Feasible evaluation of quantifying soil preferential flow in an oasis agroecosys

Chen Rui, Lin Haixia, Wang Zhenhua

(1.College of Water Resources and Architectural Engineering, Shihezi University, Shihezi 832000, China; 2.Key Laboratory of Modern Water-Saving Irrigation of Xinjiang Production and Construction Corps, Shihezi 832000, China; 3.Key Laboratory of Northwest Oasis Water-Saving Agriculture, Ministry of Agriculture and Rural Affairs, Shihezi 832000, China)

Abstract: Preferential flow in the vadose zone brings challenges in characterizing water and solute transportation, mainly ascribed to the high spatial variability and anisotropy of porous medium.The dynamic instability of the preferential movement in soil lessens the contacting duration between the soil matrix and the solutes, arising the risk of deep leakage of chemicals from the surface to the groundwater.Consequently, in agricultural practice, the preferential transportation will negatively impact the hydraulic activities in fields thus generating low water-fertilization utilization efficiency.Oasis agroecosystems as the economic pump in Xinjiang have been neglected regarding the preferential flow phenomenon, which potentially affects water use efficiency and groundwater pollution, eventually restricting agricultural development.In these regions, the consecutive land-reclamation has made some achievements for the increased irrigated arable land.Nevertheless, pressures coexist due to the desertification, the limited precipitation and poor land fertility; whether the natural conditions or the agronomic management will affect the soil preferential flow is uncertain.Therefore, a specific oasis planting area of Regiment 121 (R121) in Xinjiang of China was selected in this study to seek and discuss the underlying factors possibly causing the preferential flow and the proper quantifying techniques.In R121, generally, the potential initiating drivers, including the freeze-thaw cycle, residual film, cultivation modes, and soil salinization will change the soil porosity, and alter the soil structure, thus shaping the preferential flow pathways concerning the macropore flow.During the constant freezing-thawing processes, the soil proportion of solid soil water increases, putting on the soil porosity and soil volume; but the effects of freeze-thaw on the soil depend on the collecting time and conditions of the soil samples.In addition, the accumulating plastic fragments imped the subsurface water movement, retard the horizontal movement, thus accelerating the downward movement of the soil water.Furthermore, the returning cotton straws into the fields will signally have an impact on the water storage during sowing seasons, enhancing the penetration and permeability of the soil; on the other hand,the straws in the soil also elevate the development of soil pores which then are transferred to bigger pores, thus increasing the total porosity; with the extension of reclamation time, these effects will become much more obvious.Additionally, the soil type in this region is loam, whose water-repellency will weaken the influence of the capillary of the soil conduit, force the flow unstably selecting the area with higher water conductivity, and may trigger the finger flow.The techniques of quantifying the preferential flow have also been summarized and evaluated, in which the dye tracing imaging methods can appraise well the preferential flow for its direct expression of non-uniform infiltration process in the soil.Other methods such as micro-tension measurement and acoustic detection may not be applicable in this region.Moreover, future research needs to properly combine the micro-observation with the large-scale investigation for a thorough evaluation.This review is expected to provide a reference for the research on preferential flow in arid oasis ecosystems of Xinjiang and other similar parts around the world.

Key words: agriculture; ecosystems; reviews; oasis agroecosystems; preferential flow; evaluation

0 Introduction

Through favored paths, the soil water, and solute rounding most parts of the porous medium, travel unusually faster downward and are reported to respond at more flowing rates to the input water under many circumstances.Affected by various drivers, this preferential process occurs prevailingly whose hydrology activities vary actively, thus leading to spatial variability and anisotropy of soil medium.It is also highly pivotal to discern the forms of preferential flow (PRF) according to the inducing mechanism.Generally, the preferential flow is often closely related to the macropore individual channels with solid water conductivitysuch as the plant root path, soil fauna’s holes, and long-narrow fissuresallowing flow to infiltrate downward rapidlywhile bypassing the soil matrix with dense structure and low permeability.However, even though it is devoid of apparent bio-pores or fractures, the instability and nonlinearityof flow will not be neglected concerning the precipitation, soil architecture, and flowing dynamic properties.Additionally, the complexity of capturing the transient flow, which has a short duration and fast response, being highly inhomogeneous, requires detailed categorization.Therefore, this unsteady flow can be further divided into the funnel and finger flow.The formation mechanism of both funnel flow and finger flow is mainly attributed to the soil water repellency and gravity with limited influence by capillarity, causing the increase of the hydraulic conductivity vertically alongside the infiltration direction.Consequently, the finger-shaped channels will even preferentially expand, proceeding through conductive paths because of the redistribution of soil water and the enclosure of the air in the soil.

