersen et al., 1997). In the field, wind drift was prevented by a wooden frame surrounding the treated area. 2.4. Sampling

The plots were excavated one or two days after dye application. An excavator was used to dig a trench in front of the treated area of each plot. Soil below the treated area

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was loosened with hand spades and vertical cross sections were prepared for detailed description. Each plot was excavated systematically in 11 parallel vertical cross sections separated 10 cm from each other. Flow patterns and relevant structural features appearing on the cross sections were described and photographed with a 35 mm camera. A 100 by 100 cm square metal frame was placed on the cross section before taking photos meant for image analysis. Soil samples were taken, in 5-10 cm depths, just outside the treated areas, for determination of initial moisture content. During the excavation soil samples were taken in 5-10 and 25-35 cm depth for determination of moisture content of the soil after infiltration (Table 3). The shown soil moisture contents were all close to the field capacities.

2.5. Image analysis All stained patterns on the photos appearing within the metal frame were transferred manually with a fine black pen to transparent plastic sheets. The only distinction made in this process was whether blue dye was visible or not. A representation of the metal frame (size on the photos: 10 by 10 cm) was transferred as well. The new black, or transparent, representations of the flow patterns were digitized with a resolution of 300 dpi (dots per inch) using either an Agfa Arcus, or a Hewlett-Packard scanner. The resulting binary representations were read into a computer program and displayed on a data screen. The four comers of the metal frame, with known coordinates, were marked on the screen. After this, the program made a projective geometric transformation of that part of the image being inside the frame to a quadratic representation with 464 true horizontal rows and 464 true vertical columns. For each row the computer counted the number of pixels representing stained areas, and the number of transitions from pixels representing unstained areas to pixels representing stained areas. Average values were calculated for each profile depth increment of 8 rows in order to reduce the random error. From these data it was straightforward to calculate the average dye coverage (DC) and average number of stained flow pathways or channels (NC) as function of profile depth. The least significant difference (LSD) between treatments was calculated for each variable and depth interval from one-side analysis of variation assuming variation homogeneity inside each treatment.

3. Results

3.1. Experiments with newly tilled soil in 1994 Digitized, geometrically corrected images of flow patterns found in the newly tilled Hojbakkegaard and Snubbekorsgaard soils, respectively, are shown in Fig. 1 for the two irrigation intensities (10 and 50 mm h- ‘) applied. Each of the four cross sections was chosen arbitrarily, within the given restrictions. Profiles of average dye coverage (DC) and number of stained flow pathways or channels (NC) are shown in Fig. 2.

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145

W

20 40 60 80 100

0

Fig. 1. Digitized flow patterns from vertical 100 by 100 cm cross sections in newly tilled soil 1994. (a) and (b): examples from the Snubbekorsgaard; (c) and cd): examples from the Hejbakkegaard. (a) and 6): 10 mm h-‘;(b)and(d):50mmh-‘.

Most of the soil aggregates were completely dyed in the uppermost 3-5 cm. Geometrically complex but well-defined stained areas (flow pathways) were found in the topsoil between about 5 cm and the interface created during the last plough

0

0 M

20

E 2o 5Y 40 $

60

n

80

40

E

100

5

60

5

80

p

100 0

20

40 60 DC, %

80

1000

5

10

15

NC

2; (cl

0

40

40

60

60

5

80

80

p

100 m 0

20 5

100 20

40 60 DC, %

80

100 NC

Fig, 2. Depth functions showing ((a) and (c)): the average degree of dye coverage, DC and c(b) and cd)): the number of separately stained pathways or channels, NC. Results from newly tilled soil 1994 for ((a) and (b)): the H@jbakkegarud soil and ((c) and cd)): the Snubbekorsgaard soil. (-):50mmh-‘;(. . .I: 1Omm h-‘.

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operation. This interface is located at a depth between 20 and 25 cm. Patterns in the subsoil below 30-35 cm were dominated by stained, vertically oriented earthworm channels. Relatively few vertically oriented stained flow pathways were found in the transition zone between topsoil and subsoil at depths of about 25-35 cm. This well defined transition zone is rarely tilled but is subject to soil compaction. The transition zone connects stained areas in the disturbed O-25 cm layer, with stained earthworm channels in the undisturbed subsoil. Thus, all the depth profiles for NC and DC have local minima at about 30 cm depth (Fig. 2). For both soils a general tendency appeared towards a decreasing degree of dye coverage (DC) in the O-25 cm topsoil layer with increasing irrigation intensity, and in the 35-100 cm subsoil a general tendency to an increase in the number of stained channels (NC) with increasing irrigation intensity. However, when comparing the difference in DC and NC between treatments (Table 4) the differences often turned out to be non-significant, except for the Hojbakkegaard below 75 cm depth (P < 0.05) and for the Snubbekorsgaard in the 50-75 cm layer (P < 0.01). Below the 30 cm depth both DC and NC increased to local maximum values at about 40-50 cm depth. Under this depth the variables decreased with depth although another local maximum value apparently occurred at 80-85 cm depth in the Snubbekorsgaard soil. In the 35-100 cm layer NC was significantly larger for the Snubbekorsgaard soil than for the Hojbakkegaard soil, except in the 50-75 cm sublayer for the application intensity 50 mm h-’ (Table 4). 3.2. Experiments with second year grass field at Hgjbakkegaard,

