Estimation method for photosynthetically active radiation absorption ratio of emergent aquatic vegetation

Abstract

The invention discloses an emergent aquatic vegetation photosynthetically active radiation absorption ratio estimation method, and belongs to the technical field of ecological remote sensing and environment monitoring. In order to solve the problems that the vertical layered structure of the emergent aquatic plant is complex and the photosynthetically active radiation absorption ratio is difficult to accurately estimate by underwater light propagation characteristics, the method mainly adopts a technical means of combining layered modeling and a two-flow approximate radiation transmission model, and integrates multiple scattering calculation, interface refraction and reflection correction and stalk contribution compensation. And accurate estimation of the FAPAR value of each structural layer of the emergent aquatic vegetation is realized. The method can improve the quantitative monitoring precision of the photosynthetic activity and the ecological function of the emergent aquatic vegetation, and is suitable for remote sensing application scenes of typical aquatic vegetation areas such as wetlands and lakes.

Description

Technical Field

[0001] This invention relates to a method for estimating the photosynthetically active radiation absorption ratio of emergent vegetation, belonging to the field of ecological remote sensing and environmental monitoring technology. Background Technology

[0002] Wetland vegetation is a crucial carbon sink in wetland systems and an indicator of wetland ecological conditions. Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) refers to the proportion of photosynthetically active radiation absorbed by vegetation in the 400-700 nm wavelength range to the total incident photosynthetically active radiation. As a direct indicator of vegetation photosynthesis, FAPAR serves as a "bridge" connecting absorbed photosynthetically active radiation with vegetation primary productivity, and has been identified by the United Nations Global Climate Observatory as one of the 50 key parameters reflecting global climate change. Therefore, accurately and quantitatively obtaining wetland vegetation FAPAR is essential for a deeper understanding of wetland vegetation's photosynthetic and carbon sequestration characteristics, and is a crucial prerequisite for accurately estimating wetland vegetation carbon sinks. Remote sensing is the most direct way to obtain large-scale FAPAR. However, existing FAPAR remote sensing inversion methods are mainly developed for terrestrial vegetation and cannot be applied to emergent vegetation in wet soil or water bodies as a background. While some studies have made progress in water radiative transfer modeling and vegetation reflectance simulation, none have specifically modeled and estimated the FAPAR of emergent vegetation in detail, and there is a significant deficiency in considering the radiative transfer process in the context of water bodies and wet soil. Summary of the Invention

[0003] The purpose of this invention is to overcome the problem of insufficient accuracy in estimating the photosynthetically active radiation absorption ratio (FAPAR) of emergent vegetation in the prior art, and to propose a method for estimating FAPAR of emergent vegetation under the conditions of water body or wet soil. This method can achieve fine modeling of radiation transfer process and high-precision estimation of FAPAR, and provide reliable parameter support for remote sensing assessment of light energy utilization efficiency and carbon sequestration capacity of wetland vegetation.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0005] A method for estimating the photosynthetically active radiation absorption ratio of emergent vegetation includes the following steps:

[0006] 1) Obtain the vertical stratification information of emergent vegetation, and determine the radiation absorption coefficient and scattering coefficient of each structural layer based on the vegetation composition, optical properties and distribution parameters of each layer.

[0007] 2) Based on the absorption coefficient and scattering coefficient, a two-flow approximate radiative transfer model for emergent vegetation is constructed, and the upward and downward irradiance in each structural layer is calculated.

[0008] 3) Based on the boundary conditions and interlayer interface connection conditions, and combined with the refraction and reflection characteristics between layers, determine the boundary irradiance parameters required for light radiation propagation in each structural layer.

[0009] 4) Using the multiple scattering theory and the scattering transfer matrix method, solve the radiative transfer model in step 2) to obtain the scattering irradiance distribution in each structural layer;

[0010] 5) Based on the total irradiance distribution and absorption coefficient of each structural layer, the absorbed photosynthetically active radiation flux in each structural layer is calculated according to the photosynthetically active radiation absorption ratio (FAPAR) algorithm, and a preliminary FAPAR estimate is obtained accordingly.

[0011] 6) Based on the spatial distribution density and optical characteristics of stems in emergent vegetation, the preliminary FAPAR estimate is corrected to obtain the FAPAR value that includes the contribution of stems;

[0012] 7) Based on the FAPAR value obtained in step 6), the FAPAR value of the underwater vegetation is corrected according to the refraction law of light propagation in water and Fresnel's law of reflection, so as to obtain the final estimated FAPAR value of emergent vegetation.

[0013] Further, before step 1), three-dimensional lidar point cloud data or multispectral remote sensing images of emergent vegetation areas are acquired to extract the spatial thickness and vegetation parameters of each structural layer.