In most unsaturated soil, the PRF has the features as follows: 1) The PRF in soil porous medium is a laminar flow or a transition flow from laminar flow to turbulent flow, which Darcy’s law cannot appraise well about the problem of soil infiltration and redistribution with preferential pathways for the law regards the soil medium as a comparative uniform system; 2) The PRF can penetrate the soil quickly with capillary force having little translocation power to water flow, so various soil solutes in PRF can reach the soil bottom layer or groundwater rapidly; 3) The lateral infiltration of the PRF distributes the soil water and solute in all soil layers, presenting a radial shape of the solute with the macropores as the center, which indicates that the closer to the center of the macropore the higher the solute concentration.4) The interaction between water flow and soil is weak, the solutes carried by water only interplay partially with the matrix part.5) The solute breakthrough curve of the PRF has asymmetric and hysteresis phenomena.

So far, myriad indoor or field experiments have been carried out to quantify and understand this process, and the profound implication of the soil preferential flow has been documented and ascertained.Some accepted theories are introduced to conceptualize the inherent characteristics of the non-equilibrium flow.Collectively, it is fairly agreed that there is the following principal controlling factors regarding the preferential flow:

1) Soil structure alteration.The elongated pores, as a capillary shield, separate the matrix flow and PRF region, stabilizing the water flow in the soil matrix while allowing high-speed movement in the macropores; beyond that, these macropores also provide the point sources for deeper supplementary.

2) Soil properties and initial conditions.The soil water repellency such as sand or clay prevents the ground from being moisture, forcing the solute and water to pass the unsaturated zone through more hydraulic-favored paths unstably driven mainly by gravity.In addition, the initial water content essentially determining the degree of PRF has been recognized based on the findings that deeper percolation of the dye tracer happened in comparatively moist soil.However, lower water content is also reported to have a higher non-uniformity degree when the irrigation rate and the precipitation intensity surpass the soil water infiltration rate.

3) The boundary conditions.In agricultural practice, different irrigation modes will affect the infiltration by modifying the infiltrating status.The infiltration depth in ponding water was shallower than in the sprinkling experiment.However, the prioritized migration speed of ponding water was faster than that of the sprinkling condition considering the surface fissures.Moreover, the ways of applying fertilizer also have an impact on the deepest infiltration depth in structured soils with more macropores.

4) Instability of soil water flow.When the pressure gradient of the wetting front is opposite to that of the water flow direction, this instability will appear.Several possible preconditions that the unstable flows may occur have been put forward: ①soil is challenging to be moist; ②the hydraulic conductivity increases by soil depth; ③higher air pressure in front of the wet front.Specifically, the less capillary pressure from the wetting curve than the air entering the capillary force of the drainage curve in the water characteristic curve triggers the formation of the finger flow; once the unstable flow has been shaped, the finger flow recurs along the same paths during raining seasons.

Agricultural significance is prominent both economically and ecologically; the preferential flow has increased concerns over the agroecosystems, especially in arid regions such as Xinjiang, where the relevant research about this issue is under-recognized and limited-investigated.Xinjiang, the center of the Eurasian Continent, locates deeply in northwest China.The Tianshan Mountains, a significant mountain range flanked by the Junggar Basin in the north and the Taklimakan Desert in the south, stretch from east to west in central Xinjiang.Despite the wide distribution of the deserts accounting for 22.5% of land in Xinjiang, the cash crop production still flourishes depending on the rich sunshine and heat resources locally based on the piedmont oases.Since the 1960 s, a continuous land-reclamation revolution has been launched throughout Xinjiang to consider the livelihood of a growing population and social-economic development.By the end of 2018, the irrigated arable land had increased from 2.37×10to 7.07×10hm, and Xinjiang has achieved some excellent results indeed, with cotton and grapes planting area and output top-ranking in China.Efforts and practices have been taken for better oasis agricultural development.However, we need to face the challenges given pressure from desertification, the lack of precipitation, and poor land fertility inhibiting agricultural growth.Whether the natural conditions or the long periods of systematic management can genuinely affect the soil preferential flow is uncertain.Hence, it is scientifically essential to quantify the preferential flow in oasis agroecosystems aiming at sustainability.

