Article Information

Chinese Academy of Forestry, Haidian District, Xiangshan Rd, Beijing, China

^*Corresponding Author: Mingyuan Wang, Chinese Academy of Forestry, Haidian District, Xiangshan Rd, Beijing, China.

Received: 28 June 2024; Accepted: 23 July 2024; Published: 30 July 2024

Citation: Mingyuan Wang. The Theory of Sandy Land Soil Hydrology and Mode of Low Coverage Afforestation in Northwest China. International Journal of Plant, Animal and Environmental Sciences. 14 (2024): 42-58.

Abstract

Keywords

Northwest China; Sandy land; Soil hydrology; Afforestation

Article Details

1. Introduction

There are two figures for desertification area in northwest China: 2.18 million km² and 600,000 km². The first data covers deserts, gobi and desert formed in geological history. The scope of the second data only refers to the desertification land caused by human causes and with the conditions to control it, namely in semi-humid, semi-arid areas, due to the effect of the natural and human factors and interference, to form similar to desert landscape types, called desertification land or sandy land [1]. The distribution of sandy land in China is bounded by Helan Mountain and has obvious regional characteristics. The area west of Helan Mountain, mainly including Xinjiang, Gansu, northwest Tibet and western Inner Mongolia, has annual precipitation of less than 200 mm, which belongs to the "arid zone". Sandy land mainly distributed on the edge of the desert and near the oasis, and its distribution area accounting for 30.7% of the total area of the sandy land, its climate belongs to arid and it is difficult to control desertification, and poor self-reversal ability of desertified land. Therefore, it is necessary to pay attention to the rational use of water resources in oasis and areas with better natural conditions in the process of controlling desertification (Figure 1). The area east of Helan Mountain mainly includes the eastern part of Inner Mongolia Plateau, the western part of Loess Plateau and the central part of Qinghai-Tibet Plateau. It is the most widely distributed and harmful area of all kinds of desertification land, accounting for 69.3% of the total area of sandy land (Table 1). Most of the area belong to semi-arid area and the average annual rainfall is about 250-500 mm. Sandy land has the potential to reverse itself when man-made pressure is removed [2].

Table 1: Basic characteristics of sandy land in northwest China.

Figure 1: Distribution of desert and sandy land in Northwest China.

Data source: Status of Desertification in China, State Forestry Administration, 2015.

2. Sand Fixation and Afforestation in Mobile Dunes

The surface morphology of mobile dunes is mostly composed of dunes and interdune lowlands (sand bay). Dunes are extremely arid, with high mobility and low water holding capacity. Direct plantation is easy to be eroded by wind and difficult to achieve results. Due to the low topography, sand accumulation environment and good moisture conditions, plants can survive in the interdune lowlands without artificial sand barriers. Afforestation and sand fixation should be carried out first in interdune lowlands and then in the dunes. Of course, it is feasible to carry out both technologies at the same time. For a mobile dune, afforestation of both the front and back interdune lowlands constitutes the sand fixation afforestation technology of "front blocking and back pulling".

3. Forest Belt Serving for Windbreak and Sand Resistance

The periphery of oasis and the zone adjacent to desert, gobi and wind-eroded land are the most important areas causing mobile dune and wind-eroded sand to the damage of oasis. Building large wind-resistant sand-forest belt in these areas have significant effects on preventing the invasion of mobile dune to oasis and weakening the occurrence of sandstorm in sand area.The windproof sand forest belt should be interwoven with patch forest, block forest and belt forest, so it is not necessary to force uniformity. Afforestation is mainly in the interdune land, wind-eroded land and gentle sandy land.Afforestation around the oasis first, and gradually expand and widen outwards.The dunes are separated and surrounded by clumps of interdune forests.With the dune moving forward, the dune top is flattened by the wind, and afforestation should be carried out at the retreating sand bank to expand the area of interdune forest and form a denser windproof and sand-blocking belt.

