Calculation method for determining reasonable ecological pattern of ecologically fragile zone in semi-arid region

Abstract

The invention relates to a calculation method for determining a reasonable ecological pattern of an ecologically fragile zone in a semi-arid region. The method comprises the following steps: determining a current agricultural and pastoral pattern distribution situation of a research region; calculating the critical burial depth of groundwater recharge vegetation; determining the thickness of the natural grassland degradation buffer zone influenced by the irrigation area; performing natural grassland expansion analysis based on a source-sink theory; analyzing the stability of the expansion area of the natural grassland; the ecological fragile zone is reasonable in ecological pattern. According to the method, the vegetation critical burial depth is supplied through underground water, the supporting effect of the underground water on the natural grassland is clarified, and the influence buffer zone thickness of the irrigated area on the natural grassland is determined; after the natural grassland is adjusted by using landscape ecology, whether the underground water condition of the adjusted area is enough to support the growth of vegetation of the natural grassland or not and whether the expansion area is in a buffer zone or not are further analyzed, so that the actual recoverable range of the natural grassland is analyzed.

Description

Technical Field

[0001] The invention belongs to the technical field of ecological management, and in particular relates to a calculation method for determining a reasonable ecological pattern of an ecologically fragile zone in a semi-arid area. Background Art

[0002] The distribution range of my country's semi-arid areas lies between the arid areas of the northwest inland rivers and the semi-humid areas of northeastern North China. It stretches from the eastern part of Inner Mongolia to the southwest to the eastern foot of the Helan Mountains, and spans the two major geomorphic units of the Inner Mongolia Plateau and the Loess Plateau. The semi-arid areas are covered with grasslands and have a large population of ethnic minorities. Historically, they were dominated by traditional agriculture and animal husbandry, and the industry was single. In recent decades, irrigation agriculture has developed rapidly and is an important agricultural development area in my country, forming the so-called agricultural and pastoral transition zone, which has led to changes in its hydrological cycle and ecological effects. The relationship between groundwater recharge and discharge has changed, and the ecological pattern has changed from being dominated by natural grasslands to being distributed in an alternating manner among irrigation areas, natural grasslands, successional grasslands, artificial grasslands, and sandy lands. With the continuous and rapid development of irrigation agriculture, the water resources and ecological environment problems faced are becoming increasingly complex, posing huge challenges. In particular, large-scale groundwater exploitation has caused the groundwater level to continue to decline, resulting in problems such as river interruption, grassland degradation, and desertification. It has become one of the sources of sand for sandstorms, directly threatening the ecological security of the capital economic circle. However, for a long time, there has been no systematic theoretical and technical research on the hydrological cycle and ecological effects after the transition from agriculture to animal husbandry in semi-arid areas.

[0003] The core element of the hydrological cycle and ecological security in semi-arid areas is groundwater. The natural attributes of the hydrological cycle in semi-arid areas determine the basic pattern of the ecosystem, and its natural ecology is non-zonal grassland vegetation supported by groundwater. Driven by the development and utilization of water and soil resources, the depth of groundwater continues to decline, and the structure of groundwater formation and dissipation has changed accordingly, resulting in the continuous shrinkage of natural grassland area, the succession of vegetation communities, the decline of species diversity, and a significant decline in the natural attributes of the ecosystem. The natural grassland has split into a diversified ecological pattern including agricultural irrigation areas, natural grasslands, successional grasslands, artificial grasslands, and sandy land.

[0004] Therefore, the key to this technical method is to reasonably determine the critical buried depth of groundwater recharge vegetation and the thickness of the buffer zone Δh that affects the degradation of natural grasslands in irrigation areas and use them as control conditions to determine the rationality of ecological pattern adjustment.

[0005] Current grassland ecological protection measures ignore the supporting role of groundwater on grassland ecology and fail to consider the boundary range of irrigation areas affecting natural grassland degradation, resulting in limited grassland restoration effects and the area of ​​natural grassland is still shrinking.

[0006] Therefore, it is urgent to propose a calculation method for determining the reasonable ecological pattern of the ecologically fragile zone in the agricultural-pastoral transition zone in the semi-arid region, and in view of this, the present invention is specially proposed. Summary of the invention

[0007] The technical problem to be solved by the present invention is to extract and identify the bands of the measured points of natural grassland, degraded grassland, severely degraded grassland and cultivated land in the agricultural-pastoral transition zone based on multispectral UAV, and combine the remote sensing satellite data analysis to obtain the current agricultural-pastoral distribution pattern of the ecologically fragile zone more accurately than the land use data; in view of the current grassland ecological protection measures, the supporting role of groundwater on grassland ecology is ignored, and the boundary range of the irrigation area affecting the degradation of natural grassland is not considered, resulting in limited grassland restoration effect and the problem that the area of ​​natural grassland is still shrinking. A calculation method for determining the reasonable ecological pattern of the agricultural-pastoral transition zone in semi-arid areas is proposed:

[0008] A calculation method for determining a reasonable ecological pattern of an ecologically fragile zone in a semi-arid region comprises the following steps:

[0009] Step S1: extract and identify the bands of natural grassland, degraded grassland, severely degraded grassland and cultivated land in the agricultural-pastoral transition zone based on multispectral UAV, and determine the current agricultural-pastoral distribution pattern in the ecologically fragile zone based on multispectral band data combined with remote sensing satellite data, that is, the area of ​​natural grassland is A, the area of ​​degraded grassland is B, the area of ​​severely degraded grassland is C, and the area of ​​cultivated land is D;