[0014] Furthermore, the structural layers described in step 1) include a canopy layer, a water surface layer, an underwater vegetation layer, and a bottom water layer, wherein the absorption coefficient and scattering coefficient of each layer are calculated based on the vegetation type, optical characteristics, and leaf area index of that layer, respectively.

[0015] Furthermore, the two-stream approximate radiative transfer model described in step 2) includes a set of coupled differential equations that describe the changes in upward and downward luminous flux, respectively. The two-stream approximate radiative transfer model models the attenuation process of direct radiation and the propagation process of scattered radiation in each structural layer to simulate the overall irradiance distribution.

[0016] Furthermore, the boundary conditions described in step 3) include the incident angle and intensity of the incident light at the top layer, the reflection coefficient of the lower boundary, and the interface connection conditions include the refractive index ratio and Fresnel reflection coefficient between adjacent layers.

[0017] Furthermore, the scattering transfer matrix method described in step 4) includes establishing a local scattering transfer matrix for each structural layer and constructing the overall system transfer matrix by layer-by-layer multiplication to achieve a closed-form solution for irradiance.

[0018] Further, the calculation of the FAPAR value in step 5) includes dividing the photosynthetically active radiation absorbed by the plant tissue per unit area by the total photosynthetically active radiation incident per unit area, and then summing the results for leaves and non-leaf organs respectively.

[0019] Furthermore, the optical properties of the stalk described in step 6) include its reflectivity, transmittance, and absorptivity for different wavelengths of light. During the correction process, the initial FAPAR is compensated for gain according to the volume distribution coefficient of the stalk.

[0020] Furthermore, the FAPAR value correction for the underwater portion in step 7) includes correcting the change in the incident angle of light according to Snell's law, and using Fresnel's formula to correct the reflection loss and transmission efficiency at the water surface respectively.

[0021] Furthermore, a time series analysis was performed on the final FAPAR estimate obtained in step 7) to track the changing trend of FAPAR during the growth cycle of emergent vegetation.

[0022] The beneficial effects achieved by this invention are as follows:

[0023] 1. This invention models the growth environment characteristics of emergent vegetation, distinguishes between two background conditions: water bodies and wet soil, and has strong adaptability and specificity.

[0024] 2. Based on the layered structure of emergent vegetation, this invention constructs radiation transfer models for the above-water and underwater regions, respectively, which accurately expresses the absorption characteristics of the canopy and underwater vegetation, thus improving the physical rationality of the model.

[0025] 3. This invention adopts the two-stream approximate radiative transfer equation, introduces multiple scattering theory and matrix solution method, and can systematically simulate multiple radiative interaction processes, thereby enhancing the model's ability to express actual light field changes.

[0026] 4. This invention introduces dynamic boundary conditions and air-water interface refraction and reflection mechanisms to correct the underwater light propagation path and improve the accuracy of underwater FAPAR estimation.

[0027] 5. This invention parametrically expresses the radiative transfer behavior of multiple components such as vegetation, water bodies, and soil, including absorption coefficients, bulk density, and leaf tilt angle distribution, which is beneficial for expanding the application of the model under multi-source remote sensing data.

[0028] 6. This invention, through a hierarchical integration method, can quantitatively separate the relative contributions of above-water vegetation, below-water vegetation, and water body to PAR absorption, providing refined supporting data for wetland carbon cycle research.

[0029] 7. This invention has good scientific value and engineering application prospects, and is applicable to multiple scenarios such as FAPAR remote sensing inversion of emergent vegetation in wetland ecosystems, ecological model input, and carbon sink estimation. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the layered structure of emergent vegetation in the embodiment.

[0031] Figure 2 This is a schematic diagram of the radiative transfer process in the embodiment.

[0032] Figure 3 This is a schematic diagram of the absorption of total incident PAR in the embodiment. Detailed Implementation

[0033] To make the various technical features, advantages, or effects of the present invention more apparent and understandable, detailed descriptions are provided below through embodiments.

[0034] This invention provides a method for estimating the photosynthetically active radiation absorption ratio of emergent vegetation, comprising the following steps:

[0035] Step 1: Obtain the vertical stratification information of emergent vegetation, and determine the radiation absorption coefficient and scattering coefficient of each structural layer based on the vegetation composition, optical characteristics and distribution parameters of each layer.

[0036] Emergent vegetation exhibits a distinct vertical stratification structure, primarily comprising a canopy layer, a water surface layer, an underwater leaf layer, and a bottom layer. To characterize the radiative transfer characteristics of each structural layer, basic inputs such as leaf area density distribution, leaf tilt angle distribution, and leaf optical parameters were first obtained.

[0037] 1. Leaf Area Volume Density:

[0038]

[0039] Where u(z) is the leaf area density per unit volume at vertical height z, and h is the total height of the vegetation layer.

