subsurface flow processes in sloping cropland of purple soil

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J. Mt. Sci. (2012) 9: 1–9 DOI: 10.1007/s11629-012-2199-7 1 Abstract: Subsurface flow is a prominent runoff process in sloping lands of purple soil in the upper Yangtze River basin. However, it remains difficult to identify and quantify. In this study, in situ runoff experimental plots were used to measure soil moisture dynamics using an array of time domain reflectometry (TDR) together with overland flow and subsurface flow using isolated collecting troughs. Frequency of preferential flow during rainfall events and the controls of subsurface flow processes were investigated through combined analysis of soil properties, topography, rainfall intensity, initial wetness, and tillage. Results showed that subsurface flow was ubiquitous in purple soil profiles due to well- developed macropores, especially in surface soils while frequency of preferential flow occurrence was very low (only 2 cases in plot C) during all 22 rainfall events. Dry antecedent moisture conditions promoted the occurrence of preferential flow. However, consecutive real-time monitoring of soil moisture at different depths and various slope positions implied the possible occurrence of multiple subsurface lateral flows during intensive storms. Rainfall intensity, tillage operation, and soil properties were recognized as main controls of subsurface flow in the study area, which allows the optimization of management practices for alleviating adverse environmental effects of subsurface flow in the region. Keywords: Subsurface lateral flow; Purple soil; Soil moisture; Sloping land; Land use Introduction Rapid transport of water and chemicals in soils is associated with processes such as flow through earthworm burrows, cracks, hydrophobicity, or soil layering, leading to contamination in groundwater (Clothier et al. 2008; Allaire et al.). It has been well recognized that subsurface flow in soils contributes to rapid transport of nutrients and chemicals out of the soils (Jarvis 2007). Methods of investigating subsurface flow include dye tracing (Weiler and Flühler 2004; Anderson et al. 2009), soil trenching and excavations (Mosley 1982; Graham et al. 2010), geophysical methods (Garre et al. 2010; Oberdorster et al. 2010), and in situ monitoring (Lin and Zhou 2008). At the plot and hillslope scales, soil moisture responds to variations in vegetation (Qiu et al. 2001), soil properties (Famiglietti et al. 1998), topographically-driven lateral flow (Dunne and Black 1970), and solar radiation (Western et al. 1999). Soil and topography are widely recognized as important local controls of soil moisture variation (Zhu and Lin 2011). Soil moisture distribution in a landscape has often been conceptualized as being controlled predominantly by soil properties during dry periods, such as water repellency (Jarvis 2007), and by topography during wet periods. Subsurface Flow Processes in Sloping Cropland of Purple Soil TANG Jialilang 1,2 , ZHU Bo 1 *, WANG Tao 1 , CHENG Xunqiang 1 , GAO Meirong 1 , LIN Henry 2 1 Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610041, China 2The Pennsylvania State University, Department of Crop and Soil Sciences, University Park, PA 16802, USA * Corresponding author, [email protected] © Science Press and Institute of Mountain Hazards and Environment, CAS and Springer-Verlag Berlin Heidelberg 2012 Received: 29 May 2011 Accepted: 8 October 2011

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Page 1: Subsurface flow processes in sloping cropland of purple soil

J. Mt. Sci. (2012) 9: 1–9 DOI: 10.1007/s11629-012-2199-7

1

Abstract: Subsurface flow is a prominent runoff process in sloping lands of purple soil in the upper Yangtze River basin. However, it remains difficult to identify and quantify. In this study, in situ runoff experimental plots were used to measure soil moisture dynamics using an array of time domain reflectometry (TDR) together with overland flow and subsurface flow using isolated collecting troughs. Frequency of preferential flow during rainfall events and the controls of subsurface flow processes were investigated through combined analysis of soil properties, topography, rainfall intensity, initial wetness, and tillage. Results showed that subsurface flow was ubiquitous in purple soil profiles due to well-developed macropores, especially in surface soils while frequency of preferential flow occurrence was very low (only 2 cases in plot C) during all 22 rainfall events. Dry antecedent moisture conditions promoted the occurrence of preferential flow. However, consecutive real-time monitoring of soil moisture at different depths and various slope positions implied the possible occurrence of multiple subsurface lateral flows during intensive storms. Rainfall intensity, tillage operation, and soil properties were recognized as main controls of subsurface flow in the study area, which allows the optimization of management practices for alleviating adverse environmental effects of subsurface flow in the region. Keywords: Subsurface lateral flow; Purple soil; Soil moisture; Sloping land; Land use

Introduction

Rapid transport of water and chemicals in soils is associated with processes such as flow through earthworm burrows, cracks, hydrophobicity, or soil layering, leading to contamination in groundwater (Clothier et al. 2008; Allaire et al.). It has been well recognized that subsurface flow in soils contributes to rapid transport of nutrients and chemicals out of the soils (Jarvis 2007).