1995

Digitized and geometrically corrected images of flow patterns found in the second year grass field at Hajbakkegaard are shown in Fig. 3 for the four irrigation intensities (3.1, 6.2, 12.5, and 25 mm h-i) applied. The depth functions of DC and NC are shown in Fig. 4 for the three irrigation intensities 3.1, 6.2, and 12.5 mm h-l. The uppermost layer that was completely stained was relatively thin, l-2 cm or less. Flow patterns in the topsoil between about 5 cm and about 25 cm depth were dominated by well-defined, although geometrically complex stained areas. Furthermore, when comparing the treatments 10 mm h-i in 1994 with 12.5 mm h-r in 1995, but remembering the different amounts of dye applied, there were little difference in NC for the layer whereas DC was significantly lower in 1995 (P < 0.001). Thus, for the 5-25 cm layer, and for a fairly similar irrigation intensity, the patterns were narrower in 1995 than in experiments on newly tilled soil in 1994. Patterns in the subsoil were completely dominated by vertically oriented stained earthworm channels. Clear minimum values for NC and DC were found at 30 cm depth for the intensity 12.5 mm h-’ as in the experiments on newly tilled soil but not for the lower intensities (Fig. 4). It appears that dye coverage (DC) in the O-25 cm topsoil layer decreases, and that the number of activated channels (NC) in the subsoil (below 35 cm) increases with increasing irrigation intensity up to 12.5 mm h-’ (Fig. 4; Table 4). Below about 40 cm depth both NC and DC decrease with depth. A 30 cm wide wheel track giving rise to local surface poncling during the dye

***

a

ns ***

DC,

= DC,

ns ns ns *

refer to plot Nos., cf. Table

a Subscripts

* ns ** Il.3

2. ns: not significant.

** ** ** **

* * ns * rejected

ns ns ** ***

NC,

*** ***

*** *** NC, = NC,

DC,

DC, = DC,

* , * * , and * * * : hypothesis

* ** ns ***

Hypotheses concerning the number of stained channels a NC, = NC, NC, = NC4 NC, = NC, NC, = NC4

IlS

*

= DC,

DC, = DC3

the degree of dye coverage

ll.9

DC,

* ns

concerning

Hypotheses

DC, =DC,

2.5-35 35-50 50-75 >75

o-5 5-25

Layer (cm)

Table 4 Results from test of hypotheses

ns * nn **

NC,

** ns

DC,

= NC,

= DC,

at the 95, 99, or 99.9%

= NC,

= DC,

= NC,

= DC,

level, respectively.

ns ns ns ns

NC,

*** ***

DC,

ns ** ** **

NC,

*** ***

DC,

= NC,

= DC,

E

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Cd) 0 20 4u 60 80 100 [hl 0 20 40 60 ED 100

0 20 40 68 80 IQ0

Fig. 3. Digitized (a)-(c), (d)-(f), respectively.

flow patterns from vertical (g)-(i), (i)-(l): examples

100 by 100 cm cross sections in second year grass field 1995. from application intensities 3.1, 6.3, 12.5, and 25 mm h-‘,

0 2o 40

E s

60

g

80

p

100 0

20

40 60 DC, %

80

1000

5

10 NC

15

20

Fig. 4. Depth functions for second year grass field 1995 at test site Hprjbakkegaard showing (a): the average degree of dye coverage, DC and (b): th e number of separately stained pathways or channels, NC. (p >: 12.5 mm h-‘, (. . .): 6.3 mm h-‘, and (---)I 3.1 mm h-‘.

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application was observed in the plot receiving an irrigation intensity of 25 mm h- ‘. We believe that the ponding have caused overland flow and, consequently, flow heterogeneity at a relatively large scale. For this reason the DC and NC profiles obtained for the plot receiving 25 mm h-i have not been considered. There was no surface ponding or overland flow for the other treatments.