Nevertheless, no specific research has done relative work before in Xinjiang.Thus, in this review, first the existing quantifying methods of preferential flow were summarized, and their application values were further discussed that combining the experiences and problems of agricultural development in Xinjiang.A typical oasis irrigation area, Regiment 121, was selected as a case study to be analyzed.This work can provide a profound understanding of the preferential flow in arid areas and a reference for local agricultural development.

1 General description of the study area

Regiment 121 (R121, 85°33′-85°35′E, 44°48′-44°50′N, Fig.1) characterized by the extremely arid desert climate, affiliated with the 8division of the Xinjiang Production and Construction Corps, has strategic importance for its location in Tianshan northern slope oasis economic zone.Situated in the Manas River valley starting south from the Yilianhabilgan Mountain to the Gurbantunggut Desert in the north, the R121, which is on the edge of the desert, has now developed with the construction of water conservancy projects and reclamation of wasteland.In this region, the annual average precipitation, evaporation, sunshine duration, frost-free period, and temperature are 142 mm, 1 826 mm, 2860 h, 163 d, and 6.2 ℃, respectively.The fundamental soil’s physical properties in the R121 irrigation area present a consistent texture and structure from surface to 100 cm soil layer, the type of which is loam with average soil bulk of 1.45 g/cm.Since 1996, a water-saving technology of integrating plastic film and drip irrigation has been applied by local farmers in R121, becoming a base for high-quality cotton production.

Fig.1 Location of the study area

2 Potential factors initiating the preferential flow in oasis agroecosystems

2.1 Seasonal freeze-thaw cycles

R121 is a typical frozen district in which the frozen soil area is over 90% of the irrigation field.As a particular soil-water system containing ice crystals, seasonally frozen soil is more unstable than conventional soil, responded quickly by the external temperature.When the temperature drops below 0 ℃, the liquid water in the porous medium will be converted into loosely cemented crystalline solid water, increasing the volume of the earth and thus gradually expanding the soil.As the temperature slowly rises above 0 ℃, the solid water in the medium begins melting.The melted water, however, still stays on the soil’s surface because of its incapability of discharging in time or penetrating deep into the soil.Essentially, the mutual conversion between liquid and solid water in the soil changes with the external ambient temperature.In this case, the continual decrease of temperature diffuses the heat in the soil rapidly.Moreover, restricted by the action of the surface energy of the soil particles, the water movement in the soil presents melting during the daytime and thawing at nighttime; consequently, the soil’s internal structure and particle composition will alter with time, leading to the migration of water and temperature in the soil.Conclusively, after the freeze-thaw cycle, the soil density, water distribution in the pores, and the connections between soil particles will all change.Figure 2 shows the soil structure changes after the freeze-thaw cycle.

2.2 Plastic impaction

From ‘White revolution’ to ‘White pollution’, R121, also in most parts of Xinjiang, has gone through a bitter-sweet experience.The application of plastic films mulched over the drip irrigation lines has functioned as preserving the surface soil temperature, conserving soil degradation, and improving water fertilizer efficiency, which plays a crucial role in promoting the development and yield of crops.Nevertheless, the components of plastic films are polyethylene and polyvinyl chloride which are highly undegradable.The reclaim activities such as harvesting, plowing, and harrowing have limited influence on recovering and recycling the residual film.Therefore, after cultivation and planting years by years, more and more plastic has accumulated in the soil.Independently, local agricultural measures have broken the films into smaller fragments, which are driven from the surface soil into deeper soil.Based on the field samples and related research, the problems of residue pollution are severe in R121.For example, in fields after 15-19 years of mulching film management, the relative frequency of the smaller Residual Plastic Film (RPF) fragments (＜10 mg or 10-25 mg) increases, while that of the larger RPF fragments (25-50 mg or ＞50 mg) shows a relative decline, as shown in Fig.3.

