quantifying the preferential flow by dye tracer in the...

Journal of Earth Science, December 2014 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-014-0489-4

Wu, Q. H., Liu, C. L., Lin, W. J., et al., 2014. Quantifying the Preferential Flow by Dye Tracer in the North China Plain. Journal of Earth Science. doi:10.1007/s12583-014-0489-4

Quantifying the Preferential Flow by Dye Tracer in the North China Plain

Qinghua Wu*1, Chunlei Liu2, Wenjing Lin2, Meng Zhang2, Guiling Wang*2, Fawang Zhang3

1. Changjiang River Scientific Research Institute, Wuhan 430010, China 2. Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, Shijiazhuang 050061, China

3. Institute of Karst Geology, Chinese Academy of Geological Sciences, Guilin 541004, China

ABSTRACT: The preferential flow plays a vital role on the infiltration of irrigation or rainfall. The ob-jective of this study was to quantify preferential flow in the processing of irrigation infiltration in the field scale. Tests of different initial soil water contents and irrigation intensities were conducted using Brilliant Blue FCF (C.I.42090) dye tracer in Luancheng County of the North China Plain. The results showed that the percentages of infiltration by the preferential flow for irrigation depth of 25, 50, and 75 mm were 16.67%, 43.67%, and 34.17%, with 19.72%, 61.42%, 66.64% of dyed areas in the soil profile, respectively, which indicated that preferential flow was enhanced with increasing irrigation intensity, but reduced when the irrigation intensity was over 50 mm. The percentages of preferential flow for 75 and 180 mm previous irrigation producing different initial soil water contents were 23.26% and 18.97%, with 53.23% and 39.94% of dyed areas in the soil profile, respectively. Compared with the 75 mm without previous irrigation, the results indicated that higher initial soil water contents restrained the preferential flow in the field. Therefore, intermittent irrigation and low irrigation intensity patterns, and larger depth of plowing would be suggested to reduce the preferential flow which would increase the soil water utilization efficiency and reduce pollution risk of pesticide and fertilizer to groundwater. KEY WORDS: Brilliant Blue FCF, preferential flow, infiltration, North China Plain.

0 INTRODUCTION

The North China Plain is one of the largest agricultural bases in China, which suffered from shortage of water re-sources. The groundwater is the main source of drinking, agri-culture, and industry. The irrigation water amount accounted for 72% of the total exploited water resources (Wu, 2008). At present, over-exploitation caused a series of environmental problems, e.g., continued declining of the groundwater level, increasing cones of depression, moving down of the interface between fresh and saltine water, and superficial deposit (Fei et al., 2007), which were attributed to unreasonable management of groundwater. The soil water infiltration is vital to estimate the groundwater resources. However, the piston model was mainly applied to estimate the groundwater recharge, which is not suitable in the field where preferential flow is common. The preferential paths could improve the groundwater recharge and increase the risk of groundwater suffering from pesticides, fertilizers, and other contaminants.

The preferential flow is the phenomenon that the soil wa-ter and solute move along preferential paths, e.g., earthworm

*Corresponding author: [email protected];

[email protected] © China University of Geosciences and Springer-Verlag Berlin Heidelberg 2014 Manuscript received May 15, 2014. Manuscript accepted September 11, 2014.

burrows, channels of decayed roots, aggregate fractures, dry-wetting cracks, freezing and chemical dissolving fractures, bypassing the soil matrix. Macropore flow (Qin et al., 2000), finger flow (Bu et al., 2012), and funnel flow (Guo, 2008) are main patterns of preferential flow. The macropore flow was the most common preferential flow in fields, which was found earliest by Schumacher in 1864 during the experiment of soil water infiltration, however, which wasn’t recognized the im-portance in soil water and solute movement until in 1960s (Cheng and Zhang, 1998). Although the macropore flow has been researched at least 60 years, there isn’t a uniform standard to define the sizes of macropore. More reasonable definition is that a pore with a faster flow velocity than the soil matrix could be considered as a macropore (Li et al., 2007). The directed methods, e.g., polyethylene glycol, paraffin, gypsum (Feng and Hao, 2001), dyeing (Li et al., 2007), and computerized tomog-raphy (Feng and Hao, 2002), and undirected methods, e.g., tensiometers, breakthrough curves, and Poiseuille formula (Lü et al., 2012; Sun et al., 2012; Shi et al., 2007), were applied to measure the macropores size.