[0010] Step S2: Determine the critical depth of natural grassland vegetation growth based on groundwater recharge in the semi-arid ecologically fragile area. The calculation formula is as follows:

[0011]

[0012] Where: H is the critical depth of natural grassland vegetation growth based on groundwater recharge, in m; n is soil porosity; d is effective soil particle size, in m; σ is soil water tension, in N / m; ρ is the density of water at the lowest soil temperature before irrigation, in kg / m 3 ; g is the acceleration of gravity, unit is m / s2; D is the thickness of the root layer, unit is m;

[0013] Step S3: Identify the boundaries of the irrigation areas with different groundwater depths and groundwater exploitation intensities that affect the degradation of natural grasslands, and determine the thickness Δh of the buffer zone where the irrigation areas affect the degradation of natural grasslands;

[0014] Step S4: Analysis of natural grassland expansion based on source-sink theory. Taking natural grassland expansion as the basic principle, the "source-sink" theory method in landscape ecology is used to adjust and optimize the existing ecological pattern. After adjustment, the natural grassland can be expanded by A1, the degraded grassland area is reduced by B1, the severely degraded grassland area is reduced by C1, and the cultivated land area is reduced by D1;

[0015] Step S5: Stability analysis of natural grassland expansion area: the critical burial depth H of natural grassland vegetation growth calculated in step 2 and the buffer zone thickness Δh of the irrigation area affecting natural grassland degradation determined in step 3 are used as the determination conditions for the stability of natural grassland expansion area;

[0016] Step S6: The area of ​​the natural grassland expansion area where the groundwater depth is less than H is set as A2, and the area of ​​the area where the groundwater depth is greater than H is set as A3, that is, A1=A2+A3; the groundwater conditions of A1 can support the growth of natural grassland vegetation, and after adjustment, it becomes a potential natural grassland, and the groundwater conditions of A2 do not support the growth of natural grassland vegetation, and after adjustment, it becomes a degraded grassland;

[0017] Step S7: whether the A2 area overlaps with the thickness of the buffer zone where the irrigation area affects the degradation of natural grassland is determined. When the A2 area does not overlap with the buffer zone, the A2 area is the final natural grassland expansion area; when the A2 area overlaps with the buffer zone by an area ΔA, the natural grassland expansion area is A2-ΔA.

[0018] As a preferred embodiment of the present invention, in step S3, combined with the measured data of groundwater monitoring wells in the study area, typical irrigation areas with different groundwater depths are screened out, and the bands of natural grasslands, degraded grasslands, severely degraded grasslands and cultivated land in the agricultural-pastoral transition area are extracted and identified based on the multispectral drone in step one. The current agricultural and pastoral distribution pattern in ecologically fragile areas is determined based on the multispectral band data combined with remote sensing satellite data, the distribution of degraded grasslands around irrigation areas with different groundwater depths is analyzed, and the buffer zone thickness Δh of the irrigation area affecting the degradation of natural grassland is extracted and determined.

[0019] As a preferred implementation mode of the present invention, in step S4, the natural grassland expansion is taken as the basic principle, and the "source-sink" theory method in landscape ecology is used to adjust and optimize the existing ecological pattern.

[0020] As a preferred embodiment of the present invention, the specific method of adjustment and optimization is as follows: natural grassland, as a gene bank for grassland ecological restoration, plays the most critical role in the safety of the entire plain ecosystem, as the "source", and farmland and unused land as the "sink"; the minimum cumulative resistance model is used to describe the resistance overcome by the "source" through different landscape patches, and GIS is used to analyze the influence of different landscape patch types on the spatial diffusion of the "source", and to construct a landscape resistance surface; the resistance coefficient is set in the range of 1-10, and the larger the resistance coefficient, the less the landscape patch utilizes the expansion and merging of the "source"; the resistance value of urban construction land, large irrigation areas and other areas involving urban development planning that cannot be adjusted is the largest, with a value of 10; farmland is second, with a value of 7, and unused land is The value is 5, and the successional grassland takes a value of 1; by using 1 / 2 standard variance to perform hierarchical statistics on the cumulative resistance values ​​in GIS, the critical threshold is determined according to the frequency mutation of the resistance values ​​and the spatial distribution characteristics to divide the corresponding resistance value interval, and the expandable area of ​​the natural grassland is determined; by expanding the area of ​​natural grassland and merging landscape patches, the connectivity of natural grassland habitat patches is increased, the flow of plant species between natural grassland and successional grassland is promoted, and the migration rate of natural grassland plant species to successional grassland is increased, thereby improving the plant species diversity of successional grassland, thereby continuously expanding the area of ​​natural grassland and maintaining certain natural attributes. After adjustment, the expandable area of ​​natural grassland is A1, the area of ​​degraded grassland is reduced by B1, the area of ​​severely degraded grassland is reduced by C1, and the area of ​​cultivated land is reduced by D1.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The present invention illustrates the supporting role of groundwater on natural grasslands through the critical burial depth of groundwater recharge vegetation, and determines the thickness of the buffer zone of the irrigation area on natural grasslands; after adjusting the natural grasslands using landscape ecology, it further analyzes whether the groundwater conditions in the adjustment area are sufficient to support the growth of natural grassland vegetation, and whether the expansion area is in the buffer zone, thereby analyzing the actual recoverable range of natural grasslands; the physical process and action mechanism of this method are clear, and the evaluation calculation method is simple and easy to operate, providing reliable theoretical and technical support for guiding water resources management.