4. Analysis on the Basic Principle of Rainfall -Infiltration Process in Sandy Land

The soil in sandy land is mainly composed of fine sand and medium sand. Such particle size composition determines that there are less capillary pores and more large pores in sandy land. The dry sand layer formed by surface evaporation after rainfall can effectively reduce the evaporation loss in the soil layer below. When the sandy soil is in unsaturated state, it is generally divided into dry sand layer, wet sand layer and free water layer from top to bottom. During the period of rainfall, the bound water and film water of dry sand layer are rapidly replenished. The bound water and film water of wet sand layer below are saturated, deep sandy soil with particle size distribution of non-uniform coefficient between 1.2 and 1.5, pore size close to consistent, fast penetration and weak soil water-holding capacity. The diffusion of water movement controlled by the capillary force compared to that under gravity is negligible. Water infiltration is controlled by gravity and resistance caused by the flow, and the better drainage performance. There is an obvious wet front when rainfall infiltration. The depth of the wet front of the soil infiltrated by rainfall can be continuously moved down. Although the soil water flow speed is faster, the seepage is laminar flow, seepage Reynolds number Re=Vd/ν < 1, in the formula, V represents the flow velocity, ν is the viscosity of liquid and d is the average particle size of the soil. The infiltration rate (or velocity V) and water potential gradient ψ show a linear relationship, and Darcy's law is valid, namely:

Where K_L is hydraulic conductivity (infiltration rate), γ is specific gravity of fluid, μ is dynamic viscosity of fluid, and C is constant. In the formula, γ and μ are only related to fluid, and the kinematic viscosity decreases and K_L increases with the increase of temperature. Cd² is only related to soil media in sandy land. For real soil, parameter C must cover the influence of other media characteristics unrelated to average particle size on fluid, such as particle size distribution, particle sphericity and roundness, and density of their filling. If defined:

If K_L is called hydraulic conductivity, the parameter K is called "specific permeability" or "intrinsic permeability". Specific permeability is the decisive factor of soil water conductivity and water holding capacity in sandy land. It has the following two meanings: (1) from the perspective of the infiltration path of soil water, it is mainly represented by the distribution of number (porosity) and size (d) through the pores and gaps existing in the soil. According to pore size, it can be divided into soil macropore, > 100μm, is the pore that can provide preferential water flow path, ventilation and drainage. Medium pore, 100-30μm, is characted as water conductivity high and water movement fast. Small pores, 30-3μm, is characted as water holding capacity and capillary water diffusion movement. Water enters the soil from macropores and mesopores with a positive pressure head, also known as "channel flow". The seepage flow in sandy land is uniform and consistent in all directions. The excavation of soil animals and the penetration of plant roots make the infiltration path of "channel flow" warped in sandy land, so the hydraulic gradient is uncertain. The change of "channel flow" through soil profile of sandy land determines the size of water infiltration from the surface. Water infiltration through capillary action diffuses horizontally and vertically into smaller pores in the soil. As capillary flow increases with hydraulic gradient (suction/infiltration process), the film water thickness decreases. Infiltration is actually a combination of two seepage systems: channel flow and capillary flow. (2) Pore structure directly affects the path and mode of water migration in sandy soil surface and soil body and determining the hydraulic properties of soil such as water holding capacity, permeability and conductivity. Quantitative index parameters that can evaluate pore structure characteristics include pore equivalent diameter, pore number, porosity, pore connectivity and hydraulic radius. In a rainfall process, from the perspective of equivalent pore size, the number of small pore size increases, the number of large pore size decreases, and the pore area ratio decreases. The changes of soil pores in the topsoil layer are most obvious due to the direct impact of raindrops. From the perspective of soil connectivity, the topsoil layer is more affected, and the average Euler number increased (Euler number = (number of pores) - (number of fragments -1)). For the relationship of soil profile depth and the vertical spatial variability of porosity, generally, porosity decreases with the increase of sampling depth, that is, porosity can be used as an index to reflect the profile depth. In a rainfall process, porosity and hydraulic radius continue to decrease, while Euler number increases, indicating that the connectivity and conductivity of pores decrease with the duration of rainfall, and less conductive to water and solutes. In conclusion, soil hydraulic conductivity K_L varies with the specific permeability K structure in the whole soil profile space. At the same point of specific permeability structure, hydraulic conductivity will also change with different measurement directions (x,y,z). The former property is called "heterogeneity" and the latter "anisotropy". The characteristic of sandy soil is that the specific permeability structure is anisotropic in the x or y direction of soil surface (0-10cm). The soil profile below the surface layer shows interlayer homogeneity and isotropy (see Figure 2).

Figure 2: Four combinations of homogeneity, heterogeneity, isotropy and anisotropy.

