Physical inversion method for root layer soil moisture based on vegetation transpiration

Abstract

The invention discloses a root layer soil moisture physical inversion method based on vegetation transpiration, and relates to the technical field of surface parameter quantitative inversion, and the method comprises the steps: S1, determining a quantitative relation between a vegetation transpiration ratio and a root layer soil volume moisture content, and carrying out the normalization of the root layer soil moisture; s2, determining a pressure water head upper limit for converting vegetation transpiration from energy limitation to moisture limitation; s3, calculating wilting coefficients and critical soil moisture under the conditions of different soil types, vegetation parameters and meteorological factors; s4, obtaining a transpiration ratio through a ratio of vegetation transpiration to potential vegetation transpiration; and S5, converting the normalized root layer soil moisture into the volumetric water content of the root layer soil by using the wilting coefficient and the critical soil moisture. The root layer soil moisture physical inversion method based on vegetation transpiration has high generalization ability and can meet the high-precision requirements of the fields of modern agriculture, water resources, climate change and the like for root layer soil moisture.

Description

Technical Field

[0001] The invention relates to the technical field of quantitative inversion of surface parameters, and in particular to a physical inversion method of root layer soil moisture based on vegetation transpiration. Background Art

[0002] Root layer soil moisture is a key parameter driving the three major circulation systems of water, energy and carbon on the earth's surface. It has an important feedback effect on the carbon sink efficiency and temperature regulation of the surrounding environment by controlling the development and growth of vegetation and physiological mechanisms (transpiration, water absorption and stomatal opening and closing, etc.), and greatly affects the surface-atmosphere interaction and energy exchange process. In the context of the dual core driving forces of global change and population explosion, the dynamic changes of root layer soil moisture are of great significance to the research of agriculture, hydrology, ecology, climate change and social economy.

[0003] At present, water shortage has become a major challenge that needs to be solved urgently worldwide, especially in densely populated areas, where groundwater resources are experiencing a severe decline year by year. Agriculture, as the main consumer of water resources, accounts for about 70% of the world's available water resources. Scientific water resource management policies based on root layer soil moisture are of great significance for alleviating water resource pressure and promoting the rational allocation and efficient use of water resources. Therefore, obtaining the spatiotemporal distribution and change information of root layer soil moisture has important scientific significance and application value for agricultural monitoring, water resource management, and global climate change research, and is one of the frontier topics in the fields of agriculture, water resources, and climate change.

[0004] Traditional root layer soil moisture monitoring mainly relies on direct point measurement of soil profiles or the expansion of point observation data to the grid scale by grid interpolation. This method can roughly reflect the spatial distribution of root layer soil moisture, but the accuracy of the interpolation result is affected by the number and distribution of observation points, and the cost is high. In the context of the big data era, data assimilation or data-driven methods that rely on surface soil moisture observation information have many challenges, including the decrease in accuracy caused by the accumulation of initial errors in observation data, insufficient spatial resolution to capture fine terrain features, complex error sources that are difficult to quantify accurately, and the parameter calibration process is easily disturbed by multiple factors such as surface characteristics and vegetation cover. The inversion methods based on remote sensing technology generally have the problem of low accuracy and fail to consider the impact of vegetation on the "saturation" of root layer soil moisture. This limitation is mainly attributed to the insufficient explanation of the complex process mechanism of surface water pressure (indicators such as potential evaporation ratio, evaporation ratio and transpiration ratio). In general, the current root layer soil moisture inversion algorithm is difficult to meet the high-precision requirements for root layer soil moisture in modern agriculture, water resources and climate change. Therefore, developing a physical inversion method for root layer soil moisture that is independent of surface soil moisture information and has high generalization ability and high precision is not only the basic prerequisite for obtaining high-precision root layer soil moisture data at regional and even global scales, but also the inevitable trend and core direction for promoting the development of future quantitative inversion technology for root layer soil moisture. Summary of the invention

[0005] The purpose of the present invention is to provide a physical inversion method for root layer soil moisture based on vegetation transpiration, which is independent of surface soil moisture information, has high generalization ability, and can meet the high-precision requirements of root layer soil moisture in modern agriculture, water resources, and climate change.