[0023] The specific implementation modes of the present invention are further described in detail below in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In the attached picture:

[0025] Figure 1 It is a flow chart of the calculation method for determining the reasonable ecological pattern of the agricultural-pastoral transition zone in the semi-arid region of the present invention;

[0026] Figure 2 Schematic diagram of the vadose zone model for recharging vegetation for phreatic evaporation;

[0027] Figure 3 This is a schematic diagram of the landscape pattern adjustment of the Xiliaohe Plain;

[0028] Figure 4 This is a distribution map of the expansion of natural grassland in the Xiliaohe Plain. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The following embodiments are used to illustrate the present invention.

[0030] like Figures 1 to 4 As shown, a calculation method for determining the reasonable ecological pattern of the ecologically fragile zone in the semi-arid area includes the following steps:

[0031] Step S1: extract and identify the bands of natural grassland, degraded grassland, severely degraded grassland and cultivated land in the agricultural-pastoral transition zone based on multispectral UAV, and determine the current agricultural-pastoral distribution pattern in the ecologically fragile zone based on multispectral band data combined with remote sensing satellite data, that is, the area of ​​natural grassland is A, the area of ​​degraded grassland is B, the area of ​​severely degraded grassland is C, and the area of ​​cultivated land is D;

[0032] Step S2: Determine the critical depth of natural grassland vegetation growth based on groundwater recharge in the semi-arid ecologically fragile area. The calculation formula is as follows:

[0033]

[0034] Where: H is the critical depth of natural grassland vegetation growth based on groundwater recharge, in m; n is soil porosity; d is effective soil particle size, in m; σ is soil water tension, in N / m; ρ is the density of water at the lowest soil temperature before irrigation, in kg / m 3 ; g is the acceleration of gravity, unit is m / s2; D is the thickness of the root layer, unit is m;

[0035] Step S3: Identify the boundaries of the irrigation areas with different groundwater depths and groundwater exploitation intensities that affect the degradation of natural grasslands, and determine the thickness Δh of the buffer zone where the irrigation areas affect the degradation of natural grasslands;

[0036] Step S4: Analysis of natural grassland expansion based on source-sink theory. Taking natural grassland expansion as the basic principle, the "source-sink" theory method in landscape ecology is used to adjust and optimize the existing ecological pattern. After adjustment, the natural grassland can be expanded by A1, the degraded grassland area is reduced by B1, the severely degraded grassland area is reduced by C1, and the cultivated land area is reduced by D1;

[0037] Step S5: Stability analysis of natural grassland expansion area: the critical burial depth H of natural grassland vegetation growth calculated in step 2 and the buffer zone thickness Δh of the irrigation area affecting natural grassland degradation determined in step 3 are used as the determination conditions for the stability of natural grassland expansion area;

[0038] Step S6: The area of ​​the natural grassland expansion area where the groundwater depth is less than H is set as A2, and the area of ​​the area where the groundwater depth is greater than H is set as A3, that is, A1=A2+A3; the groundwater conditions of A1 can support the growth of natural grassland vegetation, and after adjustment, it becomes a potential natural grassland, and the groundwater conditions of A2 do not support the growth of natural grassland vegetation, and after adjustment, it becomes a degraded grassland;

[0039] Step S7: whether the A2 area overlaps with the thickness of the buffer zone where the irrigation area affects the degradation of natural grassland is determined. When the A2 area does not overlap with the buffer zone, the A2 area is the final natural grassland expansion area; when the A2 area overlaps with the buffer zone by an area ΔA, the natural grassland expansion area is A2-ΔA.

[0040] In a specific implementation manner, in step S3, combined with the measured data of groundwater monitoring wells in the study area, typical irrigation areas with different groundwater depths are screened out, and the bands of natural grasslands, degraded grasslands, severely degraded grasslands and cultivated land in the agricultural and pastoral transition zone are extracted and identified based on the multispectral drone in step one. The current agricultural and pastoral distribution pattern in ecologically fragile areas is determined based on the multispectral band data combined with remote sensing satellite data, the distribution of degraded grasslands around irrigation areas with different groundwater depths is analyzed, and the buffer zone thickness Δh of the irrigation area affecting the degradation of natural grassland is extracted and determined.

[0041] Furthermore, in step S4, the expansion of natural grassland is taken as the basic principle, and the "source-sink" theory method in landscape ecology is used to adjust and optimize the existing ecological pattern. The specific method of the adjustment and optimization is as follows: natural grassland, as the gene bank of grassland ecological restoration, plays the most critical role in the safety of the entire plain ecosystem, as the "source", and farmland and unused land as the "sink"; the minimum cumulative resistance model is used to describe the resistance overcome by the "source" through different landscape patches, and GIS is used to analyze the influence of different landscape patch types on the spatial diffusion of the "source", and the landscape resistance surface is constructed based on this; the resistance coefficient is set to take a value range of 1-10, and the larger the resistance coefficient, the less the landscape patch uses the expansion and merging of the "source"; the urban construction land, large irrigation areas, etc. involve urban development planning and the resistance value of the area that cannot be adjusted is the smallest The largest resistance value is 10; farmland is second, with a value of 7, unused land is 5, and successional grassland is 1; by using 1 / 2 standard variance to perform hierarchical statistics on the cumulative resistance values ​​in GIS, the critical threshold is determined according to the frequency mutation and spatial distribution characteristics of the resistance values ​​to divide the corresponding resistance value interval, and the expandable area of ​​the natural grassland is determined; by expanding the area of ​​natural grassland and merging landscape patches, the connectivity of natural grassland habitat patches is increased, the flow of plant species between natural grassland and successional grassland is promoted, and the migration rate of plant species in natural grassland to successional grassland is increased, thereby improving the diversity of plant species in successional grassland, thereby continuously expanding the area of ​​natural grassland and maintaining certain natural attributes. After adjustment, the expandable area of ​​natural grassland is A1, the area of ​​degraded grassland is reduced by B1, the area of ​​severely degraded grassland is reduced by C1, and the area of ​​cultivated land is reduced by D1.

