A method for estimating actual evapotranspiration in different irrigation zones of farmland based on temperature

Abstract

This invention discloses a method for estimating the actual evapotranspiration of different irrigation zones of farmland based on temperature. The method first measures meteorological parameters of a reference irrigation zone and calculates the sensible heat flux and evapotranspiration of the reference irrigation zone. Characteristic parameters of the irrigation zone to be tested and the reference irrigation zone are then obtained. Based on the difference in the characteristic parameters between the irrigation zone to be tested and the reference irrigation zone, the sensible heat flux and evapotranspiration of the irrigation zone to be tested are estimated. This method can effectively calculate the actual evapotranspiration of different irrigation zones of farmland.

Description

Technical Field

[0001] The present invention relates to a method for acquiring surface evapotranspiration data, and in particular to a method for estimating actual evapotranspiration of different irrigation zones of farmland based on temperature. Background Art

[0002] Surface evapotranspiration refers to the process of water entering the atmosphere in gaseous form, mainly including surface soil evaporation, vegetation transpiration and the interception and evaporation of precipitation by vegetation canopy. It is the main indicator for evaluating regional surface energy, climate change and water balance, and is an important part of ecological environment and water resources assessment.

[0003] Methods for obtaining evapotranspiration can be categorized into hydrological, remote sensing, plant physiological, and micrometeorological methods. The most commonly used hydrological method is the lysimeter, a container containing soil and vegetation that simultaneously measures evaporation and seepage. It offers high measurement accuracy and can automatically record data on an hourly or even minute-by-minute scale. However, its measurement area is small, and due to the oasis effect, it cannot accurately estimate evapotranspiration for entire fields. Remote sensing methods offer a lower cost for obtaining the same spatial information and account for the spatial heterogeneity of the underlying surface and climatic conditions, making them more suitable for long-term observations. However, their spatial resolution is too coarse, and variations in evapotranspiration caused by small-scale geological variations cannot be accurately inferred. Plant physiological methods are suitable for measuring complex terrain and favor small plots or single plant families. However, their sample representativeness is significantly problematic, and it is impossible to infer total evapotranspiration over a large area from a few plants. Micrometeorological methods, on the other hand, provide a key approach for studying processes at the canopy scale. They are a direct method for measuring water and heat exchange between the canopy and the atmosphere, offering significant advantages in accurately measuring evapotranspiration over small areas.

[0004] Micrometeorological methods mainly include aerodynamics, Bowen ratio energy balance, and eddy covariance. The aerodynamics method is based on the gradient diffusion theory of the surface boundary layer. The water and heat flux is calculated based on the gradient and profile equations of temperature, humidity, and wind speed. However, it is not suitable for non-uniform underlying surfaces with horizontal inversion, plant cover with large roughness, and the interior of plant canopies. The Bowen ratio energy balance method is based on the water and heat exchange of the underlying surface. This method can only be used to measure the water and heat flux of farmland under the assumption that the heat exchange coefficient and the turbulent exchange coefficient of water vapor are equal. The eddy covariance method is based on the eddy covariance theory. It directly measures the pulsation of water vapor, temperature, and wind speed at the atmospheric boundary and calculates the time-averaged covariance to obtain the water and heat flux. The equipment required for this method is too expensive. Summary of the Invention

[0005] In view of the shortcomings in the prior art, the present invention provides a method for estimating the actual evapotranspiration of different irrigation zones of farmland based on temperature.

[0006] The present invention achieves the above technical objectives through the following technical means.

[0007] A method for estimating actual evapotranspiration in different irrigation zones of farmland based on temperature:

[0008] Measure meteorological parameters of the reference irrigation zone and calculate sensible heat flux and evapotranspiration of the reference irrigation zone;

[0009] The characterization parameters of the irrigation zone to be tested and the reference irrigation zone are obtained, and the sensible heat flux and evapotranspiration of the irrigation zone to be tested are estimated based on the difference in the characterization parameters between the irrigation zone to be tested and the reference irrigation zone.

[0010] A further technical solution is that the meteorological parameters include plant canopy temperature, friction wind speed and net radiation at twice the canopy height, soil heat flux 8 cm underground, and soil temperature and humidity at g1 and g2 underground, and g1 and g2 are both less than 8 cm.

[0011] In a further technical solution, the sensible heat flux of the reference irrigation zone is:

[0012]

[0013] Where: H is the sensible heat flux, α and β are empirical coefficients, ρ is the air density, c p is the specific heat of air, S represents the third-order structure function, Δt m is the time interval, u * is the friction wind speed, z is the measurement height of the friction wind speed, and h is the canopy height.