Fig.2 Seasonal freeze-thaw (FT) alters the soil structure

Fig.3 Distribution of residual film in soil with different plastic film mulching periods

2.3 Cultivating mode

A cultivating mode of deep plowing and returning cotton stalk has been widely applied as a primary autumn tillage method in R121 (Fig.4).This farming measure loosens the soil compactness of the ploughing layer by breaking the plow bottom layer, thus changing the local climate of the underlying surface.In addition, this mode buries the soil and vegetation residuals downward containing diseases and insect pests from the upper layer, playing a positive role in the prevention and control of verticillium wilt and cotton bollworm.Moreover, the stalks residues covering the soil surface reduce the field evaporation and erosion of the soil by rain, snow, and sandy, leading to the improvement of the physical, chemical, and microbial properties of the soil.The positive effect of land consolidation and management on cotton yield and gross production is obvious.Accordingly, the deep plowing and returning cotton stalk become a popular form by policymakers in R121 on the heels of sustaining development.

Fig.4 The cultivating mode of deep plowing and returning cotton stalk

2.4 Soil salinization

Before the introduction of mulched drip irrigation, flood irrigation was primarily applied.Yet the lack of drainage systems and the neglect of overall scheming regarding the environmental factors in R121 has raised the groundwater level and encouraged secondary salinization.Although the adoption of drip irrigation saves water application, the problem of soil salinization is still aggravating.On the one hand, while the salt content of the root zone is regulated and reduced by the mulched drip irrigation during growth seasons, the salt will redistribute and accumulate in bare land without irrigation or films (as shown in Fig.5); on the other hand, the climate conditions and the high salinity of groundwater in this region will invariably have an impact on the agriculture development in Xinjiang.

Fig.5 Long-term mulching films of 15 years, has caused salt accumulation on the bare surface between mulching films while no obvious salt accumulation after 5 years of mulched drip irrigation[70]

3 Discussion

Oasis agroecosystems support a fragile eco-environment.Some potential factors that can trigger the preferential flow in soil have been identified.It is noticed that all four mentioned factors have heightened the alteration of soil structure, which shapes preferential pathways, thus affecting the infiltration process within the soil medium.During the repeat freezing-thawing processes of the soil, the proportion of solid soil water increases, putting on the soil porosity and soil volume; however, the soil bulk density gradually decreases to a stable level after this cycle.These effects are more evident with the greater freeze-thaw intensity.In some other cases, the porosity of loess shows a trend of increasing first and then decreasing, indicating that discrepancy appears when determining the effects of freeze-thaw due to the different sample collecting time or other experimental conditions.

The soil architecture has also been significantly impacted by the persisting plastic film through decreasing soil porosity, impeding subsurface water movement, and eventually inhibiting the water using efficiency; additionally, the accumulating plastic fragments increase the retardation of horizontal movement of soil moisture and simultaneously cut down the horizontal wetting front transport rate, cumulative infiltration, and soil moisture diffusivity.In addition, the application of cotton straws returning to the fields signally influence the water storage during sowing seasons, promoting penetration and permeability of the soil.Apart from that, the straws in the soil also elevate the development of soil pores which then are transferred to bigger pores, thus increasing the total porosity; with the extension of reclamation time, these effects will become much more obvious.In conclusion, we have prudently speculated that in R121 the increasing porosity and hydrology conductivity in the soil will be associated with macropore flow, thus accelerating the movement of matter in the PRF pathways.Beyond that, due to the loam soil massively distributes in R121, whose water repellency as already mentioned will also activate the finger flow.

To comprehensively quantify the macropores flow requires very careful scheming of the expertise, equipment, and instrumentation for dictating the sites and the scale at which PRF is measured.In R121, the intensive agricultural management and land consolidation also expand the research scale and bring about the sophistication of quantifying PRF in this region.Therefore, in the next chapter, some quantifying techniques have been summarized whose application values are evaluated.