[0042] Furthermore, the calculation formula for the critical depth of vegetation for groundwater recharge in step 2 of this scheme is derived as follows:

[0043] Water is a key factor in the growth of grassland vegetation in semi-arid areas. After natural rainfall infiltrates into the soil, it forms soil water. Part of the soil water continues to infiltrate and replenish groundwater, and part of it is retained in the soil for absorption by vegetation roots to achieve evaporation or evaporation from bare soil. Under the level of average rainfall over many years, when the water absorbed by vegetation comes only from soil moisture replenished by rainfall, the surface vegetation is mainly grassland with low coverage. When the water absorbed by vegetation comes not only from soil water replenished by rainfall, but also from groundwater replenishment, a natural pasture grassland with high coverage is formed.

[0044] As an important support for natural vegetation, groundwater replenishes vegetation through groundwater rise. At the groundwater surface, under the action of the surface tension of the groundwater surface and the capillary of the soil pores, groundwater rises into the soil to form capillary rise water, forming a groundwater influence layer of a certain thickness; and the upper vegetation roots have a root action layer of a certain thickness. When the upper edge of the groundwater influence layer intersects with the lower edge of the root action layer, the water absorbed by the vegetation roots not only comes from the water retained in the soil by rainfall, but also includes the groundwater in the groundwater influence layer, thereby realizing the groundwater replenishment of vegetation. On the contrary, when the two do not intersect, vegetation cannot absorb groundwater, and groundwater cannot replenish vegetation. In the long run, the soil begins to become desertified, which is also the key to soil desertification.

[0045] Phreatic evaporation is the core issue of grassland ecology in semi-arid areas, so it is necessary to deeply analyze the mechanism of phreatic evaporation replenishing vegetation. Phreatic water does not directly affect surface vegetation, but replenishes surface vegetation through the interaction of two processes: phreatic evaporation to form soil water and plant root absorption. By defining the concepts of vegetation root action layer and phreatic influence layer, the vadose zone is decomposed as follows: Figure 2 , Figure 2 Schematic diagram of the vadose zone model for phreatic evaporation to recharge vegetation.

[0046] When the phreatic layer intersects with the vegetation root layer, groundwater can provide water to the surface vegetation; when the phreatic layer is separated from the vegetation root layer, groundwater cannot provide water to the surface vegetation. According to the above principle, when the phreatic layer contacts the root layer, it meets the definition of the critical burial depth of groundwater supplying vegetation. Therefore, the sum of the thickness of the root layer and the phreatic layer is taken as the critical burial depth of groundwater supplying vegetation.

[0047] (1) Phreatic influence layer

[0048] Definition: The sum of soil water formed by the capillary rise of soil water occurring along the water table, which is distributed in layers. Its connotation includes the thickness of the water table influence layer and the internal soil water distribution. The thickness depends on the maximum rise height of the capillary water.

[0049] The essence of phreatic evaporation is that due to the capillary force above the phreatic surface, water rises from the phreatic surface to the vadose zone to form capillary water. The phreatic influence layer is the sum of all capillary water, so the maximum range of the phreatic surface depends on the maximum rise of capillary water, which is defined as the thickness of the phreatic influence layer. The calculation of the phreatic influence layer thickness is the calculation of the maximum rise of capillary water.

[0050] Calculate the maximum rise height H1 of the capillary tube according to the Laplace formula:

[0051]

[0052] Where: H1 is the capillary water rise height, unit is m; σ is the surface tension coefficient, unit is N / m; ρ is the liquid density, unit is kg / m 3 ; g is the acceleration due to gravity, in m / s 2 .

[0053] The main factors affecting the height of capillary water rise are the effective pore size of the soil and the surface tension of the water body. The effective pore size of the soil capillary is difficult to obtain, which limits the direct calculation using formulas. This study has made a major breakthrough in this regard.

[0054] (2) Calculation of effective pore size of soil:

[0055] The effective pore size of soil capillary is the key to the formula for calculating the maximum rise height of soil capillary water. Soil structure is usually expressed by two parameters: effective particle size d and porosity n, which reflect the relationship between soil texture and the spatial distribution of soil particles under external pressure. How to convert it into the effective pore size R of soil capillary is difficult, that is, to establish the following functional relationship:

[0056] R=f(n,d) (2)

[0057] Soil particle size is an inherent characteristic of soil materials, reflecting the material properties of soil particles; soil pores are closely related to the external environment, reflecting the spatial arrangement of particles under external pressure and other environmental conditions, and are not dependent on soil particle size. Therefore, they can be regarded as two independent variables. Equation (3) can be written as:

[0058] R=f1(n)·f2(d) (3)

[0059] On the other hand, from the perspective of dimensional analysis, the properties of effective pore size and effective particle size are the same. Therefore, the above function form can be expressed as follows:

[0060] R=ξ(n)·d (4)

[0061] Therefore, the soil capillary pore size depends on the soil particle size and the soil pore characteristics ξ(n), which is called the pore characteristic function. Solving the soil pore characteristics ξ(n) is the key to solving the capillary pore size problem. It should be noted that the derivation of this formula is suitable for calculations in large areas with less artificial interference in the field. In irrigation areas, due to the large interference of artificial interference on the physical parameters of the soil itself, the relevant physical properties are changed.