Methods of investigating subsurface flow include dye tracing (Weiler and Flühler 2004; Anderson et al. 2009), soil trenching and excavations (Mosley 1982; Graham et al. 2010), geophysical methods (Garre et al. 2010; Oberdorster et al. 2010), and in situ monitoring (Lin and Zhou 2008).

At the plot and hillslope scales, soil moisture responds to variations in vegetation (Qiu et al. 2001), soil properties (Famiglietti et al. 1998), topographically-driven lateral flow (Dunne and Black 1970), and solar radiation (Western et al. 1999). Soil and topography are widely recognized as important local controls of soil moisture variation (Zhu and Lin 2011). Soil moisture distribution in a landscape has often been conceptualized as being controlled predominantly by soil properties during dry periods, such as water repellency (Jarvis 2007), and by topography during wet periods.

Subsurface Flow Processes in Sloping Cropland of Purple

Soil

TANG Jialilang 1,2, ZHU Bo1*, WANG Tao1, CHENG Xunqiang1, GAO Meirong1, LIN Henry2

1 Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610041, China

2The Pennsylvania State University, Department of Crop and Soil Sciences, University Park, PA 16802, USA

* Corresponding author, [email protected]

© Science Press and Institute of Mountain Hazards and Environment, CAS and Springer-Verlag Berlin Heidelberg 2012

Received: 29 May 2011 Accepted: 8 October 2011

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Since it is difficult to measure and quantify the components of subsurface flow, subsurface hydrologic pathways remain unclear for assessing environmental impacts from subsurface contaminant transport (Jarvis 2007; Lin and Zhou 2008; Allaire et al. 2009), especially in intensively cropped catchments (Tang et al. 2008) where relatively flat topography and less deep organic layers (O or A horizon) exist as compared to forested catchments.

In an ecologically sensitive area in upper reaches of the Yangtze River, where Sichuan Basin is located, more than 65% of the total cropland is on slopes (Zhu et al. 2009). Sichuan Basin is an important agribusiness area in southwestern China. The basin occupies 7% of the national cropland and supplies 10% of the agricultural products of China. The typical soil type (called purple soil locally) in Sichuan Basin is fertile, supporting high crop production (Zhu et al. 2008). Purple soil is widely distributed in Sichuan Basin, with an area of 160,000 km2 (Li et al. 1991). However, the soil is particularly vulnerable to erosion owing to intensive cultivation and the monsoon climate in this area. Consequently, runoff carrying large amounts of sediment and nutrients flows into the branches and confluences of the Yangtze River, and finally reaches the main stream and the Three Gorges Reservoir (TGR). A great deal of attention has been paid to N transport through overland flows and sedimentation (Huang et al. 1998; Fu et al. 2003). At the meantime, significance of N leaching from the croplands in the areas had been elucidated (Zhu et al. 2009) and a high export of N via subsurface flow was estimated as 36 kg ha-1, annually.

In order to prevent the adverse effects of solute and clay-associated compounds transport via subsurface flow, identification of subsurface flow occurrence and its controls are important for implementing management practices. It is hypothesized that subsurface lateral flow might occur above plow layer (restricting layer) in sloping cropland in the purple soil area. The specific objectives of this study were 1) to investigate the soil hydraulic properties of this locally unique purple soil; and 2) to identify the relationship between the occurrence of subsurface flow in sloping cropland and their possible controls such as soil properties, tillage operations, and rainfall

intensity.