[0014] In a further technical solution, the third-order structure function satisfies:

[0015]

[0016] Where N is the total number of canopy temperature sample points, i is the canopy temperature sample number, j is the canopy temperature sample delay, and T represents the canopy temperature.

[0017] In a further technical solution, the evaporation is:

[0018]

[0019] Where: LE is the latent heat flux, and LE satisfies R n =H+LE+G,R n is the net radiation, and G is the surface soil heat flux, which is calculated from the soil heat flux and soil temperature and humidity.

[0020] In a further technical solution, the characterization parameters include leaf area, terrain slope and leaf area index.

[0021] In a further technical solution, the sensible heat flux of the irrigation zone to be measured is:

[0022]

[0023] Where: LAI1 and LAI2 are the leaf area index of the tested irrigation zone and the reference irrigation zone, respectively; W1 and W2 are the leaf areas of the tested irrigation zone and the reference irrigation zone, respectively; cosα and cosβ are the terrain slopes of the tested irrigation zone and the reference irrigation zone, respectively.

[0024] In a further technical solution, the evapotranspiration of the irrigation zone to be measured is:

[0025]

[0026] Where LE satisfies R n =H0+LE+G.

[0027] The beneficial effects of the present invention are as follows: the present invention measures the meteorological parameters of the reference irrigation zone to calculate the sensible heat flux and evapotranspiration of the reference irrigation zone; based on the difference in the characterization parameters between the irrigation zone to be measured and the reference irrigation zone, the sensible heat flux and evapotranspiration of the irrigation zone to be measured are estimated, thereby realizing effective calculation of the actual evapotranspiration of different irrigation zones of farmland, eliminating the need to install meteorological detection equipment in each irrigation zone, saving data collection costs, and providing theoretical guidance for zoned irrigation of large-scale farmland. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Schematic diagram of a weather station in an embodiment of the present invention;

[0029] Figure 2 This is a schematic diagram of leaf area measurement in an embodiment of the present invention;

[0030] FIG3( a ) is a side view of a canopy shot in an embodiment of the present invention;

[0031] FIG3( b ) is a top view of the canopy in an embodiment of the present invention. DETAILED DESCRIPTION

[0032] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited thereto.

[0033] A method for estimating actual evapotranspiration of different irrigation zones of farmland based on temperature, comprising the following steps:

[0034] Step 1: Measure the meteorological parameters of the reference irrigation zone, calculate the sensible heat flux of the reference irrigation zone using the third-order temperature structure function based on vorticity theory, and calculate the evapotranspiration of the reference irrigation zone based on the energy balance equation.

[0035] Meteorological parameters measured for the reference irrigation zones include: plant canopy temperature, frictional wind speed and net radiation at twice the canopy height, soil heat flux 8 cm below ground, and soil temperature and humidity at g1 and g2 below ground, with g1 and g2 both less than 8 cm. The canopy temperature is measured using a thermocouple with a sampling frequency of 4 Hz or higher. The frictional wind speed at twice the canopy height is measured using a three-dimensional ultrasonic anemometer, and the net radiation at twice the canopy height is measured using a net radiometer. Soil heat flux is measured using a soil heat flux plate. Soil temperature and humidity are measured using a soil temperature and humidity sensor.

[0036] (1) Based on the measured soil heat flux and soil temperature and humidity, the surface soil heat flux G is expressed as:

[0037]

[0038] Where G′ is the soil heat flux at 8 cm, T g1 is the soil temperature at underground g1, T g2 is the soil temperature at underground g2, ρ b is the soil bulk density, ρ w is the water density, and θ is the volumetric water content of the soil at 8 cm.

[0039] (2) Based on the third-order temperature structure function of vorticity theory, the calculation formula of sensible heat flux H is:

[0040]

[0041] Among them: α, β are empirical coefficients, ρ is air density, c p is the specific heat of air, Δt m is the time interval, u * is the friction wind speed, z is the measurement height of the friction wind speed, h is the canopy height, and S represents the third-order structure function;

[0042] The third-order structure function is calculated by the following formula:

[0043]

[0044] Where N is the total number of canopy temperature sample points, i is the canopy temperature sample number, j is the canopy temperature sample delay, and T represents the canopy temperature.

[0045] (3) The empirical coefficient is calculated as follows:

[0046] The temperature ramp model amplitude a satisfies the following cubic equation:

[0047]

[0048] Solve the cubic equation of formula (4) to obtain the maximum real number solution of the amplitude a of the temperature ramp model;

[0049] Next, define the intermediate quantity τ as:

[0050]

[0051] Depend on The slope of the fitted straight line is the empirical coefficient β.