[0062] Soil pore characteristics are closely related to the spatial arrangement of soil particles. The soil particle crystal structure model is used to simulate the distribution and arrangement of soil crystals. The appropriate arrangement structure is selected to calibrate the pore characteristic function.

[0063] Through the study and analysis of multiple soil samples, it was found that there are two most common structural arrangements, one is a regular triangle and the other is a regular quadrilateral, whose total porosity parameters can cover various common soil types. Therefore, these two soil particle structure models are used as the basic models for analyzing soil pore structure.

[0064] For soil with a particle size of d, the soil structure with regular triangular arrangement can be regarded as a regular triangular prism with a height of d and a length of d on the upper and lower sides, with three 1 / 6 spheres embedded in it, and the rest is pores. The soil structure with regular quadrilateral arrangement can be regarded as a cube with a side length of d, with four 1 / 4 spheres embedded in it. Both soil structures contain spherical particles and pores, among which the cylindrical pores inscribed in the pores between particles can be regarded as effective pores in the soil.

[0065] Each particle arrangement structure corresponds to a total porosity n, which represents the unique identity of this arrangement structure and can therefore be considered a model parameter. Assuming the volume of a soil unit v of a certain arrangement structure and the volume of soil particles in the unit w, the model parameter calculation formula is:

[0066] n=(vw) / v=1-w / v (5)

[0067] The parameters of the regular triangle arrangement structure model are:

[0068]

[0069] The parameters of the regular quadrilateral arrangement structure model are:

[0070]

[0071] In the soil particle crystal model, soil capillaries are effective pores and are part of the total pores. The relationship between the effective pore size R and the particle size d of the model is further obtained. For the regular triangle arrangement, we have:

[0072]

[0073] For regular quadrilateral arrangements, we have:

[0074]

[0075] For any soil (n, d), linear interpolation can be performed using the model parameters and equations (8) and (9) to obtain an approximate expression for the soil pore characteristic function ξ(n);

[0076] ξ(n)=1.581(n-39.5%)+0.079 (10)

[0077] In this study, based on the soil particle crystal model with regular triangle structure and regular quadrilateral structure, the effective pore size of soil capillary tube was calculated by the following formula:

[0078] R=[1.581(n-39.5%)+0.079]·d (11)

[0079] Where: R is the effective pore size, unit is m; n is the soil porosity, dimensionless; d is the soil particle size, unit is m.

[0080] Through the above formula, under the conditions of known soil porosity and effective particle size, a reasonable soil structure can be selected and the effective pore size can be calculated, providing reliable parameters for the capillary water calculation formula to calculate the maximum rising height of capillary water.

[0081] (2) Calculation of the thickness of the diving influence layer

[0082] Substituting equation 11 into equation 1, the calculation formula for the thickness of the diving influence layer is as follows:

[0083]

[0084] (3) Vegetation root action layer

[0085] Definition and concept: It includes all the roots and hairs in the vegetation community, forming the root system of the community. Under natural conditions, affected by multiple factors such as climate, soil moisture, fertility, and symbiotic environment, the vegetation root system has experienced various living conditions, fully developed and grown lateral roots and hair roots, and exerted the ability to absorb deep water downward, and the thickness is close to a constant. This is the result of natural selection.

[0086] In the same community, the root depths are different due to different plant types. For the same type of plants, their root depths are also different due to water conditions. Generally, through field surveys of vegetation and root depths, the most representative root depths of the main plants widely distributed in the study area are selected as the vegetation root layer thickness D.

[0087] (4) Critical depth of vegetation for groundwater recharge H

[0088] Combined with the thickness of the groundwater influence layer of different soils and the vegetation root system, the critical burial depth H of groundwater recharge vegetation is analyzed and calculated, as shown in formula (13):

[0089] Specific embodiment:

[0091] my country's semi-arid areas are mainly in the Inner Mongolia Plateau and most of the Loess Plateau. Although the semi-arid areas have similar climate elements such as precipitation and evaporation, due to the large span from east to west, the soil and vegetation types and hydrogeological structures are complex and diverse. Even if a comprehensive analysis is carried out, there is a possibility of generalization. For this reason, this study takes the Inner Mongolia Plateau, where the vegetation degradation and soil desertification problems in the semi-arid agricultural and pastoral transition zone are more prominent, as the research object. Among them, the Xiliaohe Plain was selected as a typical research area because of its complete and independent basin, and quantitatively analyzed the critical burial depth of groundwater recharge vegetation in different regions.