[0040] 2. Leaf Projection Function:

[0041]

[0042] Wherein, g(θ)L ) is the leaf tilt angle distribution function, and ψ is the angle between the leaf surface normal and the direction of the light ray.

[0043] 3. Leaf single-scattering albedo:

[0044] ω Leaf =ρ Leaf +τ Leaf

[0045] Where, ρ Leaf τ is the reflectivity of the blade. Leaf is the blade transmittance, used to characterize the optical properties of the blade.

[0046] 4. Extinction coefficient σ e (Extinction Coefficient) and scattering coefficient σ s The (ScatteringCoefficient) is defined as follows:

[0047]

[0048] Where, σ e The unit is m -1 , σ s The unit is m -1 μ = cosθ is the cosine of the zenith angle of the ray.

[0049] Using the above parameters, an absorption and scattering optical property model can be constructed for each layer in the emergent vegetation stratification structure, providing input for subsequent radiative transfer calculations.

[0050] Step 2: Based on the absorption coefficient and scattering coefficient, construct a two-flow approximate radiative transfer model for emergent vegetation, and calculate the upward and downward irradiance in each structural layer.

[0051] This step uses a two-flow approximation model to simplify the original three-dimensional radiative transfer problem into two irradiance flux variables, one upward and one downward, in the vertical direction. The irradiance is obtained by integrating the radiance L(r,Ω) in the hemispherical space.

[0052] 1. The fundamental equation for radiative transfer is as follows:

[0053]

[0054] Where L(r,Ω) is the radiance along the direction Ω at position r, with units of W·m. -2 ·sr -1; r is the position vector, Ω is the unit vector of the photon's direction of motion, da is the path length element along the direction Ω; p(Ω,Ω′) is the scattering phase function, which describes the probability of a photon being scattered from the direction Ω′ to the direction Ω.

[0055] 2. Downward irradiance E ↓ (z) and upward irradiance E ↑ (z) is defined as:

[0056]

[0057] Where μ = cosθ is the cosine of the zenith angle of the light ray, and θ = 0° means vertically downward.

[0058] 3. Diffuse Downward Radiative Transfer Equation:

[0059]

[0060] 4. Diffuse Upward Radiative Transfer Equation:

[0061]

[0062] in, The absorption coefficient is... The scattering coefficient is... This is the normalization factor.

[0063] Using the above equations, E can be adjusted within each structural layer. ↓ (z) and E ↑ (z) Perform analytical or numerical solutions to lay the foundation for further estimation of FAPAR.

[0064] Equations 7 and 8 above are obtained by integrating the following downward irradiance equation (Equation 9) and upward irradiance equation (Equation 10) in the downward / upward hemisphere.

[0065]

[0066] Where, p ↑ ,p ↓ These represent the proportions of light scattered to the upward and downward hemispheres, respectively.

[0067] III. Based on boundary conditions and interlayer interface connection conditions, and combined with the refraction and reflection characteristics between layers, determine the boundary irradiance parameters required for light radiation propagation in each structural layer.

[0068] Figure 1This is a schematic diagram of the layered structure of emergent vegetation. To simulate the vertical propagation paths of light radiation and the boundary energy flow relationship of each layer in the vertical structure of emergent vegetation, it is necessary to establish radiation transfer equations for the above-water and underwater parts separately, and set boundary conditions for the top, interface, and bottom.

[0069] 1. Radiative transfer equation for the above-water portion (z>0)

[0070] The above-water portion consists of an emergent vegetation canopy, and its vertical propagation can be approximated using a two-flow model, establishing the following differential form:

[0071]

[0072] Where the subscript 'a' represents the area above the water surface, and 'k' represents the area above the water surface. abs,aveg k is the absorption coefficient of above-water vegetation. sca,aveg The scattering coefficient of aquatic vegetation is given in m. -1 ; The albedo is the single-scattering reflectance of aquatic vegetation.

[0073] 2. Radiative transfer equations for the underwater portion (z<0)

[0074] The underwater component comprises a pure aquatic medium and submerged plants or root systems. Ignoring multiple scattering coupling between water and vegetation, the following approximate two-flow transport equation is constructed:

[0075]

[0076]

[0077] Among them, the total absorption coefficient k of the underwater part abs,total and total scattering coefficient k b,total These are respectively represented as the weighted superposition of water body and underwater vegetation:

[0078] k abs,btotal (z)=k abs,w +k abs,bveg ·B(z) (15)

[0079] k sca,btotal (z)=k sca,w +k sca,bveg ·B(z) (16)

[0080]

[0081] Where the subscript 'b' indicates the underwater area (below water); k abs,w and k sca,wThese are the absorption coefficient and scattering coefficient of pure water, respectively, in m. -1 ;k abs,bveg and k sca,bveg These are the absorption coefficient and scattering coefficient of underwater vegetation, respectively, in m. -1 B(z) represents the leaf area density per unit volume of underwater vegetation at depth z, in m³ / s. 2 / m 3 (e.g., 1m) 2 Leaf area / 1m 3 (Water body).