Beven and Germann (1982) investigated the effect of macropores on the rainfall infiltration. The preferential flow in sand soils caused lower soil water content and larger ground-water recharge (Larsson et al., 1999). The effect of microor-ganism in macropores paths was analyzed, the results showed that the microbe preferred to reside in preferential paths where the concentrations of organic carbon and nitrate were higher than the soil matrix (Bundt et al., 2001). Machine compaction

2 Qinghua Wu, Chunlei Liu, Wenjing Lin, Meng Zhang, Guiling Wang and Fawang Zhang

could improve preferential flow in the sand soil, but restrain in the clay (Mooney and Nipattasuk, 2003). In the saturated sand soil, most of the preferential paths were activated, but except for only few structured pores, attributing to hydrophobic effect (Jarvis et al., 2008). Soil water patterns in the cultivated and fallow fields suggested that the plowing could destroy most of preferential paths in the root zone, which cut down the macropores between root zone and deeper soil (Wuest, 2009). The preferential flow in forest soil was stronger than grass soil (Alaoui et al., 2011), and kinds of plants in forest showed dif-ferent levels of preferential flow (Zhang et al., 2012; Xiao et al., 2011; Chen and Hui, 2006). The artificial macropores in the experiment were applied to analyze the effect of preferential flow on the soil water infiltration (Wu et al., 2009; Guo, 2008). Gravel mulch with a diameter of over 2.0 mm could make for development of macropores, reducing evaporation (Yu and Huang, 2012). The degree of preferential flow increased with both of higher initial soil water content and irrigation intensity, but increased extent was limited (Sheng and Fang, 2012; Weiler and Flühler, 2004).

Dyeing is a common method to investigate the preferential flow in the field. Brilliant Blue FCF (C.I.42090) dye tracer was applied successfully, owing to bright dyeing, low absorbability and toxicity, low cost, and applicability. Forrer et al. (2000) introduced the procedure of processing the dyeing images to quantify the preferential flow. Different kinds of land utiliza-tion were studied by the Brilliant Blue FCF, and the results showed that the preferential flow could be found even in ho-mogenous soils. The variability of soil water flow increased with larger space scales (Wang et al., 2007). The results of dyeing by the Brilliant Blue FCF indicated the plants roots was the main preferential paths in the field (Jian et al., 2011).

Environmental isotopes, lysimeters, and soil columns were employed to quantify the percentages of preferential flow. The 2H and 18O concentrations of precipitation, soil water and groundwater in Burkina Faso were compared to quantify the preferential flow, the results indicated that the degree of groundwater recharge by preferential flow was 70% (Renaud and Thierry, 1996). Chloride ion and 18O were used to investi-gate the preferential flow, showing 85% of preferential flow (Shurbaji and Campbell, 1997). Comparing soil water flow patterns of bromide ion to nanoscale carbon grain with a diam-eter of 2–5 nm, the effect of preferential flow could be analyzed (Subramanian et al., 2013). The preferential flow accounted for 96% in an undisturbed large scale soil column of clay (Jørgensen et al., 2002), while 77.3%–91.3% in the soil infiltra-tion by lysimeters (Guo, 2008). A comparison of the recharges of single porous model with measured values of lysimeters showed that the percentages of preferential flow were 10%–80% (Qi et al., 2007).

The previous researches have made a great progress in the preferential flow. Most works to quantify the preferential flow degree were conducted in laboratory scales via tracer test or numerical modeling; however, few researches on using the dyeing tracer in the fields were reported. The main purpose of this paper was to quantify the preferential flow in the water infiltration for the cultivated field by applying the Brilliant Blue FCF dye tracer. The influences of initial soil water content

and irrigation intensity on the preferential flow were evaluated as well. 1 MATERIAL AND METHODS 1.1 Site Description

The study area is located at Luancheng experimental site of Chinese Academy of Science in arid and semiarid region, with a monsoon-dominated climate in the North China Plain (as shown in Fig. 1). The mean annual precipitation was 450 mm (1990–2010), and over 80% between July and September. The mean annual temperature and evaporation rate were 15 ºC and 1 470 mm (1990–2010), respectively. The groundwater depth declined continually from 10 m in 1975 to 37 m in 2010, with a mean velocity of 0.7 m·a-1. The soil is silt loam with a depth of 0– -5.0 m (positive upward, 0 cm at the soil surface), and the physical parameters were shown in Table 1. The main plants are summer corn with 2–3 irrigations, and winter wheat with 4–6 irrigations each year. Flooding was the main irrigation type in this area.