[0006] To achieve the above object, the present invention provides a root layer soil moisture physical inversion method based on vegetation transpiration, the steps are as follows: S1. Based on the principle of water balance, the contribution of surface soil moisture and root layer soil moisture in soil evaporation and vegetation transpiration is distinguished. The response function between vegetation transpiration ratio and root layer soil pressure head is constructed through the Feddes root water absorption model. The van Genuchten-Mualem soil water retention curve model is introduced to convert soil pressure head into volumetric water content, thereby determining the quantitative relationship between transpiration ratio and root layer soil volumetric water content; the quantitative relationship between vegetation transpiration ratio and root layer soil volumetric water content is simplified by normalizing root layer soil moisture; S2. Based on empirical data, construct a pressure head database of the Feddes model under different vegetation conditions, and determine the upper limit of the pressure head when vegetation transpiration changes from energy limitation to water limitation according to different underlying surfaces and meteorological conditions; S3, based on the conversion function of the Rosetta model, the soil hydraulic parameters of the van Genuchten-Mualem model are inferred through soil texture information, and the wilting coefficient and critical soil moisture under different soil types, vegetation parameters and meteorological factors are calculated through the van Genuchten-Mualem model using soil hydraulic parameters and the Feddes model pressure head database; S4. Using the data of vegetation gross primary productivity and saturated vapor pressure difference of meteorological factors, the bottom water use efficiency model is used to separate the actual evapotranspiration to obtain vegetation transpiration; the Penman formula is used to calculate the potential evapotranspiration without water evaporation pressure under given radiation and meteorological conditions; based on the Lambert-Beer law, the leaf area index and potential evapotranspiration are used to derive the potential vegetation transpiration, and the transpiration ratio is obtained by the ratio of vegetation transpiration to potential vegetation transpiration; S5. The normalized root layer soil moisture model constructed by S1, the Feddes model parameters determined by S2, and the van Genuchten-Mualem model parameters determined by S3 are used to calculate the normalized root layer soil moisture using the transpiration ratio; finally, the normalized root layer soil moisture is converted into the root layer soil volume moisture content using the wilting coefficient and critical soil moisture.

[0007] Preferably, S1.1, based on the principle of water balance, distinguish the contribution of surface soil moisture and root layer soil moisture in soil evaporation and vegetation transpiration, and construct a response function between vegetation transpiration ratio and root layer soil pressure head through the Feddes root water absorption model, specifically including: According to the water balance, assuming that the water absorbed by the vegetation roots is completely used for transpiration, the response function of the root layer soil pressure head and vegetation transpiration is derived as follows: (1) In the formula, T r is the transpiration ratio; f is the water absorption efficiency of the root system; h is the pressure head; h 3 It is the upper limit of soil pressure head when vegetation is limited by water; h 4 is the pressure head of the wilting coefficient.

[0008] Preferably, S1.2, the van Genuchten-Mualem soil water retention curve model is introduced to convert the soil pressure head into volumetric water content, thereby determining the quantitative relationship between the transpiration ratio and the volumetric water content of the root layer soil, which is expressed as follows: (2) In the formula, i r is the residual water content; i s is the saturated water content; α is a shape parameter, which is related to the slope of the soil pore distribution curve and determines the shape of the curve when it is close to the saturated zone and the dry zone; n is another shape parameter, which is related to the uniformity of soil pore distribution. n The larger the value, the more uniform the pore distribution and the smoother the curve; m=1-1 / n; θ(h) is the volumetric water content, h is the soil pressure head.

[0009] Preferably, S1.3, the quantitative relationship between the vegetation transpiration ratio and the root layer soil volume water content is simplified by normalizing the root layer soil water content, which is expressed as follows: (3) (4) Normalized root zone soil moisture oh root After the expression, further assume that: (5) Then the normalized root layer soil moisture model related terms can be expressed as: (6) The simple expression of normalized root layer soil moisture is obtained by simplification of the model: (7) (8) in, a , b , c and d is a coefficient, which depends on soil, vegetation and atmospheric demand conditions; ψ This is the error term caused by the simplification process.

[0010] Preferably, in S2, the upper limit of the soil pressure head when the vegetation is limited by water is h 3 It is expressed as: (9) In the formula, T p is the potential transpiration; h 3l Indicates the upper limit of pressure head when the atmospheric water vapor demand is low. T p = T 3l ; h 3hIndicates the upper limit of pressure head when the atmospheric water vapor demand is high. T p = T 3h ; T 3l The value is set to 1 mmd -1 , T 3h The value is set to 5mmd -1 .