[0082] 3. Upper boundary condition (z = h)

[0083] Let the incident irradiance received at the top of the canopy be defined as an externally given constant E0:

[0084]

[0085] Where h is the height of the top of the vegetation canopy, in meters (m); E0 is the incident irradiance, in W·m³. -2 .

[0086] 4. Air-water interface bonding condition (z=0)

[0087] At the water-air interface, the upward and downward irradiances satisfy the Fresnel reflection / transmission model relationship:

[0088] Lateral air irradiance (z=0) + ):

[0089]

[0090] Water-side down irradiance (z=0) - ):

[0091]

[0092] Among them, 0 + This indicates the position immediately adjacent to the interface above the water surface (on the air side) (z approaches 0 from the positive direction); 0 - This indicates the position immediately adjacent to the interface below the water surface (water side) (z approaches 0 from the negative direction); r 12 τ represents the reflectivity of air to water, and the calculation formula is based on the Fresnel equation; 12 τ represents the transmittance from air to water. 12 =1-r 12 (Energy conservation, absorption ignored); r 21 τ represents the reflectivity of water to air, and its calculation formula is based on the Fresnel equation; 21 τ represents the transmittance of water to air. 21 =1-r21 (Energy conservation, absorption ignored).

[0093] 5. Lower boundary condition (z = -d)

[0094] Different reflection models are set depending on whether the water is shallow or deep:

[0095] Deep water conditions:

[0096] Assuming the water is deep enough that there is no reflection at the bottom, then:

[0097]

[0098] Or, assuming the water is deep enough, consider the bottom reflectance as r. deep ,but:

[0099]

[0100] Shallow water: The bottom reflectance is ρ b ,but:

[0101]

[0102] Where d represents the water depth, in meters (m).

[0103] If the background is wet soil, the reflectance of the wet soil background can be obtained using the following empirical model:

[0104] ρ b =ρ d ·exp(-a·ε) (24)

[0105] Where, ρ d ε is the dry soil reflectance, a is the soil absorption coefficient, and ε is the equivalent soil water thickness parameter.

[0106] IV. Establish a multiple scattering radiation transfer model in the emergent vegetation structure layer, and use the matrix solution method to calculate the upward and downward scattering irradiance distribution of each layer.

[0107] Figure 2 This is a schematic diagram of the radiative transfer process. To accurately estimate the impact of multiple scattering on light energy distribution within the vegetation structure, it is necessary to establish the radiative coupling relationship between different layers by combining the actual leaf tilt characteristics and the attenuation law of direct sunlight within the layer, and then use the matrix method to solve for the scattered irradiance distribution. Details are as follows:

[0108] 1. Establishment of the extinction coefficient of direct light

[0109] The leaf tilt angle distribution characteristics of emergent vegetation directly affect its ability to block direct sunlight. By introducing the projection function G(θ) to characterize the interception ability of leaf tilt angle against sunlight, the vertical extinction coefficient of direct sunlight at depth z is calculated as follows:

[0110]

[0111] Where, θ s G(θ) is the solar zenith angle (the angle between the incident light and the perpendicular direction); s The leaf tilt angle is distributed in the direction of the sun at θ. s The projection function; u(z) is the leaf area volume density (m³ / s). 2 / m 3 ); Ω is the aggregation index (corrected non-random distribution), which can be determined by field measurement of the leaf angle distribution of vegetation.

[0112] To accommodate the leaf characteristics of emergent wetland vegetation, the G function form of the ellipsoidal leaf inclination angle distribution is as follows:

[0113]

[0114] in, χ is the average leaf tilt angle, and its value can be determined by fitting the angle distribution data of vegetation leaves measured in the field.

[0115] 2. Separation and processing of direct and scattered light

[0116] The entire irradiance field can be decomposed into a direct component E. dir and scattering component E diff Two parts:

[0117] E(z)=E dir (z)+E diff (z) (27)

[0118] Among them, the attenuation of direct sunlight in vegetation follows the Beer-Lambert law:

[0119]

[0120] Where, k(θ) s ,z′) is the extinction coefficient along the path, and the calculation method is shown in Formula 25.