Figure 1. Position of the study area in the North China Plain. 1.2 Experimental Setup

The dyeing tests, which contain two parts, e.g., (A) irriga-tion intensity experiment and (B) initial soil content experiment (as shown in Table 2), were conducted in winter during De-cember 1, 2012 to December 18, 2012. The experiments of irrigation intensity were conducted with 25, 50, and 75 mm irrigation, respectively, with the same initial soil content (i.e., no previous irrigation), and the experiments were numbered by 1, 2 and 3, respectively. The choosing infiltrating amounts of water, i.e., 75, 50, and 25 mm, represent amounts of water for plants (winter wheat and summer corn) in heavy drought, drought, and a bit drought periods, respectively, since the plants need more amounts of water when the soil was more drought. In order to obtain the different initial soil content, 0, 75 and 180

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Quantifying the Preferential Flow by Dye Tracer in the North China Plain 3

mm water without tracer were irrigated in the three areas with a size of 2×2 m in 10 d before the dyeing experiment. The ex-periments of initial soil content were conducted with 0, 75 and 180 mm previous irrigation, respectively, with the same irriga-tion intensity (i.e., 75 mm), and the numbers of the experiment were 3, 4 and 5, respectively. Avoiding to interfering each other, the 5 plots were set in a 4.0 m space, and the micro-landform, soil structure, and plants in the each plot were the same. The roots of winter wheat decreased exponentially from the soil surface to -9.0 m. Each irrigated process last for 30 min, re-sulting in no ponding water on the surface of the 1 plot, 0.5 cm

height for the 2-plot lasting for 12 minutes, and 1, 1, and 2 cm height water on the surface of the 3, 4, and 5 plots, respectively, all lasting for 15 min. No runoff appeared on all plots’ soil surface. Owing to the low temperature, the test area was cov-ered by polyethylene film and cotton blanket to prevent the soil water from freezing a week before the dyeing experiment. Bril-liant Blue FCF (C.I.42090) with concentration of 4 g·L-1 was injected into a rainfall simulator with a size of 1.0×1.0 m which had 81 pores with a diameter of 1.5 mm for infiltration. The winter wheat with a height of 3.0 cm wasn’t removed, remain-ing the original soil surface.

Table 1 Physical parameters of soil in Luancheng site

Depth (cm) Soil type★ Particle distribution (μm) Bulk density (g·cm-3) Porosity

>50 2–50 <2

-10– -25 Silt loam 19.4 71.02 9.58 1.55** 0.49**

-40– -55 Silt loam 15.1 72.6 12.3 1.51** 0.46**

-80– -95 Silt loam 14.22 72.82 12.96 1.61** 0.46**

* Data from the Institute of Hydrogeology and Environmental Geology, CAGS; ** Data from Luancheng site of Chinese Academy of Science; ★. According to USDA.

Table 2 Experimental setup

No. Irrigation intensity (mm)

Initial soil water content Duration of irrigation Experiment period Experiment type

1 25 No previous irrigation 30 min Dec. 09–Dec. 10 A 2 50 No previous irrigation 30 min Dec. 11–Dec. 12 A 3 75 No previous irrigation 30 min Dec. 13–Dec. 14 A and B 4 75 75 mm previous irrigation 30 min Dec. 15–Dec. 16 B 5 75 180 mm previous irrigation 30 min Dec. 17–Dec. 18 B

A. irrigation intensity experiment of 25, 50 and 75 mm irrigation on the condition of the same initial soil water content (no previous irrigation); B. initial soil water content experiment of 0, 75 and 180 mm previous irrigation on the condition of the same irrigation intensity (75 mm).