[0092] Soil data of Xiliaohe Plain can be obtained according to land use maps and soil type distribution maps of different regions. After consulting soil species and combining field survey data, the effective particle size and porosity of typical soils in the region were determined. The effective pore size calculation formula was used to substitute the Laplace formula to calculate the maximum rise height of capillary water. Among them, the surface tension coefficient, according to the investigation and analysis of the temperature of different soil layers, the temperature range is 20℃~23℃, and the surface tension change caused by soil temperature change is very small, so the surface tension coefficient is 72.5×10-3N / m 2 . The calculation results are shown in Table 1. It can be seen from Table 1 that there is a sufficient theoretical basis for using the Laplace formula to calculate the capillary water rise height. However, when the soil particle size is less than 0.01m, the direct calculation formula fails. However, actual observations show that capillary water still exists. Speculation: This fine-grained clay may have an "agglomeration effect", which aggregates into larger composite "agglomerated particles" in a certain order, and the Laplace calculation formula is used based on the effective pore size of the agglomerated particles. Since the agglomeration effect is an issue that requires special in-depth research, it is beyond the scope of the study. This study directly obtained the capillary rise height of small-particle soil through observation.

[0093] Table 1 Calculation results of the thickness of the diving influence layer

[0094]

[0095] Empirical analysis of capillary water rise height:

[0096] Field demonstration was conducted in the Xiliaohe Plain. Based on field observation sampling data, the soil moisture content and soil depth change diagram of different soil types were plotted. The soil moisture content changed dramatically from large to small starting from the water table until it stabilized. The height difference was the actual capillary water rise height. At the same time, a survey of the groundwater depth of typical vegetation communities was conducted.

[0097] A total of 6 points were selected for the soil moisture survey in the grassland sandy soil area, and the survey results are shown in Table 2. The data from the sampling points consistently showed that the capillary water rise height was 1m or slightly above 1m, proving that the calculated maximum capillary water rise height of 1.25m is reasonable and reliable.

[0098] Table 2 Observation results of capillary water in the sandy soil area of ​​Xiliaohe Plain grassland

[0099]

[0100] Vegetation root action layer:

[0101] In the same community, the root depths are different due to different plant types. For the same type of plants, their root depths are also different due to water conditions. This paper gives the most representative root depths of the main plants widely distributed in the study area. Combined with the survey results of the vegetation root system in the Xiliaohe Plain, with reference to "Inner Mongolia Grassland Plant Root Types" and "North China Grassland Plant Root System", the plant root depths, types and plant characteristics are systematically summarized. Sand vegetation is the main vegetation type in the region. The root system of herbaceous vegetation is basically determined to be within 0.50m, and the root system depth of herbaceous vegetation is 0.50m as the thickness of the vegetation root action layer; the root system depth of shrub and semi-shrub vegetation is determined to be about 1.50m, and the root system depth of shrub is 1.50m as the thickness of the vegetation root action layer.

[0102] Critical depth of vegetation for groundwater recharge:

[0103] Based on the thickness of the groundwater influence layer and the vegetation root system of different soils, the critical burial depth of groundwater recharge vegetation was analyzed and calculated (see Table 3).

[0104] Table 3 Critical depth of groundwater recharge vegetation in the grassland of Xiliaohe Plain

[0105]

[0106]

[0107] According to the principle of groundwater recharge vegetation, the optimal burial depth of groundwater recharge vegetation is the herb root depth plus the thickness of the phreatic layer, and the critical burial depth of groundwater recharge vegetation is the sum of the shrub root depth as the thickness of the vegetation root action layer and the phreatic layer thickness. Taking the Xiliaohe Plain as an example, the optimal burial depth of groundwater recharge vegetation is 1 to 2 meters, and the critical burial depth is 2 to 3 meters.

[0108] Through field survey data, the relationship between different types of meadow and shrub vegetation and groundwater depth in the Xiliaohe Plain was mainly observed, and the critical depth of groundwater in native non-zonal vegetation was verified. As shown in Table 4, the field observation results of different types of vegetation and groundwater depth show that the groundwater depth corresponding to native non-zonal vegetation is within 2m for herbs and within 3m for shrubs and semi-shrubs. The groundwater depth corresponding to successional vegetation is greater than 3m. Field observations show that the critical depth of groundwater recharge for non-zonal vegetation is less than 2-3m, which is consistent with theoretical analysis and calculation.

[0109]

[0110] Landscape pattern adjustment:

[0111] According to the principles of restoration ecology, the adjustment of the ecological pattern of the agricultural-pastoral transition zone in the Xiliaohe Plain should be based on the topological expansion of natural grasslands as the basic principle. Through area expansion and landscape patch merging, the connectivity of grassland habitat patches can be increased, the flow of plant species between natural grasslands and degraded grasslands can be promoted, and the migration rate of natural grassland plant species to degraded grasslands can be increased, thereby improving the diversity of plant species in degraded grasslands. At the same time, for vegetation communities with a narrow distribution range, area expansion allows community species within patches to survive and continue, and provides a stable habitat for plant communities that lack spatial diffusion capabilities. In this way, the area of ​​natural grasslands continues to expand while maintaining certain natural attributes. While expanding natural grasslands, the landscape pattern of degraded grasslands should also be optimized, the continuity of ecological landscapes should be considered, ecological nodes should be set up, and ecological corridors should be established to ensure the connectivity of natural ecological landscapes and prevent fragmentation. Establish corridors that connect natural grasslands and degraded grasslands, promote the flow of plant species between natural grasslands and degraded grasslands, and increase the migration rate of natural grassland plant species to degraded grasslands. Please refer to Figure 3 , Figure 3 This is a schematic diagram of the landscape pattern adjustment of the Xiliaohe Plain.