[0121] 3. Multi-layer discretization of vegetation profiles

[0122] To determine the scattered irradiance in different structural layers, a vertically layered modeling method is employed. The vegetation is divided into n layers from the top z = h to the bottom (or water surface), with the thickness of the i-th layer being Δz. i Each layer can be considered to have uniform parameters, and the downward scattered irradiance is denoted as... Upward scattered irradiance is denoted as

[0123] This method can dynamically adapt to different vertical structural features:

[0124] For regions with complex structures or abrupt gradient changes, they can be further subdivided into more thin layers;

[0125] For uniform regions, the number of layers can be reduced to simplify calculations.

[0126] 4. Matrix Modeling of Scattered Radiative Transfer

[0127] The matrix transfer equations with 2n variables are constructed as follows:

[0128] M·E=S (29)

[0129] Where, vector denoted by , representing the upward and downward scattered irradiance in all layers; M is a 2n×2n coefficient matrix, representing the energy coupling and transmission / reflection relationship between layers; S is the source term vector, representing the new source irradiance generated by the scattering of direct light within each layer.

[0130] The coupling relationship between any two adjacent layers i and i+1 is expressed as follows:

[0131]

[0132] Where, τ i ρ is the transmittance of the i-th layer; i Let be the reflectivity of the i-th layer; and These are the source terms that are reflected upwards and transmitted downwards, respectively, by the scattering of direct light.

[0133] 4. Estimation methods for the scattering source term

[0134] The direct light scattering source terms of each layer can be estimated using the following formula:

[0135]

[0136] Where, σ sca,i f is the scattering coefficient of the i-th layer; d,i E represents the proportion of downward-scattered light in the i-th layer, typically estimated by the scattering phase function or the blade angle distribution function; dir,i Δz represents the average direct irradiance of the i-th layer; i Let be the thickness of the i-th layer.

[0137] V. Based on the total irradiance distribution and absorption coefficient of each structural layer, the absorbed photosynthetically active radiation flux in each structural layer is calculated according to the FAPAR algorithm, and a preliminary FAPAR estimate is obtained accordingly.

[0138] Figure 3This diagram illustrates the absorption of total incident PAR. To assess the actual absorption capacity of emergent vegetation for photosynthetically active radiation (PAR, 400–700 nm) in different vertical structural layers, the radiation absorption at each layer above and below water needs to be calculated separately, and the overall system FAPAR needs to be synthesized. This includes the following:

[0139] 1. Definition of Overall FAPAR

[0140] The total photosynthetically active radiation (FAPAR) of emergent vegetation systems is defined as the proportion of incident photosynthetically active radiation absorbed, and the calculation formula is:

[0141]

[0142] Furthermore, in actual calculations, the PAR band is integrated to obtain:

[0143]

[0144] Where E0(λ) is the unit incident irradiance at wavelength λ, FAPAR(λ) is the light energy absorptivity at wavelength λ, and PAR... absorbed PAR is the photosynthetically active radiation absorbed by vegetation. incident The incident photosynthetically active radiation.

[0145] 2. FAPAR calculation for each layer above water (from the canopy to the water surface)

[0146] From the top of the canopy to the water surface (the part above water):

[0147] The region from the top of the canopy (z = h) to the water surface (z = 0) is the aquatic vegetation region. The FAPAR calculation formula for the i-th layer in this region is:

[0148]

[0149] Among them, E 0,PAR The incident PAR irradiance at the top of the canopy (wavelength integral); k abs,aveg (z) represents the absorption coefficient of aquatic vegetation (m -1 It mainly depends on the chlorophyll content and leaf structure. abs,aveg (z)=u(z)·k abs,leaf ; and These represent the downward and upward irradiance (PAR band integral) at position z above water, respectively.

[0150] 3. FAPAR calculation for each layer in the underwater section (from water surface to bottom)

[0151] The area from the water surface z = 0 to the bottom z = -d is the underwater region, and the effects of water absorption and underwater vegetation absorption need to be comprehensively considered. The total FAPAR of the i-th underwater layer is:

[0152]

[0153] If it is necessary to extract the absorption contribution of underwater vegetation itself, the FAPAR calculation formula for this layer of underwater vegetation is as follows:

[0154]

[0155] Where, k abs,w The absorption coefficient of pure water (m) -1 );k abs,bveg The absorption coefficient of underwater vegetation leaf material (m -1 Similar to terrestrial vegetation, the density of underwater vegetation is determined by its physiological characteristics, but is influenced by the aquatic environment, and its value is usually lower than that of terrestrial vegetation; B(z) is the bulk density of underwater vegetation; k abs,bveg B(z) is the absorption coefficient of underwater vegetation at location z; and These represent the downward and upward irradiance (PAR band integral) at position z in the underwater portion, respectively.