The most significant factors affecting the process from rainfall to infiltration in bare sandy land are rainfall, rainfall process and rain type, soil texture in sandy land, homogenization degree of soil profile in x, y and z directions, soil water content (soil water potential) and distribution of soil surface crust layer. Soil moisture movementin different depth profile along the x, y, z three directions is a complicated process. It involves the dynamic flow process of water, air and water vapor in soil saturated and unsaturated zones driven by the physical mechanisms such as hydraulic gradient (see Figure 3), temperature gradient, concentration gradient and seepage gradient. It is the process by which the soil aeration zone redistributes rainwater. In each direction of the underlying surface (x, y, z), different soil moisture components are generated (some rainwater replenishes soil moisture, while others generate free water that is not absorbed by the soil, such as surface runoff, soil flow or subsurface flow).

Figure 3: A diagram showing the relationship between flux and hydraulic gradient. Hydraulic conductivity (K) is the slope that defines the relationship. The dotted lines show that at equal hydraulic gradients, soils with higher conductivity have higher flux.

Concepts such as Darcy's law, hydraulic head (water potential) and hydraulic conductivity are based on saturated media (all soil voids are filled with water). It is difficult to carry out the experiment of unsaturated water flow law (experiment of equal soil water content and equal soil water potential gradient). Theoretically, according to the regularity of vertical one-dimensional infiltration rate in unsaturated soil, it is lack of the general effective formula of soil water characteristic function such as water conductivity (K_L(θ), θ is soil water content) and diffusivity (D (θ)). However, the constant value or linear function hypothesis, such as water retention curve of power function model and unsaturated hydraulic conductivity curve (see Figure 4), are adopted to simulate soil infiltration of quantitative rainfall. In reality, the obvious difference between unsaturated flow and saturated flow is that the water flux (hydraulic conductivity) on different sections of the whole unsaturated zone is not equal. The observation test of rainfall in sandy land proves that there are two conditions in the process of infiltration: (1) When the wet front extends downward through the surface soil but does not reach the groundwater level completely,the boundary is in unsaturated infiltration state. When the wet front reaches the groundwater table, the soil is saturated, and the surface infiltration rate is equal to saturated permeability (K_L). (2) When the rainfall intensity is less than the saturated permeability coefficient of soil, or the depth of the wet front formed by rainfall infiltration fails to reach the groundwater level, the final infiltration can only be in the state of unsaturated infiltration which the gradient of wetting front is larger compared with the rainfall infiltration front which is larger than saturated permeability. In arid and semi-arid areas, there is strong evaporation and little rainfall in sandy land. In case of precipitation, the water deficit caused by soil drought needs to be replenished first. Except for deep seepage caused by channel flow in dune lowland and sandy land, there is seldom deep seepage from sandy land infiltration to free water table.

Figure 4: Diagram of comparison of hydraulic conductivity and soil water tension curves of different soil types [1].

In conclusion, infiltration intensity varies with rain type (rain intensity varies with time) and physical characteristics of infiltration interface during a rainfall process. The ability to drain through the soil:

Table 2: Comparison of infiltration characteristics of soil free gravity water and capillary water.

Figure 5: Diagram of the relationship between interlayer heterogeneity and anisotropic heterogeneity

The spatial variation of soil water content is mainly studied in horizontal (x or y) and vertical (z) directions. In the horizontal directions (x or y) of sandy topsoil, there are differences in surface crust, psammophyte cover, herb and shrub cover, macro topography (dune and interdune lowland) and micro topography (concave and convex), due to water repellency of the surface of the crust, litter water imbibition, and highland water delivery and lowland water collection, all the above form source and reservoir water movement process, therefore, leading to the spatial variabilities of soil infiltration process and water storage. In soil profile of vertical direction (z), the topography of the underlying strata of sand, mezzanine, eolian dune deposition, the transitional characteristics of underlying fine sand strata and palaeosoil interbedded soil, etc. come into being sandy layers of different thickness and the differences of the development of soil profile (Table 3). Because soil mechanical composition at different depths and in different directions affects soil water infiltration and storage capabilities (see Figure 5), which results in spatial variability of the relation and transformation between soil water and groundwater.

It can be summarized as follows:

Table 3: The characteristics analysis of rainfall-infiltration paths in sandy land.