The experiment was conducted in a semitransparent tent to

attain a uniform light, and then the dyeing pattern of preferen-tial flow in the soil profile could be clearly visible. A trench was manually excavated to a depth -1.2 m and a width of 1.2 m in 24 h after irrigations. The designs of slices’ excavation and sampling were shown in Fig. 2. The vertical slices of the soil profiles were dug at a distance of 0, 10, 20, 30, 40, and 50 cm from the boundary of each dyeing zone, in other words, the positions of vertical slices were Y=0, 10, 20, 30, 40, and 50 cm, respectively, where Y axis was the horizontal direction of ex-cavation, as shown in Fig. 2a, while the horizontal slices were 0.1 m increments to a depth of -0.5 m, i.e., at a depth of Z=0, -10, -20, -30, -40, and -50 cm, respectively, where Z axis was the vertical direction of excavation, shown in Fig. 2b. In totally, 60 vertical and horizontal sections were obtained. Avoid smearing the no-dyeing zone in the soil profiles, a spade was used to remove the soil up to the 0.02 m from the pre-sections, then the 0.02 m was remove carefully by a long knife, which could make the soil profile remain original color and macropores structures, e.g., earthworm burrows and the chan-nel of plant roots. Each section was recorded by a camera (Ni-

kon D7000) with a focal length of 45 mm, fixed on a tripod. The pictures with a resolution of 300 dip were stored in the format of .NEF which could record the original information without compression.

Samples were just obtained from the vertical sections at Y=45 cm in each plot, shown in Fig. 2c. Before sampling, a frame of 1.0×1.0 m with 225 small grids (grid resolution 0.075×0.075 m) was fixed on the vertical soil profile in order to design the position of each sample. Only the entirely or partly stained grids were sampled with a knife, and one grid was pre-pared for only one sample. Each sample with a mean weight of 30 g was collected in an aluminum box with a diameter of 3.0 and 2.0 cm of height to measure the soil water content and concentration of Brilliant Blue FCF. The initial soil water con-tent samples of each plot were collected before dyeing experi-ment, 19 of which were analyzed for soil particle distribution. Therefore, 750 samples were obtained. The soil water content was measured by drying-weight at 105 ºC for 24 h, then the sample were milled, sieved with a mesh of 1 mm, dissolved into deionized water with a mass ratio of 1 : 5 (dry soil vs. wa-ter). The mixture solute was oscillated on a swing bed (TS-1)


with a frequency of 200 rpm/min for 2 h, then centrifugated at a rate of 4 000 rpm/min for 10 min (XiangYi L-550). The super-natant of the mixture solute was extracted by a medical injector of 20 mL with filter membranes of 0.45 μm diameter, and col-lected in a centrifuge tube of 10 mL for the measurement of

Brilliant Blue FCF concentrations by an ultraviolet- spectro-photometer (UV 277) at a wavelength of 630 nm. The particle distributions of soil were obtained by volume method (Hydro 2000MU).

Figrue 2. The design of dyeing section excavation and sampling. 1.3 Image Processing

The images of soil profiles suffered from geometric dis-tortion, inhomogeneous illumination, and color difference. Therefore, the images needed to be adjusted or corrected. 1.3.1 Geometric correction

A frame of 1.0×1.0 m with rulers on the four edges was fixed on the each section for the adjustment of geometry dis-torting. Geometric distortion contained linear distortion result-ing from no-perpendicularity between the soil profile and the camera, and pillow distortion resulting from camera lens. In this study, the direction of photographing was vertical to soil profile, and the distance between camera and soil profile was kept to a constant of 3.0 m, which could eliminate the linear distortion and reduce the pillow distortion, for the vertical pro-file rather than for the horizontal. The pillow distortions of both vertical and horizontal images were adjusted by the special software of Capture NX2 produced by Nikon Company. The lineal distortion was corrected by the remote sensing software ENVI 4.2, according the procedure that 30 coordinate control positions were chosen uniformly in the four edges, and then the correction was made by the method of Delaunay triangulation with resampling method of cubic convolution. 1.3.2 Correction of inhomogeneous illumination

Owing to the trench size wasn’t enough large, the shape formed clearly in the soil profile resulting from that the bound-ary of trench sheltered against sunlight, which influenced seri-ously the imaging quantity. A semitransparent tent was used to attain a uniform light, although Forrer et al. (2000) discussed a standard procedure for correction of inhomogeneous illumina-tion by a grey frame. 1.3.3 Color adjustment

In order to obtain the true color of images, a white board of 8×15 cm was fixed on the side of the frame which had been photographed together with the dyed soil profiles. The color of the images (.NEF format) was adjusted by the white balance function of the Capture NX2 software.

1.4 Dyed Area Statistic The corrected images were saved in the format of .jpg by

Photoshop CS. The stained areas were chosen to replace into black, remaining the unstained soil original color, with adjust-ing the brightness to -100 and color difference to -5%. Then the images were transformed into gray model, and the threshold values were adjusted to -255. The images became into black-white model, i.e., binary image. Black and white zones represented the stained and unstained areas, respectively, shown in Fig. 3. Finally, the images were stored as bitmap for-mat with a resolution of 100 pixels per 1 cm2. The information of dyeing images could be analyzed by the software of Matlab 7.0.