4 Quantifying techniques of the PRF

4.1 Micro-tension measurement technology

The specific device is the time domain reflectometer (TDR).Originated in the 1960s, the TDR can generate time voltage propagating along the transmission line, detect the reflection caused by silicon, measure the input to reflected voltage ratio, and then calculate the discontinuous impedance.The time-domain reflection theory is also applied to detect soil moisture gauging the content of water and solute in soil without destroying the soil structure.High efficiency, high precision, low labor consumption, and easy operation are other advantages of the TDR soil moisture meter.Therefore, the TDR has been utilized in massive indoor and field experiments.

Additionally, TDR is an essential tool for getting the necessary parameters to establish the PRF models.Germann et al.first introduced the TDR into the investigation of the PRF and concluded that the continuity of monitoring and sensitivity would help the researchers quantify the PRF and capture the PRF phenomenon effectively.Ritsema et al.used TDR to track preferential paths in water-repellent soil under seven heavy rain conditions.Lee et al.attainted the breakthrough curves by applying the TDR technique in the undisturbed soil columns with 20 cm in height and 12 cm in diameter; the parameters from the breakthrough curves were used to estimate the mobile-immobile region model.However, there is a discrepancy with the actual situation because the TDR needs intensive point measurement which may not be easily realized in R121 for the planting of cotton, causing the results not to be too accurate sometimes.

4.2 Acoustic detection technology

Acoustic detection technology is one of the main methods of non-destructive detection.In 1954, Brandtfirst used acoustic detection technology in the study of the propagation velocity of sound waves in porous granular media.Later, the systematic research on the relationship between shallow unsaturated soils and solute content was conducted by Geertsma and Smit; In 1964, Brutsaertapplied this technology to explore the influence of clay content and pore characteristics in the sand on acoustic velocity in the medium.

By the end of the 20th century, acoustic detection as an auxiliary tool combined with TDR technology had been widely used in the study of unsaturated seepage problems to determine the distribution of water and solute in unsaturated soils.Flammer et al.first studied the water flow pattern in unsaturated soil by acoustic detection technique; he used the transmission rate of the acoustic pulse signal passing through the earth and the soil's absorption of the acoustic wave to reflect the change of soil moisture content, and then the Brutsaert model was performed for numerical simulation on the experimental data; by comparing the obtained data with the water distribution gained using the tracer, he thought that the tracer test is in good agreement with the acoustic pulse test.Blum et al.converted the velocity distribution of acoustic waves into the soil moisture content through 21-time series to determine the preferential paths of soil.However, this combination needs loads of preparation work and operating requirements, which will be time-consuming and may not be suitable in R121.

4.3 Scanning techniques

CT technology is a non-destructive measurement technology that can scan the undisturbed soil, determine different deep pores’ shapes, numbers, and connectivity, and get soil fault images and two-dimensional matrix CTN figures (i.e., slight scale fault density variation); CT scanning is intuitive, fast and convenient, and can also directly study three-dimensional pore structure.Consequently, quantitative analysis of PRF applied in R121 has the excellent potential.

The current research concentrates on the determination of macropore distribution by CT scanning technology to study the paths of the PRF.Anderson et al.applied the CT to scan the columns from the cultivated soil and forest soil and compared their distributions; he concluded that the CT technology had a good effect on the evaluation of soil structure.The fractal dimension was used by Zeng et al.to quantify the soil structure on a small scale, and the data were obtained by CT scanning technology.Warner et al.received the number, size, shape, and other information of macropores with the help of CT imaging technology, and the visualization and quantification of macropores were realized.

Beyond that, CT imaging technology focuses on analyzing PRF paths and the real-time distribution of water and solute in undisturbed soil.Because the dye has the advantage of sharp contrast, CT technology is often combined with tracer methods to achieve a qualitative and quantitative analysis of the PRF.CT technique was used to plot the flow process of dye tracer in PRF paths.Mooney and Morrisused Chloride and Brilliant Blue as dye tracers to track the PRF path, and then CT scanning was performed.A multi-scale method was proposed to analyze the migration mechanism using visualization technology.Wildenschild et al.utilized CT to scan intact undisturbed soil columns of 10 cm in diameter and 30 cm in height and attained images of the internal structures of soil; they chose two key location points and caught the real-time distribution conditions of soil solutes.