4. Discussion

In general a number of well-defined stained pathways could be traced from the topsoil and deep into the subsoil. This deep penetration of dye solution proves the general occurrence of preferential flow and indicates activation of transport processes which are different from those assumed to occur in homogeneous soil. In the subsoil below about 30 cm the preferential flow pathways were completely dominated by earthworm channels. Many of the patterns in the topsoil were cut off vertically and extended horizontally (up to lo-15 cm) at the interface created during the last plough operation. This applies in particular to experiments conducted in 1994 with application of 50 mm dye solution. Thus the patterns indicate a general decrease in small scale hydraulic conductivity in vertical direction at the interface. This is in accordance with measured saturated hydraulic conductivity (Table 2). 4.1. Influence of irrigation intensity

At the low irrigation intensity water flow in the top soil took place through a relatively great proportion of the soil matrix. In this case preferential flow reaching the subsoil was initiated at a few spots, only. However, at higher irrigation intensity localized ponding apparently developed more extensively within the soil initiating preferential flow through a greater number of macropores. Clear local minimum values for NC and DC at about 30 cm depth were observed in all experiments on bare soil in 1994 and for the application intensity (12.5 mm h- ’ > in 1995. This may well indicate a general activation of channel flow in the transition zone between topsoil and subsoil supported by a relatively general pressure build-up in the soil water caused by low hydraulic conductivity at least in the grass field (Table 2). This interpretation of data is supported by the occurrence of a number of stained, inclined root channels in the transition zone and by the occurrence of many horizontally oriented dye patterns at the interface between tilled and untilled soil layers in these plots. 4.2. Influence of earthworm channels

A vast majority of the vertically oriented earthworm channels found in the subsoil ended somewhere in, or right below, the transition zone between tilled and untilled soil in 25-35 cm depth Only during the excavations in 1995 was it possible to trace a minority of earthworm channels all the way from the subsoil to the soil surface. A number of deeply penetrating stained earthworm channels were discovered by

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careful excavation with a knife. In 1995 several of these channels could be traced to the soil surface penetrating the topsoil in gently bent zigzag courses. Stained earthworm channels penetrating to more than 90 cm depth in the plot receiving the lowest irrigation intensity (3.1 mm h-l> all originated at the soil surface. At the interface created during the last plough operation some of the earthworm channels turned sharply taking an almost horizontal direction for up to a few centimeters. Then they gradually turned back to the vertical plane. It was also observed that there were more horizontally oriented root channels right above the interface than below the interface. These phenomena may contribute to the explanation of the local minimum for NC at about 30 cm depth. However, it is realized that the suggested explanations do not sufficiently explain the observed flow patterns. The dynamic nature of the hydraulic properties governing preferential flow from topsoil to subsoil is subject to further investigations. Below about 30-35 cm the earthworm channels were vertically oriented without pronounced ramifications. The channel diameters varied between 2 and 8 mm. Several of the earthworm channels were continuous down to 90-100 cm depth and some penetrated the subsoil vertically down to at least 135 cm depth. In 1994 there were no fresh plant roots in any of the earthworm channels. In 1995 many of the earthworm channels were encircling one or two fresh grass roots. It was observed that the probability of finding fresh roots in stained earthworm channels in 50 cm depth (being about 80%) was much higher than in channels without dye adsorbed on the walls. It is however difficult to conclude whether such fresh roots are playing an important role in directing water to the earthworm channels or not. It is well known that plant roots prefer to follow earthworm channels or other pathways with low mechanical resistance when growing through compacted soil layers (Tardieu, 1994). In the experiment 1994 on the bare Snubbekorsgaard soil it was observed that a relatively large proportion of the earthworm channels terminated at about 80-85 cm depth (Table 1). This is reflected in the depth profile of NC (Fig. 2) which decreased sharply below that depth. Thus, it is likely that the phenomena of subsurface infiltration, or internal catchment, took place (Bouma, 1991). The larger number of stained channels in the subsoil found in 1994 in the Snubbekorsgaard as compared with the Hejbakkegaard soil may reflect the larger number of earthworm channels observed in the E-horizon of the Snubbekorsgaard soil (Table 1). 4.3. Znjluence of soil tillage By ploughing, all the continuous vertically oriented earthworm channels in the plough layer were disrupted and entrances to macroporous pathways at the ploughing depth may become more or less sealed by the action of the plough share or the furrow wheel. Although ploughing creates new macropores these can generally be characterized as unstable, random, and discontinuous. The discontinuity promotes internal catchment and redistribution in the plow layer. The seed bed harrowings imply the uppermost soil layer being relatively homogeneous and they are thereby further promoting the redistribution processes. Thus in the early experiments in 1994 water transport in the top soil of the newly tilled soil took place in a relatively great part of the soil matrix (Fig. 1). The DC in the upper 25 cm of the topsoil was significantly larger as compared with DC

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obtained for fairly similar irrigation intensity in the experiment in 1995 on undisturbed, grass covered soil (Table 4). 5. Conclusions