Figure 3. Dyeing model and binary image. 1.5 Method of Quantifying Preferential Flow

Comparing to dyeing pattern, the concentration profile of Brilliant Blue FCF dye tracer could quantify accurately the infiltration capacity by preferential flow, following the formu-las (1) and (2)

0ad( )d

ZR R Z Z

(1)

bSW

wad

irrigation=

mCR

C

(2)

where R is the total infiltration capacity, L; Rad is the infiltrated intensity of preferential flow per unit depth in the soil profile,


LL-1; Z is the depth of the soil profile, L; Csw is the concentra-tion of Brilliant Blue FCF, ML-3; ρw and ρb are soil water den-sity and bulk density of soil, ML-3, respectively; θm is the soil water content, MM-1; Cirrigation is the concentration of dye, ML-3; ρw is 1.0 g·cm-3. And ρb of 0– -20, -20– -60, and -60– -100 cm are 1.45, 1.51, and 1.56 g·cm-3, respectively. Csw was measured by the ultraviolet spectrophotometer according to a standard curve. The standard curve was obtained by linear fitting the known concentrations of Brilliant Blue FCF with absorption values of the standard samples at the wavelength of 630 nm. The highest and lowest concentrations of standard samples were 15.6 and 0.125 mg·L-1, respectively. The relationship was Abs=0.247 6C–0.003 1, R2=1.0, where Abs was the absorption value, and C was the dye tracer’s concentration of standard samples. Since the systematical errors of the spectrophotometer varied for each switching-shutting operation, all samples were measured in a day without closing the spectrophotometer dur-ing the experiment. 2 RESULTS AND DISCUSSION 2.1 Preferential Flow Phenomenon in the Soil Profiles

The preferential paths were common in the field. The earthworm burrows and plant roots were investigated in the stained area to the depth of -0.8 m where most of irrigation water arrived, shown in Table 3. The number of earthworm burrows in the depth of 0– -15 cm was zero, resulting from that plowing destroyed completely the earthworm burrows and no fresh formed during the winter. The mean density of earthworm burrow was 21 per 100 cm2, with diameters of 0.3–1.0 cm. The density was higher in the depth of -20– -60 cm than over -70 cm. In the dyed region, most of stained macropores were earthworm burrows rather than the channels of plant roots be-low depth of -20 cm, showing that most of the channels of plants roots hadn’t the ability to transfer water preferentially (shown in Table 3 and Fig. 4). The diameter of macropore might be the main influence factor in preferential flow devel-opment. In this study, the diameters of roots were 0.03–0.5 cm at the depth of 0– -70 cm. Other researchers found that the roots of plants with bigger diameters were the main path of preferential flow (Zhang et al., 2012; Xiao et al., 2011; Chen and Hui, 2006). Although the statistical zone was chosen in the main stained region, few of earthworm burrows weren’t acti-vated. Macropores stained by Brilliant Blue FCF were consid-ered as activated, and unstained macropores were inactivated, shown in Fig. 4b. Whether the macropores were activated or not was decided mainly by connectivity and filling degree of macropores at the bottom of the plowing zone where most of the preferential paths were cut down, therefore, only the macropores without matter filled or partly filled were dyed by Brilliant Blue FCF, i.e., activated. We also found that the acti-vated earthworm burrows often appeared together, resulting from the habit of earthworms.

2.2 Dyeing Pattern

The distributions of Brilliant Blue FCF were extremely heterogeneous in the soil profile according to the 60 sections. Figure 5 showed the dyeing pattern of 75 mm irrigation with previous 75 mm irrigation. The maximal infiltrated depths of

different sections were -73.8, -80, -120, -65.2, and -82.4 cm, respectively for Y=0 cm to Y=50 cm.

Table 3 Distribution of macropores in the soil profile

Depth(cm)

Earthworm per 100 cm2 Plant root per 1 cm2

Number Activated Number Activated

-10 - - 30 100%

-20 25 100% 10 30%

-30 23 100% 8 20%

-40 21 80% 8 0%

-50 24 88% 7 0%

-60 23 83% 6 0%

-70 11 0% 7 0%

Figure 4. Dyed macropores, e.g., earthworm burrows and roots of plant. (a) Earthworm burrows were stained (red circles), rather than the roots of plant (yellow triangle), at depth of -40 cm; (b) most of earthworm burrows were stained, i.e., activated (black rectangles), and only few not stained, i.e., inactivated (black circles).