[0011] Preferably, S4.1, using the vegetation gross primary productivity and the saturated water vapor pressure difference data of meteorological factors, using the bottom water use efficiency model to separate the actual evapotranspiration to obtain vegetation transpiration, specifically includes: Through the uWUE model, the evapotranspiration is indirectly estimated using the biological information GPP and the meteorological factor VPD. The evapotranspiration ET is decomposed into evaporation E and transpiration T within a larger time scale, as shown below: (10) in, uWUE a represents apparent water use efficiency, GPP represents gross primary productivity of vegetation, and VPD represents saturated vapor pressure deficit; T near ET When the vegetation coverage is high, the evaporation E can be ignored, and the uWUE model is expressed as: (11) in, uWUE p represents the potential water use efficiency, determined by statistical methods of time series data; The transpiration rate T is obtained by formula (10) and formula (11), which is expressed as: (12).

[0012] Preferably, S4.2, the Penman formula is used to calculate the potential evapotranspiration without water evaporation pressure under given radiation and meteorological conditions, which is expressed as follows: (13) in, ET p The Penman formula divides evapotranspiration into two parts: the first part is the evaporation caused by water absorbing radiation. ET rad , the second part is aerodynamically driven evaporation ET aero ; R n and Gare the surface net radiation and soil heat flux, respectively, in W / m 2 ; r c is the canopy impedance under sufficient water supply conditions, in s / m; r a is the aerodynamic impedance, in s / m; c p is the specific heat of air at constant pressure, in J / (kg℃); c is the psychrometric constant, set to 0.0658 kPa / ℃; Δ is the slope of saturated water vapor pressure and temperature, in kPa / ℃; r is the air density, in kg / m 3 .

[0013] Preferably, S4.3, based on the Beer-Lambert law, the potential vegetation transpiration is derived using the leaf area index and the potential evapotranspiration, and the transpiration ratio is obtained by the ratio of the vegetation transpiration to the potential vegetation transpiration, specifically including: Potential vegetation transpiration obtained by Lambert-Beer law T p , expressed as; (14) In the formula, LAI represents the leaf area index, k Indicates the extinction coefficient, set to 0.5-0.75; Vegetation transpiration ratio T r It is expressed as: (15).

[0014] Preferably, in S5, the shape parameters determined by S2 and S3 are used n , The upper limit of soil pressure head when vegetation is limited by water h 3 , pressure head of wilting coefficient h 4 , vegetation transpiration ratio determined by S4 Tr , the normalized root layer soil moisture is calculated by formula (7) and formula (8): oh root ; Then the root layer soil moisture was normalized using the wilting coefficient and critical soil moisture oh root Converted to root layer soil volume water content, expressed as follows: (16) in, represents the volumetric water content of the root layer soil, represents the critical soil moisture, Represents the wilting coefficient.

[0015] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects: Based on the principle of water balance, it is assumed that the water absorbed by vegetation roots is completely used for transpiration, and the contribution of surface / root soil moisture to soil evaporation / vegetation transpiration is distinguished; at the same time, the concept of critical soil moisture is introduced to fully consider the "saturation" effect of vegetation on root soil moisture, and solve the problem of the complex and changeable coupling relationship between surface evaporation and root soil moisture in different regions; this method has clear physical meaning and can be derived from the Feddes root water absorption model and the van Genuchten-Mualem soil physics model, overcoming the poor universality of traditional methods and the limitations of dependence on surface soil moisture, and is suitable for the inversion of root soil moisture under different underlying surfaces and atmospheric conditions, and is also conducive to the expansion of time scales.

[0016] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0018] Figure 1 It is a flow chart of an embodiment of a method for physical inversion of root layer soil moisture based on vegetation transpiration of the present invention; Figure 2 A comparison diagram of root layer soil moisture distribution points calculated by the present application method and the HYDRUS-1D hydrological model in an embodiment of the present invention; Figure 3 This is a root layer soil moisture result diagram of the US-ARM site of an embodiment of the present invention using an irrigation multi-crop rotation method; Figure 4 This is a result diagram of the root layer soil moisture of alfalfa planted in a rain-fed manner at the US-Bi1 site of an embodiment of the present invention; Figure 5 This is a result diagram of the root layer soil moisture of corn planted in an irrigation manner at the US-Ne1 site of an embodiment of the present invention; Figure 6 This is a result diagram of the root layer soil moisture of the US-Ne2 site in the embodiment of the present invention using the irrigated corn and soybean rotation method; Figure 7This is a result diagram of the root layer soil moisture of the US-Ne3 site in an embodiment of the present invention planted in a rain-fed corn and soybean rotation mode; Figure 8 This is a result diagram of the root layer soil moisture of the rain-fed grassland planted in the US-Var site of an embodiment of the present invention. DETAILED DESCRIPTION