[0156] 3. Integration of above-water and underwater FAPAR

[0157] By summing up the FAPAR for each layer above and below water, the following total indicators can be obtained:

[0158] FAPAR (Floating Aquatic Vegetation) section:

[0159] FAPAR above =∑ i∈above FAPAR i,above (38) Total underwater FAPAR (including water absorption and underwater vegetation absorption):

[0160] FAPAR below =∑ i∈below FAPAR i,below (39) The overall FAPAR of the system is:

[0161] FAPAR total =FAPAR above +FAPAR below (40) If we need to analyze the contribution of vegetation itself (excluding water absorption) to light energy absorption separately, then the total vegetation FAPAR should be:

[0162] FAPAR total,veg =FAPAR above +∑ i∈below FAPAR i,bveg (41)

[0163] VI. Based on the spatial distribution density and optical characteristics of stems in emergent vegetation, the preliminary FAPAR estimate is corrected to obtain a FAPAR value that includes the contribution of stems.

[0164] To more accurately assess the true absorption capacity of emergent vegetation for photosynthetically active radiation (PAR), this step further subdivides the existing stratified FAPAR calculation by vegetation organ types (leaves and stems), quantifying the contribution of each organ to light energy absorption in different regions (upper canopy and lower submerged area), thus obtaining a more physiologically and ecologically meaningful FAPAR vegetation index. veg .

[0165] This calculation method includes three parts: leaf absorption in the above-water area, stem absorption, and overall absorption by underwater vegetation. The calculation formula is as follows:

[0166]

[0167] Among them, E 0,PAR For incident photosynthetically active radiation (PAR) at the canopy apex, u leaf (z) represents the leaf area volume density at position z above water (m³). 2 / m 3 ), u stem (z) represents the stem area volume density (m³) at position z above water. 2 / m 3 ), k abs,leaf ,k abs,stem The unit absorption coefficients (m) of leaf and stem materials are respectively. -1 ); E total (z) represents the total irradiance at position z, indicating the total radiant energy after integrating all waveband directions (up and down, direct and scattered); B(z) represents the submerged plant volume density at position z in the underwater region, used to calculate the absorption of underwater leaves. In the third item, it is assumed that the absorption of underwater vegetation mainly comes from leaf tissue. If the proportion of stems in the underwater part is high, they can also be separated and modeled.

[0168] By integrating the above components, the relative contributions of different organs in the vegetation system to FAPAR can be quantitatively distinguished, providing a theoretical basis for subsequent ecological analysis, photosynthetic modeling and remote sensing inversion.

[0169] 7. Based on the laws of refraction of light in water and Fresnel's law of reflection, the FAPAR value of the underwater vegetation is corrected to obtain the final estimated FAPAR value of emergent vegetation.

[0170] In emergent vegetation systems, the air-water interface significantly affects the refraction and reflection of incident light, thus influencing underwater radiation distribution and consequently altering the accuracy of FAPAR estimation for the underwater vegetation layer. This step introduces Snell's law, Fresnel's reflection formula, and a path correction mechanism to achieve a precise simulation of the propagation path and extinction process of direct light in water.

[0171] 1. Calculation of water surface refraction angle

[0172] When incident light enters a body of water from air, it is refracted. The angle of refraction is θ. w This can be determined based on Snell's Law:

[0173]

[0174] Where, θ a θ is the angle of incidence in the air (i.e., the solar zenith angle). w The angle of refraction in water; n w This is the refractive index of water. It is about 1.33 for freshwater and about 1.34 for seawater with higher salinity. It can be corrected for the temperature and salinity of the specific water body.

[0175] 2. Direct light path and extinction correction in water

[0176] Because light rays refract in water along the direction θ w The propagation depth Δz in the vertical direction corresponds to the actual path length, which needs to be calculated as Δz / cosθ. w Correction. Therefore, the expression for the attenuation of direct light irradiance in water is:

[0177]

[0178] Among them, E dir,air (0 + E represents the direct irradiance after the transmitted light enters the water surface. dir,air (0 + )=τ 12 ·E dir,air (0 - ), τ 12 k represents the interfacial transmittance. water (z′)=k abs,w +k sca,w θ is the total extinction coefficient of water, including absorption and scattering; w The angle of refraction in water is calculated using formula 43.

[0179] When calculating the G-function and extinction coefficient k(θ) involving underwater vegetation, the angle parameter should also be replaced with the corrected underwater refraction angle θ. w .

[0180] 3. Calculation of Interface Reflection Loss (Fresnel Formula)

[0181] Some light rays are reflected at the air-water interface. This reflection loss is represented by the Fresnel reflection coefficient r(θ):

[0182]

[0183] Where θ is the angle of incidence in the air, θ t It is the angle of refraction.

[0184] When the incident light is perpendicular (θ = 0), the formula can be simplified to:

[0185]

[0186] Here, n1 and n2 are the refractive indices of air and water, respectively.