5. Analysis on the Basic Principle of Shrub Forest Evapotranspiration in Sandy Land

In arid and semi-arid sandy land in western China, the precipitation in growing season should firstly meet the water deficit caused by soil drought. In particular, deep seepage rarely occurs in the areas or dunes that have no hydraulic connection with groundwater, but there is subsurface lateral runoff (flow in soil) that rapidly absorbs precipitation, except in some local areas where deep seepage can occur under the control of gravity due to the channel flow with continuous large pores. In the areas or lowlands with hydraulic connection with groundwater, and in the areas with lateral recharge, water in the saturated water layer of soil rises to the active layer of plant roots through capillary action, which is the bridge to the groundwater. Defining: Soil gravity water storage capacity of rainfall infiltration in sandy landis that free gravity water can be generated only when rainfall infiltration through soil aeration zone (L_i) is greater than field water capacity. Therefore, gravity water storage capacity is the difference between soil saturated water content and field water capacity:

Table 4: Calculation method of soil water storage capacity replenished by rainfall infiltration in sandy land.Large areas of exposed sand, no hydraulic connection with groundwater, and the micro-topography of the sandy land surface was artificially transformed to create shrubbery. Due to the abundant lateral runoff in the sub-surface of the sandy land during rainfall, a collection reservoir was formed on the sandy land (lateral gravity water supply). These soil hydrological phenomena showed that the heterogeneity of water deficit in sandy soil profile is mathematically described as a complex plane, in which the imaginary number and the real number unit 1 are vector identifiers of opposing properties, and the negative and positive signs are quantitative identifiers of opposing properties. The point of the imaginary numbers is that they form a vector space together with the real numbers. As can be seen from Table 4, if the connotation of real unit 1 assigned to soil water storage capacity is dispersion, and the connotation of imaginary unit is polymerization, dispersion and polymerization cannot be identified simultaneously (see Table 5).

Table 5: Unique pure imaginary expression of aggregate of soil free water storage capacity (v_s-f).

Figure 6: Mapping between pure imaginary storage capacity and real storage capacity of soil moisture parameters in sandy land.

On a sandy land, water consumption by shrub transpiration is directly proportional to the storage capacity of precipitation absorbed by the sandy land, and inversely proportional to the storage capacity of precipitation output:

Where, are coefficient and index respectively, shrub transpiration (mm), accordingly, the relationship between plant water consumption (transpiration) and soil water content can be established.Natural soil moisture in sandy land varies with different landforms, the primary productive forces were distributed in blocks relying on the water characteristics of different geomorphic positions, as a result, the biomass produced in the small area accounted for most of the total biomass of the sandy land, which provided the basis for dune fixation and psammophyte community restoration and reconstruction. The formula (10) shows the principle of soil water deficit caused by shrub consumption and free gravity water replenished by precipitation, and on the whole, reflecting the mechanism of soil water movement in sandy land (Table 6).

Table 6: Relationship between rainfall infiltration and recharge in sandy land.Shrub plants in sandy land have a higher ratio of underground to surface biomass, indicating that the deep roots and horizontal malleability of plants in sandy land enlarged the available soil water space of vegetation under drought conditions, and relatively supplemented the water consumption caused by transpiration. In the process of dune changing from flowing to fixed, the relationship between soil water dynamics after sand-fixing forest formation and actual transpiration and evaporation law of shrub community reflects the water balance of different vegetation coverage and forestation density (Table 7). Afforestation on sandy land intermittently reserving bare sandy land area to collect surface and subsurface soil runoff (or internal drainage collecting source), adopting patch vegetation configuration with low coverage for afforestation method and technology popularization on sandy land is the key to the success of large-scale sand control afforestation.