[0112] Taking the expansion of natural grassland as the basic principle, the "source-sink" theory in landscape ecology is used to adjust and optimize the agro-pastoral ecological pattern in the Xiliaohe Plain. The "source-sink" theory is a commonly used theoretical method in the study of landscape ecological pattern, and is mostly used in the protection of large-scale biological species diversity. Among them, "source" refers to the landscape type that promotes the protection and restoration of biodiversity; "sink" refers to the landscape type that hinders the protection and restoration of biodiversity. As a gene bank for grassland ecological restoration, natural grassland plays the most critical role in the safety of the entire plain ecosystem. It can be used as a "source", and farmland and unused land as "sinks". And consider avoiding direct conflicts between different landscape functions such as grassland and farmland in spatial pattern, set up buffer zones in the areas where grasslands meet large irrigation areas, such as closed grasslands or artificial green belts, to reduce the impact of irrigation areas on grasslands, which is conducive to the maintenance and restoration of the natural attributes of natural grasslands.

[0113] The minimum cumulative resistance model is used to describe the resistance overcome by the “source” through different landscape patches. GIS is used to analyze the influence of different landscape patch types on the spatial diffusion of the “source”, and the landscape resistance surface is constructed based on this. The formula is as follows:

[0114] MCR=fmin∑(Dij×Ri)(i=1,2,3,...,m; j=1,2,3,...,n) (14)

[0115] In the formula, Dij refers to the distance between the "source" j and another landscape patch i; Ri refers to the resistance coefficient of landscape patch type i, which is used to describe the difficulty of landscape patch expansion.

[0116] Related studies have shown that different landscape patch types encounter different resistances during movement and migration, depending on the suitability of the landscape interface for the survival, reproduction and migration of the patch type. The resistance coefficient is also relative. As long as it can relatively reflect the differences in different resistance factors, it can be used to calculate the cost distance. The resistance coefficient is set to a range of 1-10. The larger the resistance coefficient, the less the landscape patch will use the expansion and merging of the "source". The resistance value of urban construction land, large irrigation areas and other areas that involve urban development planning and cannot be adjusted is the largest, with a value of 10; farmland is second, with a value of 7; unused land has a value of 5, and successional grassland has a value of 1.

[0117] By using 1 / 2 standard deviation to classify the cumulative resistance values ​​in GIS, the critical threshold is determined according to the frequency mutation and spatial distribution characteristics of the resistance values ​​to divide the corresponding resistance value intervals, and the impact of natural grasslands on the entire expandable area is divided into expansion core area, expansion connection area and expansion buffer area. Figure 4 , Figure 4 This is a distribution map of the expansion of natural grassland in the Xiliaohe Plain.

[0118] The core area of ​​natural grassland expansion is 8002.16km2. It is distributed around natural grasslands and has the lowest resistance level. It plays a key role in protecting natural grasslands from interference from human activities and maintaining their natural properties. It is the most direct area for natural grassland expansion. All farmland and unused land patches in this area should be adjusted to grasslands.

[0119] The natural grassland expansion connected area covers an area of ​​9835.85km2. As the natural grassland expands from the expansion core area to the periphery, it is gradually affected by human activities. This area mostly presents a landscape pattern of large areas of degraded grasslands interlaced with patched farmland and unused land. The difference between this area and the core area is that it will be difficult to adapt by simply changing the patch attributes to closed grassland or further planting the dominant species of the vegetation community in the nearby core area. It is necessary to use the critical burial depth of groundwater recharge vegetation combined with the succession path of the vegetation community in the Xiliaohe Plain under different moisture conditions analyzed above to adjust the vegetation community in the connected area.

[0120] The natural grassland expansion buffer zone covers an area of ​​4562.99km2. It is far from the natural grassland and has the greatest resistance. The current landscape is mostly large irrigation areas, highly fragmented farmland and unused land near towns, and degraded grassland interlaced areas. Due to the restrictions of urban development planning, this area is unlikely to be restored to the natural attributes of natural grasslands, and it is very easy to lose the natural attributes of grasslands under the condition of increased human interference. Therefore, it is necessary to protect the patches distributed in this area, maintain the stability of the area through the development of artificial grasslands, the closure of degraded grasslands and other measures, and provide safe boundary conditions for the restoration of grassland ecosystems in the core area and the connected area.

[0121] Reasonable ecological pattern of the agricultural-pastoral transition zone in the Xiliaohe Plain:

[0122] The critical burial depth of vegetation for groundwater recharge has been obtained in the previous project, which is 2-3m. By superimposing the contour maps of groundwater burial depth in grassland expansion area and Xiliaohe Plain in different periods, the rationality of groundwater subsurface flow field supporting the ecological pattern of agricultural and pastoral areas under different scenarios is analyzed. According to the groundwater burial depth values ​​<3m, 3-4m, >4m, the groundwater conditions in the expansion area are divided into support, basic support, and no support. Based on the thickness Δh of the buffer zone affected by irrigation areas of different burial depths on natural grassland analyzed based on the measured data, whether the natural grassland expansion area overlaps with the thickness of the buffer zone affected by irrigation areas on natural grassland degradation is determined and analyzed. When the expansion area does not overlap with the buffer zone, the expansion area supported by groundwater conditions is the final natural grassland expansion area; when the expansion area supported by groundwater conditions has an overlapping area ΔA with the buffer zone, the natural grassland expansion area should be deducted from the overlapping area.