[0052] In formula (2), ρ and c p , Δt m 、u * As , z, h, and S are all known, after substituting the empirical coefficient β, Equation (2) only becomes the relationship between H and α. Based on the eddy covariance, the standard sensible heat flux H can be obtained (the specific process is the existing technology), and the empirical coefficient α can be obtained by substituting it into the simplified Equation (2).

[0053] (4) The energy balance equation is:

[0054] R n =H+LE+G(6)

[0055] Among them, R n is the net radiation, LE is the latent heat flux;

[0056] Therefore, evapotranspiration ET is expressed as:

[0057]

[0058] Here, λ represents the latent heat of vaporization coefficient.

[0059] Step 2: Obtain the characterization parameters of the irrigation zone to be tested and the reference irrigation zone

[0060] The characterization parameters of the irrigation zone to be tested are obtained, including leaf area, terrain slope, and leaf area index of the irrigation zone to be tested and the reference irrigation zone.

[0061] Specifically, a level was used to measure the terrain slope of the tested irrigation zone and the terrain slope of the reference irrigation zone. The areas of 10 random leaves in the two zones were measured, and the average value was taken as the leaf area value of the zone. Image information of the same plant in eight directions was obtained, and the leaf area index was obtained based on machine vision.

[0062] Get the leaf area as follows:

[0063] (1) Spread the leaf flat on the table, place a 1 cm ruler next to the leaf, and shoot from above to obtain image information;

[0064] (2) Use OpenCV to obtain image information, create a mask image, perform Gaussian blur processing on it, and obtain a BGR channel format image to facilitate subsequent color recognition and extraction. Then, convert the BGR channel format image to HSV channel format, which can accurately identify specific colors; then erode the converted image to remove the noise on the image, and finally remove the part other than the target color and convert the image into a binary image;

[0065] (3) Obtain the boundary information of the binary image, draw the bounding box, calculate the area of ​​the target area and record it in the log file;

[0066] (4) Repeat the above steps to obtain the proportion of other colors in the original image, and analyze the data to obtain color ratio information;

[0067] (5) Compare with the standard length of 1 cm to obtain the actual green area as the leaf area.

[0068] Get the leaf area index as follows:

[0069] (1) Use the neroic algorithm to synthesize eight photos into a three-dimensional point cloud data;

[0070] (2) Use opencv to obtain image information, use adaptive threshold and bilateral filtering to denoise the collected image, and extract the image features; use the multi-scale pyramid method to obtain the multi-scale feature maps of each feature at different spatial scales, process the feature maps of different scales, and obtain the feature sub-saliency map; then perform linear normalization and linear superposition on the feature sub-saliency map at multiple scales to obtain the feature saliency map corresponding to each feature; merge each feature saliency map to calculate the final total saliency map, and combine the total saliency map into a single saliency map. Figure 2 The image is converted into black and white image and morphological processing is performed to remove noise and fill holes;

[0071] (3) Use OpenCV's own function to draw the leaf outline in the image and calculate the total leaf area. The total leaf area is divided by the projected area of ​​the plant on the ground to obtain the leaf area index.

[0072] Step 3: Based on the difference in characterization parameters between the irrigation zone to be tested and the reference irrigation zone, the sensible heat flux and evapotranspiration of the irrigation zone to be tested are estimated.

[0073] The sensible heat flux H0 of the irrigation zone to be tested is calculated by the following formula:

[0074]

[0075] Where: LAI1 and LAI2 are the leaf area index of the tested irrigation zone and the reference irrigation zone, respectively; W1 and W2 are the leaf areas of the tested irrigation zone and the reference irrigation zone, respectively; cosα and cosβ are the terrain slopes of the tested irrigation zone and the reference irrigation zone, respectively.

[0076] In the same area, the net radiation and surface soil heat flux can be considered to be approximately the same, and the evapotranspiration of the irrigation zone to be tested can be calculated by equations (6) and (7) (in this case, H in equation (6) is replaced by H0).

[0077] Example

[0078] The experimental site selected in this embodiment is the tea plantation of Jiangsu Danyang Yinchun Biya Co., Ltd.

[0079] In step 1, build the reference irrigation zone as shown below Figure 1 The weather station shown in the figure uses a four-component net radiometer CNR4 from Campell Scientific, USA. The friction wind speed is calculated according to formula (9) using the parameters measured by a three-dimensional anemometer. The three-dimensional anemometer is a Windmaster from Gill. An Omega E-type thermocouple is placed at canopy height. An Hfp01 soil heat flux plate is placed 8 cm underground, and a Stevens Hydra Probe II soil temperature and humidity sensor is placed at 2 cm and 6 cm.