Although CT is a modern instrument widely used in soil PRF, there is no special CT scanner for studying soil moisture and the solute transport process.Therefore, in soil scanning and real-time monitoring of PRF, it is necessary to send scanning samples, which requires professional technology and high cost.In addition, the small-scale example cannot play a representative role.In contrast, the more significant sample scanning has a series of shortcomings, such as scanning difficulty, processing difficulty, and many data errors.

The fundamental theory of magnetic resonance imaging technology (MRI) used in soil science is to use the nuclear magnetic resonance phenomenon of soil structure, process the obtained radio frequency signal by computer, and reconstruct the image of a particular layer of soil.Fantazzinireported that the MRI could also be widely used to study the diffusion and migration of water and solute in porous media as CT technology.Posadas et al.used the artificial double-layer sandy soil to simulate the layered soil; the numbers, length, and diameter of finger flow in the layered soil were obtained by nuclear magnetic resonance imaging technology to analyze the dynamic characteristics of finger flow.Jelinkova et al.used MRI and tracer techniques to analyze the soil structure and preferred paths of unmodified coarse sand, and the nickel breakthrough curve was established.Although the scanning techniques provide a platform for an accurate analysis of soil’s three-dimensional structure, the difficulty of the undisturbed soil column collection and the high cost somehow restrain their application in R121.

4.4 Ground-penetrating radar technology

Ground-penetrating radar detection technology is a broad-spectrum (1 M to 1 GHz) electromagnetic technology applied to determine the distribution of underground media.Doolittle et al.pointed out that the depth image of the groundwater table obtained by using ground radar detection technology had the characteristics of continuity and high resolution of recording results.Through experimental analysis, Harariconcluded that the ground radar detection method was an effective method to judge the discontinuous wetting front and PRF caused by the discontinuity of interlayer vertical flow and to visualize the discontinuous wetting front advancement and preferential paths of dunes.Nevertheless, there is limited applying ground-penetrating radar detection technology to PRF in R121.The main reasons may be the limited detection depths (5-10 cm) and small-scale conditions of this technology, while the study of PRF in R121 may be primarily large-scale.

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4.5 Electrical resistivity tomography technology

Electrical resistivity tomography technology (ERT) is a geophysical method to study resistivity distribution in the detection area by observing potential or potential differences in different directions in real-time.Schollarignored the influence of solute electrical conductivity on soil water content variation, gave the basic principle of this method for obtaining physical parameters, and pointed out that the study of the relationship between soil electrical conductivity and soil physical parameters by resistivity tomography mainly focused on the relationship between soil electrical conductivity and soil water content.Vanderborght et al.used the time-delayed ERT method to study the spatial relationship between soil permeability coefficient and dye migration characteristics in a two-dimensional plane by using a tracer test.Similar to the ground-penetrating radar technology, the utilization of electrical resistivity tomography is also repressed in R121 because of its shallow detection depth lower than the cotton root moisture zone.

4.6 Dye tracing imaging method

In contrast to the other techniques, the dye tracing imaging method can be promoted broadly in R121 because it directly expresses non-uniform water distribution in soil without high financial investment.The tracer dissolved in the infiltration water flow is dispersed to the ground along with the movement of the infiltration water flow.The flowing area is stained by the tracer or through different dyeing reactions, leading to a different color of the harmonious region from the soil base color itself.The place where the coloring water does not flow through is separated, and the base color of the soil is preserved.

The earliest research using the dyeing method can date back to the 1970 s when Bouma et al.applied the dye tracer to analyze the solute migration of the clay in the wild.Subsequently, due to the advantages of simple operation, relatively cheap, and the ability to visually display the distribution pattern of the PRF, the dyed tracing imaging method has been widely used in laboratory and field experimental studies.