[0187] The aforementioned reflection coefficient is used to correct the actual irradiance input at the water surface, and together with the path correction mechanism, it improves the accuracy of the underwater FAPAR model.

[0188] The specific examples provided by this invention are as follows:

[0189] This example uses emergent vegetation reeds as the subject, and estimates their FAPAR value under specific environmental conditions based on the method of this invention. The specific process is as follows:

[0190] I. Basic Settings

[0191] 1. Vegetation type and structure distribution:

[0192] Vegetation type: reeds;

[0193] Canopy height h = 2.0 m;

[0194] The underwater vegetation depth is d = 0.5m.

[0195] 2. Ambient lighting conditions:

[0196] Incident photosynthetically active radiation (PAR) E0 = 800 W·m -2 (Integral value of 400-700nm);

[0197] The incident angle θ = 30°.

[0198] 3. Water body and bottom background parameters:

[0199] Water type: Clear freshwater

[0200] Water absorption coefficient k abs,w =0.1m -1 ;

[0201] Water scattering coefficient ksca,w =0.05m -1 ;

[0202] The bottom of the water is moist soil, and the dry soil reflectance ρ dry =0.25, soil moisture parameter a =0.3, porosity ε =0.8;

[0203] The reflectivity ρ of wet soil is calculated using formula 24. soil =0.15.

[0204] II. Setting of Vegetation Optical Parameters:

[0205] 1. Above-water section (0-2m):

[0206] Leaf area and volume density u a (z)=0.5m 2 / m 3 Evenly distributed;

[0207] The average leaf tilt angle χ = 60°, and follows an ellipsoidal distribution;

[0208] Leaf average absorption coefficient k abs,aveg =0.8, scattering coefficient k sca,aveg =0.2;

[0209] The G function is calculated using formula 26.

[0210] 2. Underwater section (-0.5-0m):

[0211] Vegetation area density B(z) = 0.3m 2 / m 3 Evenly distributed;

[0212] underwater blade absorption coefficient k abs,bveg =0.7, slightly lower than the above-water portion considering the influence of water bodies.

[0213] III. Calculation process:

[0214] 1. Hierarchical discretization modeling:

[0215] The water canopy is divided into 4 layers, each with a thickness Δz = 0.5m;

[0216] The underwater vegetation layer is divided into one layer with a thickness of Δz = 0.5m.

[0217] 2. Radiative transfer calculation:

[0218] 2-1. Direct light attenuation:

[0219] The attenuation of direct sunlight from the water canopy is described by Equation 28:

[0220]

[0221] Where, k a (θ) is calculated from the leaf angle distribution (Equation 25-26).

[0222] For the underwater portion, the propagation path correction after incident light refraction is considered. The angle of refraction is obtained from Snell's law (Equation 43):

[0223] θ w =arcsin(sin30° / 1.33)≈22°

[0224] The propagation path increases by 1 / cosθ w The attenuation of direct light underwater is corrected according to formula (44).

[0225] 2-2. Scattered light transmission:

[0226] A 5×5 scattering transmission matrix M containing 4 layers above water and 1 layer below water is constructed. The scattering components of light propagating vertically in each layer are calculated using the multiple scattering theory and matrix method (Equations 29-32).

[0227] 2-3. Boundary condition settings:

[0228] Top-level irradiance

[0229] Underwater reflection boundary conditions:

[0230] 3. Stratified FAPAR calculation:

[0231] 3-1. FAPAR for each level above water is calculated according to Formula 35:

[0232]

[0233] Example: FAPAR calculation results for the top layer (1.5-2.0m) 1,above =0.12.

[0234] The underwater vegetation layer is calculated according to Formula 37:

[0235]

[0236] The water absorption portion is calculated according to Equation 36:

[0237]

[0238] 4. Calculation of total FAPAR (Equation 40-41):

[0239] Total vegetation uptake contribution:

[0240] FAPAR veg =∑FAPAR a,i+FAPAR b =0.12 + 0.10 + 0.09 + 0.08 + 0.08 = 0.47

[0241] Total FAPAR including water absorption:

[0242] FAPAR total =FAPAR veg +FAPAR water =0.47 + 0.05 = 0.52

[0243] The total FAPAR value was 0.52, of which 0.47 was photosynthetically active radiation absorbed by vegetation tissue, accounting for approximately 90.4% of the total absorption; and 0.05 was absorbed by the aquatic medium, accounting for approximately 9.6%. Furthermore, of the vegetation absorption portion, the contribution from above-water vegetation (canopy) was 0.33, accounting for approximately 70% of the total vegetation absorption.