[0019] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0020] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0021] Example A physical inversion method of root layer soil moisture based on vegetation transpiration, such as Figure 1 As shown, the steps are as follows: S1.1. Based on the principle of water balance, the contribution of surface soil moisture and root layer soil moisture in soil evaporation and vegetation transpiration is distinguished, and the response function between vegetation transpiration ratio and root layer soil pressure head is constructed through the Feddes root water absorption model, including: Assuming that the water absorbed by the vegetation roots is completely used for transpiration, the response function of the root layer soil pressure head and vegetation transpiration is derived as follows: (1)

[0022] In the formula, T r is the transpiration ratio; f is the water absorption efficiency of the root system; h is the pressure head; h 3 It is the upper limit of soil pressure head when vegetation is limited by water; h 4 is the pressure head of the wilting coefficient.

[0023] S1.2. The van Genuchten-Mualem soil water retention curve model is introduced to convert soil pressure head into volumetric water content, thereby determining the quantitative relationship between transpiration ratio and root layer soil volumetric water content, which is expressed as follows: (2) In the formula, i r is the residual water content; i s is the saturated water content; α is a shape parameter, which is related to the slope of the soil pore distribution curve and determines the shape of the curve when it is close to the saturated zone and the dry zone; n is another shape parameter, which is related to the uniformity of soil pore distribution. n The larger the value, the more uniform the pore distribution and the smoother the curve; m=1-1 / n; θ(h) is the volumetric water content, h is the soil pressure head.

[0024] S1.3. The quantitative relationship between vegetation transpiration ratio and root layer soil volume water content is simplified by normalizing the root layer soil moisture, thereby eliminating some soil parameters that are difficult to obtain in practice and enhancing the operability and flexibility of the method, as shown below: (3) (4) Normalized root zone soil moisture oh root After the expression, further assume that: (5) Then the normalized root layer soil moisture model related terms can be expressed as: (6) The simple expression of normalized root layer soil moisture is obtained by simplification of the model: (7) (8) in, a , b , c and d is a coefficient, which depends on soil, vegetation and atmospheric demand conditions; ψ This is the error term caused by the simplification process.

[0025] S2. Based on empirical data, construct a pressure head database of the Feddes model under different vegetation conditions, and determine the upper limit of the pressure head when vegetation transpiration changes from energy limitation to water limitation according to different underlying surfaces and meteorological conditions. h 3 , h 3It represents the critical point where transpiration changes from water limitation to energy limitation, taking into account the interaction of multiple factors. These factors include the physical properties of the soil (such as bulk density, porosity, etc.), plant species and their physiological characteristics (such as root distribution pattern, the correlation between transpiration rate and leaf area, etc.), and the combined influence of external environmental conditions (such as temperature, humidity, wind speed, etc.), expressed as: (9) In the formula, T p is the potential transpiration; h 3l Indicates the upper limit of pressure head when the atmospheric water vapor demand is low. T p = T 3l ; h 3h Indicates the upper limit of pressure head when the atmospheric water vapor demand is high. T p = T 3h ; T 3l The value is set to 1 mmd -1 , T 3h The value is set to 5mmd -1 .

[0026] S3, the conversion function of the Rosetta model, infers the soil hydraulic parameters of the van Genuchten-Mualem model through soil texture information, and uses the van Genuchten-Mualem model and Feddes model pressure head database to calculate the wilting coefficient and critical soil moisture under different soil types, vegetation parameters and meteorological factors; S4.1. Using the data of gross primary productivity (GPP) and vapor pressure deficit (VPD) of meteorological factors, the underlying water use efficiency (uWUE) model is used to separate the actual evapotranspiration and obtain the vegetation transpiration, including: Through the uWUE model, the evapotranspiration is indirectly estimated using the biological information GPP and the meteorological factor VPD. The evapotranspiration ET is decomposed into evaporation E and transpiration T within a larger time scale, as shown below: (10) in, uWUE arepresents apparent water use efficiency, GPP represents gross primary productivity of vegetation, and VPD represents saturated vapor pressure deficit; T near ET When the vegetation coverage is high, the evaporation E can be ignored, and the uWUE model is expressed as: (11) in, uWUE p represents the potential water use efficiency, which is determined by statistical methods of time series data, i.e., using years of uWUE a data to estimate the 95% order of magnitude for each location. uWUE a value and use this value as the evaluation uWUE p An alternative reference indicator of potential ceiling.