6. Afforestation Mode of Low Coverage Sand Fixation

Because the characteristics of infiltration and water storage in sandy land, shrubs with dense afforestation of dune (windward slope) will consume excessive water content of soil layer due to the lack of hydraulic connection with groundwater, and drought will cause vegetation decline and death, affecting vegetation stability and sand fixation effect. For flat interdune land and marshy grassland with hydraulic connection to groundwater, soil water content increases with soil depth, and reasonable shrub or tree species (tree species with high water consumption rate) and planting density can be selected according to sand fixation requirements (Table 8). In general, for the large area of sandy land, the soil moisture is mainly replenished by precipitation in dune or flat sandy land, shrub species with low water consumption should be selected for artificial sand-fixation forest, adopting low coverage or low density afforestation layout format, ensuring soil moisture balance in sandy land, maintaining the stability of vegetation and the effect of sand fixation, and promoting the benign succession of vegetation and ecological restoration. From shifting sand to semi-fixed and to fixed transformation process, after the artificial shrubbery settled on the shifting sand, the underground parts of shrub species preserved although experiencing wind erosion and sand burial have the ability to generate new plants and gradually expand the woodland area. However, for shrubs without this ability, coverage increased by seeding and self-propagation, and the sandy land tended to be semi-fixed. Stable growth of shrub forest, litter covering sand, sand environmental improvement, or to create suitable conditions for the invasion of natural vegetation that increasing vegetation coverage, and shrub communities to further enhance their capability of wind resistance, originally some wind erosion areas gradually occupied by a shrub vegetation, except for the individual sand dune areas, and vegetation joined together making the original quicksand gradually become fixed sandy land. After a certain number of years, the individual growth and reproduction ability gradually declined, the branches died, and the productivity of stand decreased. Most of the afforestation species in sandy land, such as Salix psammophila, Amorpha fruticosa, Hedysarum scoparium, and hettysaruin mongoicurn turcz, can be clipped, and the more clipped, the more exuberant with the characteristics of breeding and growth of tillering. Usually 3-6 years for the first crop replacement and the cropping cycle is 3 to 4 years for harvesting and utilization, and the biomass obtained is large.

Figure 7: Relationship between transpiration and soil moisture parameters of shrubbery on sandy land.

A shrub forest with an area of S is set, the aboveground biomass of the shrub forest is GM, the underground root biomass is RM, the soil water content available to the root is SC, the transpiration water consumption of the shrub forest is AET, and the shrub coverage is C_p:

Figure 8: Diagram of relationship between shrub coverage and root-shoot ratio.

Table 7: The biological and ecological characteristics of several important shrubs planted in sandy land in northwest China.

7. A Case of Study

Haloxylon ammodendron lives for a long time in the desert, up to 100 years, and being a very good sand-fixing shrub. Haloxylon ammodendron's roots can live in a Chinese herbal medicine called Cistanche, and it is a plant with high economic benefits among sand irrigation plants. However, it is difficult for other psammophytes to exist in the place where a single covering of the ammosault tree dominate a lot of water in the soil, so there will be the risk of ecological imbalance, and a variety of shrubs should be planted mixed (Table 10).

Table 8: The basic characteristics of 45 haloxylon survey plots in Gurbantunggut Desert presented.

Table 9: Parameter statistics of ammodendron forest coverage (%) model.

Table 10: Exhbition of analysis results of regression model for the coverage of Ammodenia forest.

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Supplementary File:

Summary

1. Research background

The semi-arid sandy land ecosystem in western and northern China is in the transition zone between sandy and loess regions, arid and semi-arid regions, as well as the interleaved zone between grassland grazing area and agricultural farming area, which is a typical area with fragile ecological environment. The damage caused by wind erosion in sandy soil in arid and semi-arid areas is far greater than the cost of controlling wind erosion. The dust source and affected area of each sandstorm will be damaged by wind erosion to varying degrees, and the depth of wind erosion of dust source can reach 10cm. It is estimated that the loss of soil fine particles caused by sandstorms in China is as high as 106-107 tons every year. The grain size of most of them is below 10μm, which has caused serious damage to the land productivity of farmland and grassland in the source area. The construction of wind-proofing and sand-fixing forest has always been an important measure of ecological restoration and environmental governance.

Most of the territory of northwest China is covered by shrubs and herbaceous plants. The hydrological ecological research on the community structure, distribution pattern and evolution process of plants under water stress is far from effective guidance for the practice of sand fixation and afforestation. If the drought index series is from P/ETP>1(the ratio of precipitation to potential evaporation) down to < 0.3, the vegetation will evolve from the potential comprehensive coverage to the broken vegetation canopy to the patchy vegetation distribution state. The most significant feature of vegetation in arid zone is low coverage. Since the 1980s, China has carried out large-scale protection forest construction in North China, Northwest China and Northeast China. In the construction of artificial vegetation to prevent and control soil erosion by water and wind, the vegetation coverage design and its configuration have been ignored, resulting in the problems of soil dryness and low stability of artificial vegetation in arid and semi-arid areas. The improper selection of some tree species causes large area loss, such as the production of a large area of soil dry layer, the occurrence of large area of pests and diseases, the phenomenon of "small old tree" and the low survival rate of afforestation.