[0123] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A calculation method for determining the reasonable ecological pattern of the ecologically fragile zone in a semi-arid area, characterized in that: The steps include: Step S1: extract and identify the bands of natural grassland, degraded grassland, severely degraded grassland and cultivated land in the agricultural-pastoral transition zone based on multispectral UAV, and determine the current agricultural-pastoral distribution pattern in the ecologically fragile zone based on multispectral band data combined with remote sensing satellite data, that is, the area of ​​natural grassland is A, the area of ​​degraded grassland is B, the area of ​​severely degraded grassland is C, and the area of ​​cultivated land is D; Step S2: Determine the critical depth of natural grassland vegetation growth based on groundwater recharge in the semi-arid ecologically fragile area. The calculation formula is as follows: Where: H is the critical burial depth for natural grassland vegetation growth based on groundwater recharge, in m; n is soil porosity; d is effective soil particle size, in m; σ is soil water tension, in N / m; ρ is the density of water at the lowest soil temperature before irrigation, in kg / m 3 ; g is the acceleration of gravity, unit is m / s2; D is the thickness of the root layer, unit is m; Step S3: Identify the boundaries of the irrigation areas with different groundwater depths and groundwater extraction intensities that affect the degradation of natural grasslands, and determine the thickness Δh of the buffer zone where the irrigation areas affect the degradation of natural grasslands; Step S4: Analysis of natural grassland expansion based on source-sink theory. Taking natural grassland expansion as the basic principle, the "source-sink" theory method in landscape ecology is used to adjust and optimize the existing ecological pattern. After adjustment, the natural grassland can be expanded by A1, the degraded grassland area is reduced by B1, the severely degraded grassland area is reduced by C1, and the cultivated land area is reduced by D1; Step S5: Stability analysis of natural grassland expansion area: the critical burial depth H of natural grassland vegetation growth calculated in step 2 and the buffer zone thickness Δh of the irrigation area affecting natural grassland degradation determined in step 3 are used as the determination conditions for the stability of natural grassland expansion area; Step S6: The area of ​​the natural grassland expansion area where the groundwater depth is less than H is set as A2, and the area of ​​the area where the groundwater depth is greater than H is set as A3, that is, A1=A2+A3; the groundwater conditions of A1 can support the growth of natural grassland vegetation, and after adjustment, it becomes a potential natural grassland, and the groundwater conditions of A2 do not support the growth of natural grassland vegetation, and after adjustment, it becomes a degraded grassland; Step S7: whether the A2 area overlaps with the thickness of the buffer zone where the irrigation area affects the degradation of natural grassland is determined. When the A2 area does not overlap with the buffer zone, the A2 area is the final natural grassland expansion area; when the A2 area overlaps with the buffer zone by an area ΔA, the natural grassland expansion area is A2-ΔA.

2. The calculation method for determining the reasonable ecological pattern of the ecologically fragile zone in a semi-arid area according to claim 1 is characterized in that: In step S3, based on the measured data of groundwater monitoring wells in the study area, typical irrigation areas with different groundwater depths are screened out, and band extraction and identification of measured points of natural grassland, degraded grassland, severely degraded grassland and cultivated land in the agricultural-pastoral transition area are performed based on the multispectral UAV in step 1. The current agricultural and pastoral distribution pattern in the ecologically fragile area is determined based on the multispectral band data combined with remote sensing satellite data, the distribution of degraded grassland around irrigation areas with different groundwater depths is analyzed, and the buffer zone thickness Δh of the irrigation area affecting the degradation of natural grassland is extracted and determined.

3. The calculation method for determining the reasonable ecological pattern of the ecologically fragile zone in a semi-arid area according to claim 1 is characterized in that: In step S4, the natural grassland expansion is taken as the basic principle, and the "source-sink" theory method in landscape ecology is used to adjust and optimize the existing ecological pattern.

4. The calculation method for determining the reasonable ecological pattern of the ecologically fragile zone in a semi-arid area according to claim 3 is characterized in that: The specific method of adjustment and optimization is as follows: natural grassland, as the gene bank of grassland ecological restoration, plays the most critical role in the safety of the entire plain ecosystem, as the "source", and farmland and unused land as the "sink"; the minimum cumulative resistance model is used to describe the resistance overcome by the "source" through different landscape patches, and GIS is used to analyze the influence of different landscape patch types on the spatial diffusion of the "source", and the landscape resistance surface is constructed based on this; the resistance coefficient is set to take a range of 1-10, and the larger the resistance coefficient, the less the landscape patch will use the expansion and merging of the "source"; urban construction land, large irrigation areas and other areas involving urban development planning that cannot be adjusted have the largest resistance value, which is 10; farmland is second, with a value of 7, unused land has a value of 5, and successional grassland has a value of 1; By using 1 / 2 standard variance to perform graded statistics on the cumulative resistance values ​​in GIS, the critical threshold is determined according to the frequency mutation and spatial distribution characteristics of the resistance values ​​to divide the corresponding resistance value interval, and the expandable area of ​​the natural grassland is determined; by expanding the area of ​​natural grassland and merging landscape patches, the connectivity of natural grassland habitat patches is increased, the flow of plant species between natural grassland and successional grassland is promoted, and the migration rate of natural grassland plant species to successional grassland is increased, thereby improving the diversity of plant species in successional grassland, thereby continuously expanding the area of ​​natural grassland and maintaining certain natural attributes. After adjustment, the expandable area of ​​natural grassland is A1, the area of ​​degraded grassland is reduced by B1, the area of ​​severely degraded grassland is reduced by C1, and the area of ​​cultivated land is reduced by D1.

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