Varied infiltrated depths indicated that the connectivity degrees of activated macropores were different, in other words, the preferential flow paths were controlled by the length of macropores. In fact, lots of earthworm burrows were stained partly, resulting from irrigation amount, exchange ability be-tween preferential paths and soil matrix, connectivity of macropores, and filling extent by incompact soil and excrement of earthworms. Isolated stained zones were found commonly in the both of vertical and horizontal soil profiles, especially for over -15 cm depth, suggesting that the variability of dyeing patterns was three-dimensional in space scale.

The essence of preferential is that the soil water flow in preferential paths is faster than the soil matrix, uniform and nonuniform flow for the matrix and preferential regions, re-spectively. In the field, the preferential paths, e.g., earthworm burrows and channels in the root zone of 0– -15 cm were de-stroyed completely by plowing, which caused the soil loose and uniform with higher hydraulic conductivity, while plentiful preferential paths are present under the plowing zone in the soil profile. In the plough region (0– -15 cm), the soil structure was destroyed by ploughing, but lots of new macropores formed and the soil were very incompact. The soil water flowing in both of new macropores and loose soil matrix all reached the bottom of plough region in a large time scale (e.g., one day),


Figure 5. Dyeing pattern of 75 mm irrigation with 75 mm previous irrigation.

although the water flowed faster in the new macropores than in the soil matrix. Therefore, the pattern of soil water flow in the region stained completely and uniformly when excavated in one day after irrigation in the horizontal direction was consid-ered as uniform flow, i.e., piston flow, and the stained and con-tinued regions were defined as uniform flow zones, shown in Fig. 3. The red line was the divide line between uniform flow and preferential flow zones. 2.3 Dye Coverage

The dye percentage of stained regions was applied to de-scribe the dye coverage in the soil profile. In generally, the dye percentages reduced with increased depth. The dye percentages at the depth of less than -15 cm were larger than 70%, and the stained regions were continual, indicating the uniformed soil flow. The percentages of preferential flow were defined as the ratio of stained area in the preferential flow zone to the whole stained area in the soil profile, shown in Table 4.

Table 4 Dyed area percentages of preferential flow

Experiment number 1 2 3 4 5

Percentage of pref-erential flow (%)

19.72 61.42 66.64 53.23 39.94

2.3.1 Irrigation intensity

Figure 6 showed the dye percentages of the three irrigation intensities, i.e., 25, 50, and 75 mm. The maximal infiltrated depths of 25, 50 and 75 mm irrigation were -49.1, -80.0, and -60.4 cm, with mean maximal infiltrated depths (each vertical section had a maximal infiltrated depth, and then the mean maximal infiltrated depth of one plot test was the mean value of all the maximal infiltrated depths of this plot test) of -25.4, -65.6, and -47.2 cm, respectively. The percentages above -2.0 cm were less than 100%, even 45%, resulting from nannorelief of the soil surface. The depths of 50% coverage referring to the depth where the dye coverage was 50% were -10.2, -22.6, and -24.5 cm, respectively. And the percentages of preferential flow were 19.72%, 61.42%, and 66.64%, respectively. In the exper-

iment of 25 mm irrigation, over 85% of region was dyed above -12.0 cm where the uniform flow happed mainly, while the dye coverage of deeper depths where the preferential flow (i.e., nonuniform flow) was the major pattern of soil water infiltra-tion, reduced rapidly. In comparison with 25 mm irrigation, the mean maximal infiltrated depth and dyed area were both higher in the 50 mm irrigation. The difference of dye percentages be-tween the sections of 75 mm irrigation was largest, but less difference between 50 and 75 mm.

Overall, higher irrigation intensity promoted preferential flow, but the increased extent was limited, shown in Table 4, agreeing with Sheng and Fang (2012). 2.3.2 Initial soil water content

The distribution of initial soil content vs. depth in the soil profile was shown in Fig. 7. The soil water contents increased with larger amount of the previous irrigation, and highest value for 180 mm previous irrigation. Overall, the soil water contents decreased with depth of the soil profile. The soil water content of above -10 cm in the case of 180 mm previous irrigation was 0.29, up to saturated content 0.30.