How to make the limited precipitation be used by vegetation and maintain the water balance of forestland in sandy land? What is the density of vegetation without water deficit? What is the distribution pattern of vegetation dependent soil water environment? How to manage the utilization of water resources and vegetation in sandy land? It is of great significance for vegetation reconstruction, ecological restoration and rapid and efficient control of wind erosion desertification. When the soil is dry or barren, plants preferentially allocate photosynthetic products to the root system, thus changing the ratio of crown to root and strengthening the ability of the root to absorb water and nutrients. Therefore, establishing the correlation among soil water movement, water absorption by roots and water consumption by plants is a new idea to build a model of transpiration and evaporation. This paper was supported by National Natural Science Foundation project "Study on Soil Water Consumption Model of Shrubbery community on Sandy Land", which belongs to part of the research content of this project. In addition, based on the support of this project, the author conducted in-depth research on the existing problems in desertification control and afforestation from the aspects of natural capital, economic capital, scientific and technological capital and social capital, and put forward the plan of Desertification Control and Integrated Ecosystem Management in China.

2. Main Achievements

Based on Darcy's law and the theory of soil water potential, the characteristics of soil water infiltration in sandy land were expounded comprehensively, deeply and systematically, and the relationship between two different seepage systems of sandy land soil capillary flow and macroporous channel flow and the depth of soil infiltration front and their mathematical models were proposed. Accordingly, a mathematical model of the equilibrium relationship between soil water storage (collection) and vegetation water consumption was derived. In essence, it reflected the correlation between plant root-shoot ratio, precipitation and soil drainage supplement. The vegetation coverage (afforestation density) of shrub on sandy land can be determined by the root-shoot ratio of shrub species. Under dry conditions, the deep roots and horizontal ductility of shrub plants on sandy land expand the available soil water space of vegetation, and relatively supplement the water consumption by transpiration. The above two models illustrate the relationship and transformation between the "source" and "reservoir" of soil water from both qualitative and quantitative aspects, and propose and prove that the technical measures of low coverage water harvesting and forest management are the key to the success of large-scale desertification control.

3. Theoretical Innovations

The relationship between the capillary flow system and the macroporous channel flow in sandy soil and the depth of soil infiltration front and their mathematical models were proposed for the first time. These hydrological phenomena showed that the heterogeneity of water deficit in sandy soil profile was mathematically described as a complex plane. If the real unit 1 of soil water storage capacity was assigned, the connotation was spread out. The connotation of the imaginary number unit i is polymerization, and both dispersion and polymerization cannot be recognized simultaneously. In view of the corresponding relationship between pure imaginary storage capacity (imaginary storage capacity) such as soil saturated water content, field water holding capacity and soil natural water content and real storage capacity (actual soil water holding and displacement) after spreading out, a mathematical model of the equilibrium relationship between soil water storage (collection) and vegetation water consumption was derived. In essence, it reflected the correlation between plant root-shoot ratio, precipitation and soil drainage supplement. A mathematical model for determining afforestation density (coverage) based on root-shoot ratio was proposed for the first time. The basic principle of low coverage afforestation in sandy land was expounded and the practice of afforestation under sand control was guided. The soil rhizosphere water consumption model and density formula of shrub community on sandy land have clear physical significance and easy parameters to determine, which has universal applicability to the study of vegetation-soil water relationship of plantation forest.

4. Application Background

Soil erosion and wind erosion are the two main causes of land degradation. They have caused 84% of the degraded land area in the world. This natural process combined with excessive erosion caused by human activities has become one of the most serious environmental problems in the world. In China, "State specially stipulated shrubbery forest" refers to the shrubbery economic forest which is distributed in arid and semi-arid areas with average annual precipitation less than 400 mm, or above the upper limit of tree distribution, or in tropical and subtropical karst areas, dry hot (arid) river valleys and other ecological and environmental fragile areas, and is operated for protection purposes and economic benefits. Shrub land, including the construction of artificial shrub land and the protection of natural shrub land, is the key problem of how to use the limited water resources of sandy land in arid and semi-arid areas to improve the vegetation coverage and prevent the damage of wind sand.