[0027] The transpiration rate T is obtained by formula (10) and formula (11), which is expressed as: (12).

[0028] S4.2. The Penman-Monteith formula is used to calculate the potential evapotranspiration without water evaporation pressure under given radiation and meteorological conditions, expressed as follows: (13) in, ET p The Penman formula divides evapotranspiration into two parts: the first part is the evaporation caused by water absorbing radiation. ET rad , the second part is aerodynamically driven evaporation ET aero ; R n and G are the surface net radiation and soil heat flux, respectively, in W / m 2 ; r c is the canopy impedance under sufficient water supply conditions, in s / m; r a is the aerodynamic impedance, in s / m; c p is the specific heat of air at constant pressure, in J / (kg℃); c is the psychrometric constant, set to 0.0658 kPa / ℃; Δ is the slope of saturated water vapor pressure and temperature, in kPa / ℃; r is the air density, in kg / m3 .

[0029] S4.3. Based on the Lambert-Beer law, the potential vegetation transpiration is derived using the Leaf Area Index (LAI) and potential evapotranspiration, and the transpiration ratio is obtained by the ratio of vegetation transpiration to potential vegetation transpiration, which includes: Potential vegetation transpiration obtained by Lambert-Beer law T p , expressed as; (14) In the formula, LAI represents the leaf area index, k Indicates the extinction coefficient, set to 0.5-0.75; Vegetation transpiration ratio T r It is expressed as: (15).

[0030] S5, using the normalized root layer soil moisture model constructed by S1, the Feddes model parameters determined by S2, and the van Genuchten-Mualem model parameters determined by S3, the normalized root layer soil moisture is calculated using the transpiration ratio; finally, the normalized root layer soil moisture is converted to the root layer soil volume water content using the wilting coefficient and critical soil moisture. Using the shape parameters determined by S2 and S3 n , The upper limit of soil pressure head when vegetation is limited by water h 3 , pressure head of wilting coefficient h 4 , vegetation transpiration ratio determined by S4 Tr , the normalized root layer soil moisture is calculated by formula (7) and formula (8): oh root ; Then the root layer soil moisture was normalized using the wilting coefficient and critical soil moisture oh root Converted to root layer soil volume water content, expressed as follows: (16) in, represents the volumetric water content of the root layer soil, represents the critical soil moisture, Represents the wilting coefficient.

[0031] This embodiment uses HYDRUS-1D hydrological model simulation data and AmeriFlux (American Flux Network) field experimental data as data sources to invert the root layer soil moisture under different underlying surfaces and atmospheric conditions, including the following steps: Firstly, a root layer soil moisture database under different underlying surfaces and atmospheric conditions was constructed based on the HYDRUS-1D model, and the accuracy of the proposed method was verified using this database to demonstrate the applicability and reliability of this method in retrieving root layer soil moisture in a wide range of areas. Figure 2 As shown, Figure 2 (a) shows the scatter diagram of root layer soil moisture under clear sky conditions in semi-arid region (US-Bi1); (b) shows the scatter diagram of root layer soil moisture under cloudy conditions in semi-arid region (US-Bi1); (c) shows the scatter diagram of root layer soil moisture under clear sky conditions in semi-humid region (US-Ne3); (d) shows the scatter diagram of root layer soil moisture under cloudy conditions in semi-humid region (US-Ne3).

[0032] Overview of simulation settings: (1) Soil type: A total of 30 soil types were considered, which varied in physical, chemical, and biological properties, representing a wide range of soil type diversity; (2) Vegetation type: Five vegetation types (corn, soybean, wheat, alfalfa, and grassland) were included in the simulation, which differed in physiological and ecological characteristics, transpiration intensity, and water use efficiency; (3) Meteorological conditions: The simulation covered 24 meteorological conditions, which were derived from measured atmospheric data in semi-arid (US-Bi1) and semi-humid (US-Ne3) regions, including clear and cloudy weather, as shown in Table 1; (4) Vegetation coverage: 19 vegetation coverage levels were set with an interval of 0.05 to simulate different conditions from bare soil to full vegetation coverage. The simulation design fully considers the effects of different soil types, vegetation types, climate conditions (such as rainfall, temperature, humidity, radiation, etc.), and vegetation coverage on soil moisture dynamics.