1 Material and Methods

1.1 Site description

This study was conducted at the Yanting Agroecological Station of Purple Soil, which is a part of the Chinese Ecosystem Research Network of the Chinese Academy of Sciences. This station is situated in the middle of Sichuan Basin in southwest China (31°16' N, 105°28' E, altitude 420 m). The experimental area has a moderate subtropical monsoon climate with an annual mean temperature of 17.3°C and a mean precipitation of 826 mm from 1981 to 2006. During this time period, 5.9, 65.5, 19.7, and 8.9% of annual precipitation occurred in the spring, summer, autumn, and winter, respectively (Zhu et al. 2009). Rainfed farming has been maintained all along.

The soil type in the experimental area is typical of local fertile agricultural soils, called purple soil, which is classified as a Regosol in the FAO system, a Pup-Orthic Entisol in the Chinese Soil Taxonomy, and an Entisol in the U.S. Soil Taxonomy (Gong 1999). The soil derived from purplish shale is generally shallow, with soil thickness 30 to 80 cm. The soil has a typical “binary structure of soil–bedrock” (Xiong and Li 1986), with shallow soil overlying bedrock with poor hydraulic conductivity (Zhu et al. 2009). Summer rain storms often lead to water saturation of the soil profile, which drives overland runoff and interflow moving downward along the slope.

1.2 Experimental plots setup and monitoring

Based on the hillslope hydrologic characteristics, a total of 46 runoff plots were designed and constructed to study the fertilization effects on nutrient cycling in cropland. Among them, three plots (Plot A, B, C) treated with the same fertilization of balanced nitrogen, phosphorus and potassium were instrumented for soil moisture monitoring (Figure 1). All the plots were hydrologically isolated with partition walls filled with cement, inserted at least 60 cm into the bedrock to avoid lateral seepage from adjacent

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plots. A conflux trough was built at the soil surface and at the soil-bedrock interface to collect both overland flow and interflow above the bedrock (Figure 1). The interflow conflux trough was excavated 10 cm below the soil–bedrock interface and filled with clean arenaceous quartz and pebble to the level of the soil–bedrock interface. Buckets were installed under each conflux trough to collect water samples from both overland flow and subsurface interflow, respectively. To record the discharge of overland flow and interflow, Micro-Diver Datalogger (Schlumberger Water Services, Cleveland, Australia) were installed in the water-collecting buckets. Climate data at 1-h interval were obtained from the meteorological station at the Yanting Agro-ecological Station of Purple Soil.

Each runoff plot has an area of 8 m (length) by 4 m (width), with a slope gradient of 7°and soil depth of 60 cm (Figure 1). These experimental plots functioned as free-drainage lysimeters with a relatively large surface area. All the plots were cropped conventionally with wheat (Triticum aestivum L.) in late October to next May, and then rotated with summer maize (Zea mays L.) from May to September.

In the selected three runoff plots (A, B, C), time domain reflectometry (Soilmoisture Equipment Corporation, Santa Barbara, CA, USA) probes were installed to automatically measure

volumetric soil moisture content in three slope positions – up, middle, and low slopes, each with three replicates (Figure 1). A distance of 2 m was arranged between two neighboring slope positions. At each slope position, a pit was excavated from the soil surface to the soil-bedrock interface. In the excavated pit, soil moisture probes were installed horizontally with at least 100 cm distance away from the plot edge. Probes were installed based on soil horizons and their interfaces, with the vertical distance between probes ranging from 10 to 20 cm. Generally, the top probe in the profile is at the bottom of tillage layer at about 15 cm depth, the middle probe is in the clay illuviation layer at about 25 cm depth, and the deepest probe is within the thin layer of parent material right above the soil-rock interface.

The real time soil moisture was collected at 20-minute time interval from 8 July 2009 to 22 December 2009. Summer maize was planted from end of May through middle of September and the plots were fallowed. During this period, a total of 605.2 mm precipitation fell in the study sites and 27 events were classified (9 events exceeding 20 mm and 4 events exceeding 50 mm precipitation) based on the following principle: individual events were defined as beginning when more than 1 mm precipitation fell after greater than 24 hours of no precipitation. Once an event begins, it is

Figure 1 Design of the soil moisture monitoring system used in this study

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considered to continue until the total precipitation in any given 24 hours is less than 1 mm. After the classification, 5 events were excluded because of TDR malfunction during the monitoring period. Thus, total 22 events were used for analysis of preferential flow occurrence. Three events were illustrated (Event I- July 9th, Event II- July 31st and Event III- September 13th), indicating a time sequence of different plant covers by summer maize and different rainfall types.