Deep penetration of dye solution proved the general occurrence of preferential flow. Vertically oriented earthworm channels were always the dominating pathways of preferential flow in the subsoil below about 30 cm, whereas flow patterns above about this depth appeared to be affected by the dynamic structural changes in the topsoil, and in the transition layer between topsoil and subsoil. At low irrigation intensity water flow in the top soil took place through a relatively great proportion of the soil matrix due to great influence of capillary forces limiting preferential flow in the subsoil. At higher irrigation intensity a positive pressure potential apparently developed more extensively in the topsoil initiating preferential flow through a greater number of macropores in the subsoil. In newly tilled soil water flow in the top soil took place through a great part of the soil matrix due to modified soil structure. In a soil left untilled and grass covered for about one year the continuity of macropores in terms of earthworm channels was restored and deeply penetrating stained earthworm channels could frequently be traced all the way from the subsoil to the soil surface, in particular at low irrigation intensity. Although the methods applied here do not allow a direct quantification of transport, they appear to be well suited as an introduction to more detailed studies on transport mechanisms and spatial variability of transport phenomena in the root zone. References Beven, K. and Germann, P., 1982. Macropores and water flow in soils. Water Resources Research, 18: 1311-1325. Bicki, T.J. and Guo, L., 1991. Tillage and simulated rainfall intensity effects on bromide movement in an argiudoll. Soil Science Society of America Journal, 55: 794-799. Bouma, J., 1991, Influence of soil macroporosity on environmental quality. Advances in Agronomy, 46: l-37. Edwards, W.M., Shipitalo, M.J., Owens, L.B. and Norton, L.D., 1989. Water and nitrate movement in earthworm burrows within long-term no-till cornfields. J. Soil and Water Conservation, 44: 240-243. Edwards, W.M., Shipitalo, M.J., Dick, W.A. and Owens, L.B., 1992. Rainfall intensity affects transport of water and chemicals through macropores in no-till soil, Soil Science Society of America Journal, 56: 52-58. Ehlers, W., 1975. Observations on earthworm channels and infiltration on tilled and untilled loess soil. Soil Science, 119: 242-249. Flury, M. and Fliier, H., 1994. Brilliant Blue FCF as a dye tracer for solute transport studies - A toxicological overview. J. Environmental Quality, 23: 1108-l 112. Fhny, M. and Fltihler, H., 1995. Tracer characteristics of Brilliant Blue FCF. Soil Science Society of America Journal, 59: 22-27. Flury, M., Fitter, H., Jury, W.A. and Leuenberger, J., 1994. Susceptibility of soils to preferential flow of water: A field study. Water Resources Research, 30: 1945-1954. Germann, P.F., Edwards, W.M. and Owens, L.B., 1984. Profiles of bromide and increased soil moisture after infiltration into soils with macropores. Soil Science Society of America Journal, 48: 237-244.

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Ghodrati, M. and Jury, W.A., 1990. A field study using dyes to characterize preferential flow of water. Soil Science Society of America Journal, 54: 15581563. Ghodrati, M. and Jury, W.A., 1992. A field study of the effects of soil structure and irrigation method on preferential flow of pesticides in unsaturated soil. .I. Contaminant Hydrology, 11: 101-125. Ghodrati, M., Ernst, F.F. and Jury, W.A., 1990. Automated spray system for application of solutes to small field plots. Soil Science Society of America Journal, 54: 287-290. Homberger, G.M., Beven, K.J. and German, P.-F., 1990. Inference about solute transport in macroporous forest soils from time series models. Geoderma, 46: 249-262. Klute, A. and Dirksen, C., 1986. Hydraulic conductivity and diffusivity: laboratory methods. In: ed. A. Klute, Methods of Soil Analysis. Part I - Physical and Mineralogical Methods. Agronomy No. 9. Part I. ASA and SSSA, Madison, Wisconsin, USA, pp. 687-734. Petersen, C.T., Hansen, S. and Jensen, H.E., 1997. Tillage induced horizontal periodic&y of preferential flow in the root zone. Soil Science Society of America Journal (in print). Shapiro, S.S. and Wilk, M.B., 1965. An analysis of variance test for normality (complete samples). Biometrika, 52: 591-611. Ritsema, C.J. and Dekker, L.W., 1994. How water moves in a water repellent sandy soil. 2. Dynamics of fingered flow. Water Resources Research, 30: 2519-2531. Tardieu, F., 1994. Growth and functioning of roots and of root systems subjected to soil compaction. Towards a system with multiple signalling? Soil and Tillage Research, 30: 217-243. Trojan, M.D. and Linden, D.R., 1992. Microrelief and irrigation fall effects on water and solute movement in earthworm burrows. Soil Science Society of America Journal, 56: 727-733.