[0244] This embodiment demonstrates the complete process of FAPAR estimation in a reed-emerging aquatic vegetation scenario, verifying the ability of the model of this invention to simulate the transmission process of direct and scattered light in different structural layers. Through layered calculations, the PAR absorption contributions of above-water vegetation, underwater vegetation, and water body are quantified separately, providing accurate parameters for ecological monitoring and energy budget analysis.

[0245] In practical applications, remote sensing, lidar, or field measurement data should be combined to obtain more accurate vegetation structure parameters (such as leaf area density and tilt distribution) and environmental optical parameters (such as water absorption coefficient and sediment reflectivity) to further improve the accuracy and adaptability of FAPAR estimation.

[0246] Although the present invention has been disclosed above with reference to embodiments, it is not intended to limit the present invention. Appropriate modifications or equivalent substitutions made by those skilled in the art to the technical solutions of the present invention should be covered within the protection scope of the present invention, which is defined by the claims.

Claims

1. A method for estimating the photosynthetically active radiation absorption ratio of emergent vegetation, comprising the following steps: 1) Obtain the vertical stratification information of emergent vegetation, and determine the radiation absorption coefficient and scattering coefficient of each structural layer based on the vegetation composition, optical properties and distribution parameters of each layer. 2) Based on the absorption coefficient and scattering coefficient, a two-flow approximate radiative transfer model for emergent vegetation is constructed, and the upward and downward irradiance in each structural layer is calculated. 3) Based on the boundary conditions and interlayer interface connection conditions, and combined with the refraction and reflection characteristics between layers, determine the boundary irradiance parameters required for light radiation propagation in each structural layer. 4) Using the multiple scattering theory and the scattering transfer matrix method, solve the radiative transfer model in step 2) to obtain the scattering irradiance distribution in each structural layer; 5) Based on the total irradiance distribution and absorption coefficient of each structural layer, the absorbed photosynthetically active radiation flux in each structural layer is calculated according to the photosynthetically active radiation absorption ratio (FAPAR) algorithm, and a preliminary FAPAR estimate is obtained accordingly. 6) Based on the spatial distribution density and optical characteristics of stems in emergent vegetation, the preliminary FAPAR estimate is corrected to obtain the FAPAR value that includes the contribution of stems; 7) Based on the FAPAR value obtained in step 6), the FAPAR value of the underwater vegetation is corrected according to the refraction law of light propagation in water and Fresnel's law of reflection, so as to obtain the final estimated FAPAR value of emergent vegetation.

2. The method as described in claim 1, characterized in that, Before step 1), acquire three-dimensional lidar point cloud data or multispectral remote sensing images of emergent vegetation areas to extract the spatial thickness and vegetation parameters of each structural layer.

3. The method as described in claim 1, characterized in that, The structural layers described in step 1) include a canopy layer, a water surface layer, an underwater vegetation layer, and a bottom water layer. The absorption coefficient and scattering coefficient of each layer are calculated based on the vegetation type, optical properties, and leaf area index of that layer, respectively.

4. The method as described in claim 1, characterized in that, The two-stream approximate radiative transfer model described in step 2) includes a set of coupled differential equations that describe the changes in upward and downward luminous flux, respectively. The two-stream approximate radiative transfer model models the attenuation process of direct radiation and the propagation process of scattered radiation in each structural layer to simulate the overall irradiance distribution.

5. The method as described in claim 1, characterized in that, The boundary conditions mentioned in step 3) include the incident angle and intensity of the incident light at the top layer, the reflection coefficient of the lower boundary, and the interface connection conditions include the refractive index ratio and Fresnel reflection coefficient between adjacent layers.

6. The method as described in claim 1, characterized in that, The scattering transfer matrix method described in step 4) includes establishing a local scattering transfer matrix for each structural layer and constructing the overall system transfer matrix by multiplying layer by layer to achieve a closed-form solution for irradiance.

7. The method as described in claim 1, characterized in that, The calculation of the FAPAR value in step 5) includes dividing the photosynthetically active radiation absorbed by the plant tissue per unit area by the total photosynthetically active radiation incident per unit area, and then summing the results for leaves and non-leaf organs respectively.

8. The method as described in claim 1, characterized in that, The optical properties of the stem mentioned in step 6) include its reflectivity, transmittance and absorptivity for different wavelengths of light. During the correction process, the initial FAPAR is compensated for gain according to the volume distribution coefficient of the stem.

9. The method as described in claim 1, characterized in that, The FAPAR value correction for the underwater portion mentioned in step 7) includes correcting the change in the incident angle of light according to Snell's law, and using Fresnel's formula to correct the reflection loss and transmission efficiency at the water surface respectively.

10. The method as described in claim 1, characterized in that, A time series analysis was performed on the final FAPAR estimate obtained in step 7) to track the changing trend of FAPAR during the growth cycle of emergent vegetation.

Images