The given method (Lin and Zhou 2008) was followed to identify subsurface preferential flow when a subsurface horizon responded to a rainfall input earlier than a soil horizon above it. Intuitively, rainwater should infiltrate into the soil and pass the upper horizon(s) first before it can reach a lower horizon. Therefore, when a soil moisture sensor buried in a subsoil layer responded to a storm event earlier than other sensors above it, it was assumed the rainwater had bypassed the overlying horizon(s) or the rain water had percolated into the deeper subsoil from the upslope or sideslope areas via subsurface lateral flow, or both (Lin and Zhou 2008). In both cases a preferential subsurface flow has occurred. It was then to calculate the frequency of subsurface preferential flow in the study area during the whole monitoring period, including all 9 events.

1.3 Soil sampling and analysis

Intact soil cores (50.46 mm diameter by 50 mm height) were collected at the depths of 0-15, 15-35, and 35-50 cm under three types of land use to measure soil water retention curve. The three land uses were 1) sloping rainfed crop, 2) secondary woodland, and 3) rice paddy field

(periodically irrigated and flooded during the growing period). Soil water retention curves of sampled soil cores were measured at 0, 10, 30, 50, 100, 300, 500, 1000, 1500 kPa using pressure plates (Soilmoisture Equipment Corporation, Santa Barbara, CA, USA), and the Brooks-Corey equation (Leij et al. 1997) was used to describe the measured soil water retention data.

Meanwhile, triplicate soil cores and bulk soil samples at the depths of 0-15, 15-25, and 25-40 cm at the soil moisture monitoring plots (Figure 1) were also collected through excavating a soil pit to measure saturated hydraulic conductivity (Ksat), bulk density. The vertically-oriented soil samples were collected by digging downward to desired depths and then inserting the rings into the soil vertically. The Ksat was determined in the laboratory using the constant head method (Klute 1986). After the Ksat measurements, soil bulk density was determined for each soil core (Table 1). Soil texture was analyzed using Mastersizer 2000 (Malvern Instruments Ltd, Worcestershire, UK).

1.4 Double ring infiltration and dye tracing

A double-ring infiltrometer (Ahuja et al. 1976) was installed for the treated plots under different residue amendment. The small ring of 30-cm diameter was used as the inner ring. The outer ring was 60 in diameter to set up a buffer zone. The infiltrometer rings were installed with care so as to minimize disturbance to the soil. The rings, all 30 cm high and made of 1/3-cm galvanized steel, were leveled from outside to inside at the lower edge. They were driven 10 cm deep into the ground starting with the small ring. After the driving was

Table 1 Soil profile properties of purple soils. Numbers in the brackets are standard deviations of the means shown (n = 3)

Depth (cm) Clay content (%) Silt content (%) Sand content (%) Bulk density (g cm-3) Ksat* (cm h-1)

33.98 a# 45.43 a 20.59 a 1.19 a 645 a 0-15

(20.99) (15.39) (6.91) (0.05) (385)

53.31 a 33.10 a 13.59 a 1.64 b 58 b 15-30

(16.66) (10.74) (6.12) (0.08) (49)

29.94 a 44.45 a 25.60 a ND ND 30-50

(19.81) (14.50) (11.39) ND ND

Note: * Ksat is saturated hydraulic Conductivity, # Different letters in the column indicate significant difference among different depths (P<0.05). ND= No data

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completed, the disturbed soil adjacent to the rings was tamped firm with a hand tool. The process of infiltration was initiated by ponding water in the inner ring and in the buffer zone to a level 5 cm above the soil surface, which was maintained throughout the duration of the measurement by means of Mariotte vessels. The soil within the rings was covered with pieces of burlap to prevent direct water impact and subsequent disturbance of soil surface. The water or dye solution was supplied from supply vessel fitted with glass tubes to show the water level inside. The water level was read at 1- to 20-min intervals as a measure of cumulative infiltration in the inner ring.