[0033]

[0034] In the example of measured data, in order to verify the applicability of the method of the present application under natural variable conditions, the observation data of six US flux network sites (US-ARM, US-Bi1, US-Ne1, US-Ne2, US-Ne3, US-Var) were used to estimate the root layer soil moisture. The results are as follows Figure 3 — Figure 8 As shown in the figure, the inversion results of the proposed method are verified by using the observed soil moisture in the root layer. The weighted average of the observed soil moisture in the root layer at a fixed depth is used as the relative truth value: (17) In the above formula, represents the average root zone soil moisture, i i At different soil depths L i Observed soil moisture content.

[0035] The selection of sites in this example ensures that the evaluation of the proposed method can span different climate regions, from humid to semi-arid, to achieve a comprehensive and multi-level verification of the model performance. Vegetation types include key crop species such as wheat, corn, soybeans and alfalfa, demonstrating their unique value in agricultural research, as shown in Table 2.

[0036]

[0037] At the same time, the Aridity Index (AI) is used to distinguish the degree of drought at different sites: (18) In the above formula, PET represents the potential evaporation, P Represents precipitation.

[0038] In S2, a pressure head database under different vegetation conditions of the Feddes model is constructed, taking into account the differences in the adaptation mechanisms and ecological strategies of plants under different plant types and environmental conditions to water stress. h 3l and T 3l In low atmospheric water vapor evaporation environments, such as low radiation, high humidity, and low wind speed, the pressure on transpiration is relatively small. There is also a critical soil water pressure head threshold h 3l When the soil water potential drops below this threshold, the transpiration rate begins to decline significantly. Under low atmospheric demand conditions, when soil water is abundant, the potential transpiration rate that plants can achieve is defined as T 3l , to reflect environmental conditions with low evaporation potential.

[0039] Relatively speaking, h 3h and T 3h It is closely related to the environment with high evaporation demand. Under conditions of high radiation, low humidity and high wind speed, transpiration may be extremely strong. There is also a critical value of soil water potential at this time. h 3h When the soil water potential is lower than h 3hWhen the evaporation demand is high, the evaporation rate is severely restricted by the soil moisture. Under conditions of high surface evaporation demand, if the soil moisture is sufficient, the potential evaporation rate that plants can achieve is defined as T 3h , to reflect environmental conditions with high evaporation potential. h 3l and h 3h The specific values ​​of depend mainly on the plant type. These values ​​are obtained by consulting previous research data or conducting field experiments, which comprehensively considers various factors such as the plant's growth stage, soil type, soil moisture status, and environmental conditions, as shown in Table 3.

[0040]

[0041] In this example, the parameters of the van Genuchten-Mualem model are derived from the soil texture information of the input site by using the Rosetta model using the conversion function, thereby improving the efficiency and accuracy of parameter acquisition in the method of this application and realizing the inversion of root layer soil moisture at the field scale. h 4 Substituted into the van Genuchten-Mualem model, the critical soil moisture is obtained by h 3 Substitute it into the van Genuchten-Mualem model and calculate it. The determination of the wilting coefficient mainly depends on the soil type and vegetation type.

[0042] In this application method, by introducing the concept of critical soil moisture, the "saturation" effect of vegetation on root layer soil moisture is fully considered to solve the problem of the complex and changeable coupling relationship between vegetation transpiration and root layer soil moisture under different soil characteristics and environmental factors, such as soil texture, soil structure, vegetation type, meteorological factors, etc. i c ) is defined as the soil moisture content at which plant transpiration begins to be limited by soil moisture, and its value is between field capacity ( i fc ) and the wilting coefficient ( i wp ) to assess the adequacy of water supply during vegetation growth. i c and i fc When the water content of the plant is between 0.01 and 0.02, the plant is considered to be in an ideal water environment, and the transpiration rate is close to its potential maximum value, that is, T r= 1. At this stage, plant transpiration is not limited by soil moisture and can fully meet the growth needs of plants. It also means that vegetation transpiration loses sensitivity to changes in root layer soil moisture.

[0043] In this example, necessary data quality control was performed: (1) Given that the basic principle of the method is to reflect the dynamic changes of root layer soil moisture through the stress state of vegetation, this example focuses on the growing season of vegetation and selects dates that have the observation values ​​required for the input of the method (soil properties, vegetation parameters, and meteorological factors) for inversion; (2) Exclude situations where the root layer soil moisture may be in an unstable state, generally before and after precipitation.