Figure 8 showed the dye percentages of different initial soil water contents for 0, 75 and 180 mm previous irrigation. The mean percentages of dye coverage were 26.0%, 34.1%, and 26.8%, with the maximal infiltrated depths of -60.0, -144.0,

Figure 6. Dye percentages of different irrigation intensities, i.e., 25, 50, and 75 mm irrigation.


and -92.6 cm, respectively. However, the percentages of pref-erential flow were 66.64%, 53.23%, and 39.94%, respectively. This meant that higher initial soil water content promoted infil-trating of irrigation, causing soil water flow more uniformly in the profile. However, it was difficult for irrigation water to infiltrate into deep soil, and the horizontal flow, referring to that in the ploughing region rather than the soil surface, the irriga-tion water flowed horizontally out of the plot of 1×1 m, devel-oped resulting in less water infiltrating vertically, in the 180 mm previous irrigation. 2.4 Quantifying Infiltration by Preferential Flow

In the preferential flow zone, i.e., under the plowing zone, the infiltrated intensities of preferential flow per depth were shown in Fig. 9. The patterns were different from the dyeing profile, owing to that the resolution of dyeing was 1 pixel per mm2, while 7.5×7.5 cm for concentration pattern with a mean concentration in the area 7.5×7.5 cm. Therefore, the concentra-tion distribution was more consecutive and less difference be-tween preferential paths and soil matrix.

2.4.1 Irrigation intensity

The total infiltration amounts of 25, 50, and 75 mm irriga-tion were 20.53, 26.35, and 42.79 mm, accounting for 82.10%, 53.13%, and 57.05% of the total irrigation, respectively. Higher irrigation intensity reduced the percentages of infiltration. However, the total infiltration amounts were less than the irri-gation intensities, the main reasons were: a) the horizontal flow developed in the plowing zone, especially for 50 and 75 mm irrigation. If the irrigation intensity was higher than the infiltra-tion capacity, the horizontal flow appeared. No ponding water was found in the 25 mm irrigation, showing that the irrigation intensity was smaller than the infiltration capacity of the plow-ing region. Therefore, less water flowed horizontally out of the irrigated region 1×1 m. While 0.5 and 1 cm height of ponded water on the soil surface for the 50 and 75 mm irrigation lasting for 12 and 15 min, respectively, which indicated that the infil-

tration intensity was less than 100 mm/h resulting in horizontal flow, moreover, the larger height of ponding water accelerated the horizontal flow; b) the Brilliant Blue FCF could be ab-sorbed by soil matrix, especially for heavy clay. And sorption wasn’t reversible and sorption- desorption hysteresis appeared apparently. This resulted that the measured concentrations of Brilliant Blue FCF were lower than the real values (Morris et al., 2008). Therefore, the total infiltration intensities were lower than the irrigation intensities; c) there were conspicuous spatial variability (shown in Fig. 5) among the vertical profiles,

Figure 7. Distribution of initial soil content in the soil pro-file.

Figure 8. Dye percentages of different initial soil water con-tent for 0, 75, and 180 mm previous irrigation with 75 mm irrigation.

Figure 9. Infiltrated intensities of preferential flow per depth in the soil profiles. (a), (b), and (c) were the cases of 25, 50, and 75 mm irrigation, respectively. (d) and (e) were the cases of 75 mm irrigation with 75 and 180 mm previous irrigation, respec-tively.


which caused the infiltrated intensities at the central profile (Y=45 cm, shown in Fig. 9) couldn’t characterize exactly the all information of the whole sections in the soil profile. The infiltration intensity by preferential flow was defined by the formula (3), as following

pre-boundary

pre ad( )dZ

ZR R Z Z

(3)

where Rpre is the infiltration intensity by preferential flow, L; Zpre-boundary is the depth for the boundary of preferential flow zone and uniform flow zone, L. Then the infiltration intensi-ties by preferential flow of 25, 50, and 75 mm irrigation were 3.42, 11.6, and 14.62 mm, respectively, showing that the total infiltration intensity of preferential flow was higher with the increased irrigation intensity. However, when the irrigation changed from 25 to 50 mm, the total infiltration of preferen-tial flow was increased by 239%, but only 26 % for 50 to 75 mm, indicating that the increased extent of preferential flow was limited when the irrigation intensity was large.