Two of the 46 studied plots were selected for the dye tracing experiment, 5 L methylene blue trihydrate (1.0 g L-1) and following 15 L of tap water were applied within the inner ring as tracer for mass movement in soil profiles after the infiltration experiment, all the irrigation operations had been completed within 20 minutes in terms of fast infiltration rates. Standard digital camera (Kodak EasyShare, Z650) was used with natural light for the level stained layers every 2 cm downward until deepest stained layer in the semicircular area of inner ring, while the other half of the inner ring area was exposed with vertical face of dye stain. Visualization of dyed profiles provides direct information about soil macropore distribution and possible subsurface flow processes.

1.5 Data analysis

Rainfall events were classified according to the local rainfall-runoff characteristic with more than 20 mm of total precipitation being the criterion. T test or ANOVA was employed to investigate the significant differences of soil physical properties and soil moisture under different conditions using SPSS (SPSS Inc., 2004). The rainfall

characteristics and soil moisture conditions were calculated based on the rainfall events classification using Matlab 7.0 (The MathWorks, Inc., USA).

2 Results and Discussion

2.1 Preferential flow and its controls

Only two preferential flow events occurred at middle slope position in plot C during all the 22 events (Table 2), suggesting that sequential flow was the dominant flow type (10 events in this site and all 12 events in other sites). Generally, soil moisture in this area had no response to rainfall events with less than 6 mm precipitation (10 events). The preferential flow was controlled by antecedent wetness reflected in less previous rainfall and dry soil. However, high total precipitation and intensity are also the necessity for preferential flow occurrence.

Figure 2 showed distinct existence of

Table 2 Statistics of rainfall and antecedent soil moistures at middle slope position in plot C

Antecedent rainfall Antecedent soil moisturesFlow type TP (mm)

MI (mm/hr)

Duration (days) API1

(mm) API4 (mm)

API7 (mm)

API14 (mm)

15cm (%)

25cm (%)

40cm (%)

PL (n=2) 65.4 11.1 2.35 5.5 14.8 15.8 29.5 26.0 25.2 25.4

SF (n=10) 28.66 8.78 1.71 13.5 42.4 60.9 74.1 30.0 30.1 33.2

NF (n=10) 2.76 1.16 1.07 7.96 19.42 22.56 54.86 30.3 30.7 34.0

Note: TP=Total precipitation, MI= precipitation, PL= Preferential flow, SF= Sequential flow, NF= No flow

Figure 2 A typical preferential flow event occurred during the event on August 16th-17th at middle slope of plot C.

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preferential flow with deeper layer responding early than surface layers during this event on August 16th. As both of two preferential flow events are in July and August with highest temperature, the results indicated during dry and hot condition may enhance the occurrence of preferential flow in this study area, being consistent with other research results due to either soil cracking in desiccated soils or hydrophobicity (Taumer et al. 2006).

2.2 Soil moisture characteristics and responses to storm events

During the event I (July 9, 2009), the rainfall was initially intensive and then weakened, lasting for about 5 hours (Figure 3). The previous rainfall with 10 days was 3.4 mm (Table 3). In terms of the sequential cases, after a 6 mm’s precipitation, surface soil moisture responded immediately to the rainfall in the second hour with 14.4 mm rainfall. The second layer had about 20 minutes’ delay compared to the first layer’s response. Meanwhile the bottom layer had a delay from 0 minutes to 20 minutes (Figure 3, I), which suggested a gradual wetting process. The time reaching moisture peak was different for all the soil layers. Generally, the deeper layer took longer time to reach saturation status. Time needed for saturation in surface layers was about 60 minutes which was 20 minutes shorter than the second layer and 20- 80 minutes shorter than the bottom layer. It was verified that the bottom layer at lower slope position had slower responses than upper positions during this event (Figure 3, I) and was similar in other events (not shown), which was due to site specific dense layer at the location of plot C (Figure 1).

During event II (July 31, 2009), rainfall was more evenly distributed with sufficient antecedent rainfall (Table 3). Soil moisture curves were sequential types and the subsurface flow rise came just after the moisture peak of bottom layer (Figure 3, II). It could be inferred that in this event subsurface flow was formed by the gradual seepage of rain water.