[0044] The remaining technical features in the above embodiments can be flexibly selected by those skilled in the art according to actual conditions to meet different specific practical needs. However, it is obvious to those skilled in the art that it is not necessary to adopt these specific details to implement the present invention. In other examples, in order to avoid confusing the present invention, the well-known components, structures or parts are not specifically described, which are all within the technical protection scope defined by the technical solution claimed for protection in the claims of the present invention.

[0045] Modifications and changes made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the scope of protection of the claims attached to the present invention. In the above description, a large number of specific details are set forth in order to provide a thorough understanding of the present invention. However, it is obvious to those of ordinary skill in the art that these specific details are not necessary to practice the present invention. In other examples, in order to avoid confusing the present invention, well-known technologies, such as specific construction details, operating conditions and other technical conditions, are not specifically described.

[0046] The principles and implementation methods of the present invention are described in this article using specific examples. The description of the above embodiments is only used to help understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A physical inversion method for root layer soil moisture based on vegetation transpiration, characterized by: Here are the steps: S1. Based on the principle of water balance, the contribution of surface soil moisture and root layer soil moisture in soil evaporation and vegetation transpiration is distinguished. The response function between vegetation transpiration ratio and root layer soil pressure head is constructed through the Feddes root water absorption model. The van Genuchten-Mualem soil water retention curve model is introduced to convert soil pressure head into volumetric water content, thereby determining the quantitative relationship between transpiration ratio and root layer soil volumetric water content; the quantitative relationship between vegetation transpiration ratio and root layer soil volumetric water content is simplified by normalizing root layer soil moisture; S2. Based on empirical data, construct a pressure head database of the Feddes model under different vegetation conditions, and determine the upper limit of the pressure head when vegetation transpiration changes from energy limitation to water limitation according to different underlying surfaces and meteorological conditions; S3, based on the conversion function of the Rosetta model, the soil hydraulic parameters of the van Genuchten-Mualem model are inferred through soil texture information, and the wilting coefficient and critical soil moisture under different soil types, vegetation parameters and meteorological factors are calculated through the van Genuchten-Mualem model using soil hydraulic parameters and the Feddes model pressure head database; S4. Using the vegetation gross primary productivity and the saturated water vapor pressure deficit data of meteorological factors, the bottom water use efficiency model is used to separate the actual evapotranspiration to obtain vegetation transpiration; The Penman formula is used to calculate the potential evapotranspiration without water evaporation pressure under given radiation and meteorological conditions. Based on the Lambert-Beer law, the leaf area index and potential evapotranspiration are used to derive the potential vegetation transpiration, and the transpiration ratio is obtained by the ratio of vegetation transpiration to potential vegetation transpiration. S5. The normalized root layer soil moisture model constructed by S1, the Feddes model parameters determined by S2 and the van Genuchten-Mualem model parameters determined by S3 are used to calculate the normalized root layer soil moisture using the transpiration ratio; finally, the normalized root layer soil moisture is converted into the root layer soil volume moisture content using the wilting coefficient and critical soil moisture.

2. The root layer soil moisture physical inversion method based on vegetation transpiration according to claim 1 is characterized by: S1.

1. Based on the principle of water balance, the contribution of surface soil moisture and root layer soil moisture in soil evaporation and vegetation transpiration is distinguished, and the response function between vegetation transpiration ratio and root layer soil pressure head is constructed through the Feddes root water absorption model, including: According to the water balance, assuming that the water absorbed by the vegetation roots is completely used for transpiration, the response function of the root layer soil pressure head and vegetation transpiration is derived as follows: （1） In the formula, T r is the transpiration ratio; φ is the water absorption efficiency of the root system; h is the pressure head; h 3 It is the upper limit of soil pressure head when vegetation is limited by water; h 4 is the pressure head of the wilting coefficient.

3. The root layer soil moisture physical inversion method based on vegetation transpiration according to claim 2 is characterized by: S1.

2. The van Genuchten-Mualem soil water retention curve model is introduced to convert soil pressure head into volumetric water content, thereby determining the quantitative relationship between transpiration ratio and root layer soil volumetric water content, which is expressed as follows: （2） In the formula, θ r is the residual water content; θ s is the saturated water content; α is a shape parameter, which is related to the slope of the soil pore distribution curve and determines the shape of the curve when it is close to the saturated zone and the dry zone; n is another shape parameter, which is related to the uniformity of soil pore distribution. n The larger the value, the more uniform the pore distribution and the smoother the curve; m=1-1 / n; θ(h) is the volumetric water content, and h is the soil pressure head.