The percentages of preferential flow in the total infiltra-tion were 16.67%, 43.67%, and 34.17%, respectively. The percentages of preferential flow increased largely with higher irrigation intensity, but reduced when the infiltration intensity was over 50 mm. In the case of 25 mm irrigation, most of irrigation water infiltrated into the root zone of 0– -10 cm with less water into the preferential flow region. For the case of 50 mm, although less percentage of irrigation infiltrated into the soil profile, the percentage of preferential flow in-creased, resulting from that more macropores were activated. However, when the irrigation increased to 75 mm which was larger than the infiltration capacity causing a height of 1 cm ponding on the surface for 15 minutes, more water infiltrated into soil matrix and flowed horizontally in the ploughing region. 2.4.2 Initial soil water content

The total infiltration intensities of 75 mm irrigation with 0, 75, and 180 mm previous irrigation were 42.79, 39.53, and 29.68 mm, accounting for 57.05%, 52.70%, and 39.57% of the total irrigations, respectively. The infiltration capacity of preferential flow were 14.62, 9.2, 5.63 mm, with 34.17%, 23.26%, and 18.97% of the total irrigations, respectively. The results revealed that higher initial soil water content re-strained the preferential flow, although only 50% or less irri-gation water infiltrated into the soil profile of 1×1 m. The reason was that: 1) soil capillary of lower initial soil water content could accelerate the soil water infiltrating, resulting in soil water flowing preferentially; 2) the clay would expand when higher initial soil water contents were obtained by irri-gating before the dyeing test. The previous irrigation leaded to the soil compacting, low hydraulic conductivity, and visible horizontal flow in the plowing zone, which destroyed most of macropores in the plowing region and blocked lots of macropores’ entrances at boundary of plowing region and preferential flow region. Especially for the 180 mm previous irrigation, the soil structure was destroyed completely result-ing it was difficult to infiltrate into soil although ponding 2 cm height water for 15 min on the soil surface. Therefore, the

preferential flow was restrained by higher initial soil water content.

The mean percentage of infiltration was 56.9%, showing that the soil profile 1×1 m could capture partly the total irri-gation. Some activated macropores developed into depths -120 and -140 cm in the case of 75 mm previous irrigation at Y=20 and 30 cm, respectively. Additionally, the results in Fig. 7 were just for Y=45 cm profile in each experiment. Accord-ing to the results of dyeing pattern, the irrigation infiltration pattern and infiltration capacity of preferential flow in the soil profile varied at different sections. Moreover, the dyed earthworm burrows could be found outside of the frame 1×1 m, attributing to the variability of macropores distribution in the field. This phenomenon was distinguished in the cases of 75 and 180 mm previous irrigation and most of dyeing re-gions out of the frame were under the depth of -10 cm. 3 CONCLUSIONS

In summary, we have demonstrated that the percentage of preferential flow was larger with increased irrigation in-tensity, but reduced when the irrigation intensity was over 50 mm. High initial soil water content inhibited the preferential flow from developing in the field. The mean percentage of preferential flow in the total irrigation was 27.3%. And most of active macropores were earthworm burrows, rather than the channel of the plants’ roots in this field, and only partly marcopores were activated, which resulted mainly from the connectivity, size of diameters, filling degree of macropores at the bottom of the plowing zone, and the habit of earth-worms. Based on these findings, the macropore preferential flow plays a vital role in the infiltration of irrigation or rain-fall. Therefore, intermittent irrigation patterns and low irriga-tion intensity (i.e., increasing the frequency of irrigation with the same total irrigation) would decrease the preferential flow, and large plowing depth could destroy the macropore paths, which improved the utilization efficiency of irrigation or rainfall. However, further studies are needed to determine the mechanism of preferential flow on the condition of small interval initial soil water content and irrigation intensity. And it was necessary to investigate the effect of soil capillarity on the soil water movement, since the soils with different soil water contents have different capillarity. ACKNOWLEDGMENTS

This study was supported by the National Basic Re-search Program of China (No. 2010CB428802), the National “Twelfth Five-Year” Plan for Science & Technology Support (No. 2011BAB10B04), and the National Natural Science Foundation of China (Nos. 51279016 and 41372243). Thanks for all members who participated in the field work and of-fered helpful advice for this study. REFERENCES CITED Alaoui, A., Caduff, U., Gerke, H. H., et al., 2011. Preferential

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quantifying the preferential flow by dye tracer in the...

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