During event III (September 13, 2009), a single peaked rainstorm with wet antecedent soil moisture (Table 3) accelerated the soil water movement velocity, showing quick soil moisture responses within about 20 minutes in surface layer

and within 40-60 minutes in subsoil layers (Figure 3, III). However, the bottom layer at lower slope position showed faster response to rainfall, indicating a more preferential path for upslope lateral flow input. Time needed for saturation in surface layers was about 60 minutes which was shorter than the subsoil layers. It is illustrated that once the surface layer was saturated, the interflow rise came simultaneously while all the subsoil layers were not saturated until at least 40 minutes

Figure 3 Soil moisture changes during the three storm events investigated: I) storm event on July 9th in plot C; II) storm event on July 31st in plot B; III) storm event on September 13th in plot B. Interflow flush refers to the increasing interflow water table measured by Diver in the interflow collecting bucket, Diver was not instrumented before the event II.

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later (Figure 3, III). These relatively long lags of saturation in subsoil layers sufficiently supported the occurrence of subsurface lateral flow above the plough layer. Nevertheless, the occurrence of this type of subsurface flow might be limited in quantity and in most intensive storms as event III.

2.3 Soil properties and their implications to subsurface flow and potential pathways in purple soil area

It is quite distinct that clay illuviation contributed to a higher proportion of clay content and significantly (P<0.05) higher bulk density of the subsoil (Table 1), forming a possible restricting layer which favors subsurface lateral flow generation (Lin 2006). In addition, Figure 4

illustrates that sloping cropland soil had different water retention curves from woodland soil and paddy field soil. At low soil water tension status, the relative degree of saturation (RS) in surface layer was higher than in subsoil layers in sloping cropland, except at the middle slope position. Generally, RS of soil decreased with soil depth, indicating that more water could be retained in surface layer than in subsoil at the beginning of the wet-dry period, which accorded with coarser texture caused by illuviation process (Fedoroff 1997) and more roots and animal burrows in surface soil as well. While, slope position within small plot did not show impacts on soil water retention characteristics, which verified the previously reported conclusion (Ovalles and Collins 1986;

Figure 4 Soil water retention curve in sloping cropland, forest and paddy fields(U, M, L, F, P indicates upper slope, middle slope, lower slope of sloping cropland, forest and paddy fields, respectively; 1, 2, 3 indicates 0-15 cm, 15-35 cm and 35-50 cm depth respectively)

Table 3 Summary of monitored storm and runoff events from the experimental plots

Rainfall Antecedent soil moistures Average outflow Storm event Date Total

(mm) Peak (mm/h)

15cm (%)

25cm (%)

40cm (%)

Surface (mm)

Subsurface (mm)

I 2009-07-09 56.8 14.4 27.7 29.8 32.7 6.0 26.0

II 2009-07-31 75.6 10.4 26.4 24.4 25.6 28.5 34.7

III 2009-09-13 42.4 19.2 25.4 26.1 26.4 21.0 26.1

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Pachepsky et al. 2001). Surface layer of woodland soil and paddy field soil (Anthrosol, FAO) had lower RS value than sloping cropland soil, which indicated lower water retention capability of surface soil in woodland and paddy field. This difference is probably due to the more fractured features such as high content of rock fragments for shallow woodland soil and monostructure feature in surface layer of paddy field soil. The water retention capability of subsoils in woodland and paddy soils increased with depth, which suggested the possibility of a high water holding capacity in woodland subsoil during storms and less disturbed structure of deep layer of paddy soil. The fitted Brooks-Corey equation (Leij et al. 1997) showed that high determination coefficients (R2) had been achieved for all the soil layers, ranging from 0.82 to 0.98. The soil properties at 15-30 cm depth verified that a preliminary restricting layer, caused by compact of tillage operation and clay illuviation, was formed with significantly higher bulk density and lower Ksat value than surface layer (Table 1).

Furthermore, distribution of stained dye was more ubiquitous in the surface soil (0-15 cm) as compared to only sparsely scattered feature in subsoil below 15 cm. The distinct contrast suggested that only a very small proportion of preferential paths existed in vertical direction and subsurface lateral flow may form on the restricting layer during intensive storm events. High-intensity rain may promote the generation of lateral subsurface flow both in the soil profile above restricting layer and at the soil-rock interface of cropped hillslopes in the study area (Figure 5).