4. The method for physical inversion of root layer soil moisture based on vegetation transpiration according to claim 3 is characterized by: S1.

3. The quantitative relationship between vegetation transpiration ratio and root layer soil volume water content is simplified by normalizing the root layer soil water content, as follows: （3） （4） Normalized root zone soil moisture ω root After the expression, further assume that: （5） Then the normalized root layer soil moisture model related terms can be expressed as: （6） The simple expression of normalized root layer soil moisture is obtained by simplification of the model: （7） （8） in, a , b , c and d is a coefficient, which depends on soil, vegetation and atmospheric demand conditions; ψ This is the error term caused by the simplification process.

5. The method for physical inversion of root layer soil moisture based on vegetation transpiration according to claim 4 is characterized in that: In S2, the upper limit of soil pressure head when vegetation is limited by water h 3 It is expressed as: （9） In the formula, T p is the potential transpiration; h 3l Indicates the upper limit of pressure head when the atmospheric water vapor demand is low. T p = T 3l ; h 3h Indicates the upper limit of pressure head when the atmospheric water vapor demand is high. T p = T 3h ; T 3l The value is set to 1 mmd -1 , T 3h The value is set to 5 mmd -1 .

6. The method for physical inversion of root layer soil moisture based on vegetation transpiration according to claim 5 is characterized by: S4.

1. Using the vegetation gross primary productivity and the saturated water vapor pressure deficit data of meteorological factors, the bottom water use efficiency model is used to separate the actual evapotranspiration to obtain vegetation transpiration, including: Through the uWUE model, the evapotranspiration is indirectly estimated using the biological information GPP and the meteorological factor VPD. The evapotranspiration ET is decomposed into evaporation E and transpiration T within a larger time scale, as shown below: （10） in, uWUE a represents apparent water use efficiency, GPP represents gross primary productivity of vegetation, and VPD represents saturated vapor pressure deficit; T near ET When the vegetation coverage is high, the evaporation E can be ignored, and the uWUE model is expressed as: （11） in, uWUE p represents the potential water use efficiency, determined by statistical methods of time series data; The transpiration rate T is obtained by formula (10) and formula (11), which is expressed as: （12）。 7. The method for physical inversion of root layer soil moisture based on vegetation transpiration according to claim 6 is characterized by: S4.

2. The Penman formula is used to calculate the potential evapotranspiration without water evaporation pressure under given radiation and meteorological conditions, expressed as follows: （13） in, ET p The Penman formula divides evapotranspiration into two parts: the first part is the evaporation caused by water absorbing radiation. ET rad , the second part is aerodynamically driven evaporation ET aero ; R n and G are the net surface radiation and soil heat flux, respectively, in W / m 2 ; r c is the canopy impedance under sufficient water supply conditions, in s / m; r a is the aerodynamic impedance, in s / m; c p is the specific heat of air at constant pressure, in J / (kg℃); γ is the psychrometric constant, set to 0.0658 kPa / ℃; Δ is the slope of saturated water vapor pressure and temperature, in kPa / ℃; ρ is the air density, in kg / m 3 .

8. The method for physical inversion of root layer soil moisture based on vegetation transpiration according to claim 7 is characterized in that: S4.

3. Based on the Beer-Lambert law, the potential vegetation transpiration is derived using the leaf area index and potential evapotranspiration, and the transpiration ratio is obtained by the ratio of vegetation transpiration to potential vegetation transpiration, which includes: Potential vegetation transpiration obtained by Lambert-Beer law T p , expressed as; （14） In the formula, LAI represents the leaf area index, k Indicates the extinction coefficient, set to 0.5-0.75; Vegetation transpiration ratio T r It is expressed as: （15）。 9. The root layer soil moisture physical inversion method based on vegetation transpiration according to claim 8 is characterized by: In S5, the shape parameters determined by S2 and S3 are used n , The upper limit of soil pressure head when vegetation is limited by water h 3 , pressure head of wilting coefficient h 4 , vegetation transpiration ratio determined by S4 Tr , the normalized root layer soil moisture is calculated by formula (7) and formula (8): ω root ; Then the root layer soil moisture was normalized using the wilting coefficient and critical soil moisture ω root Converted to root layer soil volume water content, expressed as follows: （16） in, represents the volumetric water content of the root layer soil, represents the critical soil moisture, Represents the wilting coefficient.

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