The dye tracer disperses into the region where it flows through as it transfers with the water current.Due to the vivid color of the dye tracer itself, the soil can be dyed directly without additional color reaction, which significantly facilitates dyes as tracers.The frequently-used dye tracer is Brilliant Blue and Methylene blue.The food-grade dye, Brilliant Blue FCF, is commonly used during field and indoor experiments because this dyestuff is non-toxic, easily soluble in water, and easy to identify.Image processing is widely used to describe how PRF paths distribute in soil and the degree of its non-uniformity.The dye coverage via image processing can then be computed.However, clay particles can adsorb Brilliant Blue molecules, resulting in the migration rate of Brilliant Blue molecules being lower than that of infiltration water when the clay content is high, limiting the application of Brilliant Blue as a dye tracer in clay soils.

In contrast to dye tracers, chromogenic ion tracers generally do not produce color, and different chromogenic reactions are required to stain the soil.For example, iodide ions are first oxidized to iodine molecules, which react with starch to turn blue-purple to show the area of water movement.Inorganic ions are difficult to be absorbed by soil particles because their negative charges are repelled by the negative charges of soil clay particles.Among them, the iodide ion is particularly suitable for the tracing study of dyeing under clay conditions due to its low toxicity, high solubility, and easy recognition of the blue-purple formed by iodine molecules and starch.But it can be inconvenient and complicated to operate the accessional dyeing reaction by applying iodide ion and starch as tracers.Therefore, it is necessary to choose a specific tracer according to the soil properties of the test area.

Many scholars have used the dyeing imaging method to observe the moving patterns of water and solute in soils.So far, it has achieved good results.For example, the non-uniform characteristics of soil water movement, the effect of soil tillage on water flow, spatial statistical characteristics of soil water flow movement, the irrigation uniformity efficiency, and the global non-uniform distribution of solute concentration are properly analyzed by applying this method.

4.7 Resin filling method and Soil pores skeleton extraction method

The resin filling method needs to get undisturbed soil columns and dry them thoroughly.The resin is mixed with resin solvents, hardeners, catalysts, fluorescent dyes, and other additives and poured into the soil columns at a slow and uniform rate.Vacuum technology is often applied to expel the trapped gas in the soil columns.Through air drying baking, the samples can be completely hardened and cut into evenly spaced slices.Soil structure in every portion is photographed and characterized by computer image analysis.Despite its cheap and long preserved advantages, this method remains for further discussion because of the applied limitations of area and depth.

The skeleton extraction method is similar to the resin filling method.Here, the resin is replaced by the hardening liquids such as polyethylene, paraffin wax, and glass fiber.These hardening liquids can infiltrate into the soil columns and cover the macropores.In practice, however, it is not easy to get the accurate dosage of the hardening liquids to get the whole frame of the undisturbed soil columns thoroughly; thus, the soil structure cannot be adequately analyzed.Consequently, these techniques may not be applicable in R121.

5 Conclusions

In this study, the preferential flow and its potential inducing drivers in typical oasis ecosystems - Regiment 121 was first overviewed; then by highlighting the quantifying methodologies, their practicability and the possibility of applying in this region were delved into.It was proved that the dye tracing imaging method is comparatively more adaptive with less restraint considered.Notwithstanding the challenges and limitations PRF has, the proceeded studies may be of assistance to deal with current ecological problems including desertification, groundwater pollution, as well as low agricultural water use efficiency.Therefore, if the issues and debate are to be moved forward, a better understanding of PRF needs to be developed in oasis regions in Xinjiang.Future research in this field should be undertaken to establish: 1) the accurate measurement of the gas and micro-organism exchange between the PRF region and soil matrix for exploring the agricultural coordinated regulation mechanism of water, fertilizer, gas, heat, and salt under the PRF phenomenon; 2) special design and invention of a set of CT scanners for PRF research for the quantification of PRF being a research hotspot; a series of modern imaging technologies, such as photon emission (SPEC), gamma-ray and neutron imaging, will bring a new dawn to quantitative research.Experimenting with new technologies will be the development trend of PRF research in the future.

We thank to the College of Water Resources and Architectural Engineering (Shihezi University) for providing the experimental site and my colleagues for their valuable help.This research was supported by the Key Laboratory of Modern Water-Saving Irrigation of Xinjiang Production and Construction Corps, Project No.2019CB004, 2019-2021.