In this study, subsurface flow collected at the lower troughs explained large portion of total precipitation during intensive storm events, ranging from 45% to 62% (Table 3). It has been reported that subsurface lateral flow at the soil-rock interface contributed to most of subsurface flow in some cases (Lin 2006; Lin and Zhou 2008; Allaire et al. 2009). and also earlier observation in this area (Zhu et al. 2009) assumed that rapid sequential flow and then its turning into following lateral flow at the soil-rock interface was the main mechanism of subsurface flow generation. In most cases, rapid sequential response to precipitation is also a characteristic of preferential flow, such behavior can also be due to differences in particle and pressure responses (Rasmussen et al. 2000;

Torres and Alexander 2002) or simply high soil matrix permeability in some cases, however, sequential flow with longer time needed for subsoil saturation in this study was supposed to contribute to subsurface lateral flow in upper part of soil profiles above the clay-rich layer, which was also a type of infiltration-excess process (Orlandini et al. 1996; Kim et al. 2005) during intensive storm events in purple soil region.

3 Summary and Conclusions

Combined effects of soil properties, tillage operation, topography, rainfall intensity, and initial wetness on the occurrence of subsurface flow were analyzed in the purple soil area in Upper Yangtze River basin. Sequential flow is the dominant flow type in the soil profile with only very few cases of preferential flow occurred in the study area. High rainfall intensities significantly increased subsurface flow. Tillage operation increased the restricting characteristics of plow layer and thus changed soil water movement pathways. This study implied that multiple hydrological pathways might exist in the purple soil area and subsequently influence downstream water quality due to nitrate leaching and subsurface transport.

Acknowledgement

This work was funded by the Natural Science Foundation of China (Grant No. 40801101) and the

Figure 5 Two types of interfaces observed in purple soil profiles. 1) Left: soils located at the lower part of hillslope with wheat cropping have a restricting layer underneath at 15 cm depth; and 2) Right: soils located at the upper or middle part of hillslope with corn cropping have a bedrock interface at 50 cm depth.

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Chinese Academy of Sciences for supporting New PhD staff in the initial research stage. Dr. C B Graham providing the Matlab codes and Dr. XH Peng providing the saturated hydraulic

conductivity measuring apparatus are gratefully acknowledged. We also wish to acknowledge the helpful contributions made by the staff at Yanting Agro-ecological Station of Purple Soil.

References

Ahuja LR, Elswaify SA, Rahman A (1976) Measuring hydrologic properties of soil with a double-ring infiltrometer and multiple-depth tensiometers. Soil Science Society of America Journal 40: 494-499.

Allaire SE, Roulier S, Cessna AJ (2009) Quantifying preferential flow in soils: A review of different techniques. Journal of Hydrology 378: 179-204.

Anderson AE, Weiler M, Alila Y, Hudson RO (2009) Dye staining and excavation of a lateral preferential flow network. Hydrology and Earth System Sciences 13: 935-944.

Bauters TWJ, DiCarlo DA, Steenhuis TS, Parlange JY (1998) Preferential flow in water-repellent sands. Soil Science Society of America Journal 62: 1185-1190.

Clothier BE, Green SR, Deurer M (2008) Preferential flow and transport in soil: progress and prognosis. European Journal of Soil Science 59: 2-13.

Dunne T, Black RD (1970) Partial area contributions to storm runoff in a small New-England watershed. Water Resources Research 6: 1296-1311.

Famiglietti JS, Rudnicki JW, Rodell M (1998) Variability in surface moisture content along a hillslope transect: Rattlesnake Hill, Texas. Journal of Hydrology 210: 259-281.

Fedoroff N (1997) Clay illuviation in Red Mediterranean soils. Catena 28: 171-189.

Fu T, Ni JP, Wei, CF, Xie DT (2003) Research on the nutrient loss from purple soil under different rainfall intensities and slopes. Plant Nutrtion and Fertilizer Science 9: 71-74. (In Chinese)

Garre S, Koestel J, Gunther T, Javaux M, Vanderborght J, Vereecken H (2010) Comparison of heterogeneous transport processes observed with electrical resistivity tomography in two soils. Vadose Zone Journal 9: 336-349.

Gong ZT (1999) Chinese soil taxonomy. Science Press, Beijing. (In Chinese)

Graham CB, Woods RA, McDonnell JJ (2010) Hillslope threshold response to rainfall: (1) A field based forensic approach. Journal of Hydrology 393: 65-76.

Huang L, Ding SW, Dong Z, Zhang GY (1998) Study on nutrient losses of purple soil in Three Gorges Reservoir region. Chinese Journal of Soil Water Conservation 4: 8-13. (In Chinese)

Jarvis NJ (2007) A review of non-equilibrium water flow and solute transport in soil macropores: principles, controlling factors and consequences for water quality. European Journal of Soil Science 58: 523-546.

Kim HJ, Sidle RC, Moore RD (2005) Shallow lateral flow from a forested hillslope: Influence of antecedent wetness. Catena 60: 293-306.

Klute A (Ed.) (1986) Methods of soil analysis. Part 1. Agron. Monogr. 9. ASA and SSSA, Madison, WI.

Leij FJ, Russell WB, Lesch SM (1997) Closed-form expressions for water retention and conductivity data. Ground Water 35: 848-858.

Li ZM, Zhang XW, He YR, Tang SJ (1991) Purple soil in China (A). Science Press, Beijing.

Lin H (2006) Temporal stability of soil moisture spatial pattern and subsurface preferential flow pathways in the shale hills catchment. Vadose Zone Journal 5: 317-340.

Lin H, Zhou X (2008) Evidence of subsurface preferential flow using soil hydrologic monitoring in the Shale Hills catchment. European Journal of Soil Science 59: 34-49.

Mosley MP (1982) Subsurface flow velocities through selected forest soils, South Island, New Zealand. Journal of Hydrology 55: 65-92.

Oberdorster C, Vanderborght J, Kemna A, Vereecken H (2010) Investigating preferential flow processes in a forest soil using time domain reflectometry and electrical resistivity tomography. Vadose Zone Journal 9: 350-361.

Orlandini S, Mancini M, Paniconi C, Rosso R (1996) Local contributions to infiltration excess runoff for a conceptual catchment scale model. Water Resources Research 32: 2003-2012.

Ovalles FA, Collins ME (1986) Soil-landscape relationships and soil variability in North Central Florida. Soil Science Society of America Journal 50: 401-408.

Pachepsky YA, Timlin DJ, Rawls WJ (2001) Soil water retention as related to topographic variables. Soil Science Society of America Journal 65: 1787-1795.

Qiu Y, Fu BJ, Wang J, Chen LD (2001) Soil moisture variation in relation to topography and land use in a hillslope catchment of the Loess Plateau, China. Journal of Hydrology 240: 243-263.

Rasmussen TC, Baldwin RH, Dowd JF, Williams AG (2000) Tracer vs. pressure wave velocities through unsaturated saprolite. Soil Science Society of America Journal 64: 75-85.

Reynolds WD, Bowman BT, Brunke RR, Drury CF, Tan CS (2000) Comparison of tension infiltrometer, pressure infiltrometer, and soil core estimates of saturated hydraulic conductivity. Soil Science Society of America Journal 64: 478-484.

Tang JL, Zhang B, Gao C, Zepp H (2008) Hydrological pathway and source area of nutrient losses identified by a multi-scale monitoring in an agricultural catchment. Catena 72: 374-385.

Taumer K, Stoffregen H, Wessolek G (2005) Determination of repellency distribution using soil organic matter and water content. Geoderma 125: 107-115.

Taumer K, Stoffregen H, Wessolek G (2006) Seasonal dynamics of preferential flow in a water repellent soil. Vadose Zone Journal 5: 405-411.

Torres R, Alexander LJ (2002) Intensity-duration effects on drainage: Column experiments at near-zero pressure head. Water Resources Research 38, 1240, doi: 1210.1029/ 2001WR001048.

Weiler M, Flühler H (2004) Inferring flow types from dye patterns in macroporous soils. Geoderma 120: 137-153.

Xiong Y, Li QK (1986) Soils in China. Science Press, Beijing. (In Chinese)

Zhu B, Wang T, You X, Gao MR (2008) Nutrient release from weathering of purplish rock. Pedosphere 18: 257-264.

Zhu B, Wang T, Kuang FH, Luo ZX, Tang JL, Xu TP (2009) Measurements of Nitrate Leaching from a Hillslope Cropland in the Central Sichuan Basin, China. Soil Science Society of America Journal 73: 1419-1426.

Zhu Q, Lin H (2011) Influences of soil, terrain and crop growth on soil moisture variation from transect to farm level scale. Geoderma 163: 45-54.