[0080]

[0081] Among them, u′, v′ and w′ are the wind speed fluctuations in the north, east and upward directions, respectively, which are the differences between the instantaneous value and the average value during the period.

[0082] Based on the actual measured data, the sensible heat flux of the reference irrigation zone at 11:00 on November 4, 2022 is calculated to be 189.26W / m 2 , evaporation is 0.174mm / d.

[0083] In step 2, use two cover glasses to make Figure 2 The device shown randomly selects 10 leaves in the irrigation zone to be tested and the reference irrigation zone. During each measurement, the upper cover glass is pressed on the leaf to ensure that the leaf area measured each time is the area when the leaf is fully expanded.

[0084] The area of ​​10 leaves in the reference irrigation zone is 8.41 cm 3 、4.66cm 3 , 8.25cm 3 、9.94cm 3 、6.89cm 3 、9.10cm 3、8.74cm 3 、4.76cm 3 、8.69cm 3 and 9.08cm 3 , and then the leaf area of ​​this partition is calculated to be 7.85cm 3 The area of ​​10 leaves in the irrigation zone to be tested is 9.75cm 3 、9.82cm 3 、9.86cm 3 、9.30cm 3 、7.14cm 3 、9.35cm 3 、7.91cm 3 、9.84cm 3 、3.68cm 3 and 9.95cm 3 , and then the leaf area of ​​this partition is calculated to be 8.66cm 3 .

[0085] Eight canopy photographs were taken at the same angle as shown in Figures 3(a) and (b). Analysis showed that the total number of leaves per unit area in the tested irrigation zone and the reference irrigation zone was 3578 and 3694, respectively. The leaf area index of the tested irrigation zone and the reference irrigation zone was calculated to be 3.1 and 2.9, respectively.

[0086] In step 3, the parameters obtained in steps 1 and 2 are substituted into formula (8) to calculate the sensible heat flux of the irrigation zone to be tested as 219.29 W / m 2 The evapotranspiration of the irrigation area to be tested is 0.138 mm / d.

[0087] The sensible heat flux of the irrigation zone to be tested is 208.75W / m 2 , evapotranspiration is 0.151 mm / d; the error of the sensible heat flux obtained by the method of the present invention is 5.0%, and the error of the evapotranspiration obtained is 8.6%. It can be seen that the accuracy of evapotranspiration estimated by the method of the present invention is relatively high.

[0088] The embodiments described are preferred implementations of the present invention, but the present invention is not limited to the above implementations. Any obvious improvements, substitutions or modifications that can be made by those skilled in the art without departing from the essence of the present invention are within the scope of protection of the present invention.

Claims

1. A method for estimating actual evapotranspiration of different irrigation zones of farmland based on temperature, characterized by: Measure meteorological parameters of the reference irrigation zone and calculate sensible heat flux and evapotranspiration of the reference irrigation zone; Obtaining characterization parameters of the irrigation zone to be tested and the reference irrigation zone, and estimating the sensible heat flux and evapotranspiration of the irrigation zone to be tested based on the difference in characterization parameters between the irrigation zone to be tested and the reference irrigation zone; The sensible heat flux for the reference irrigation zone is: Where: H is the sensible heat flux of the reference irrigation zone, α and β are empirical coefficients, ρ is the air density, c p is the specific heat of air, S represents the third-order structure function, Δt m is the time interval, u * is the friction wind speed, z is the measurement height of the friction wind speed, and h is the canopy height; The third-order structure function satisfies: Where N is the total number of canopy temperature sample points, i is the canopy temperature sample number, j is the canopy temperature sample delay, and T represents the canopy temperature; The evapotranspiration for the reference irrigation zone is: Where: LE is the latent heat flux of the reference irrigation zone, and LE satisfies R n =H+LE+G,R n is the net radiation of the reference irrigation zone, G is the surface soil heat flux, which is calculated from the soil heat flux and soil temperature and humidity, λ is the latent heat coefficient of vaporization, and ρ w is the water density; The characterization parameters include leaf area, terrain slope and leaf area index; The sensible heat flux of the irrigation zone to be tested is: Where: LAI1 and LAI2 are the leaf area indices of the tested irrigation zone and the reference irrigation zone, respectively; W1 and W2 are the leaf areas of the tested irrigation zone and the reference irrigation zone, respectively; cosα and cosβ are the terrain slopes of the tested irrigation zone and the reference irrigation zone, respectively.

2. The method for estimating actual evapotranspiration of different irrigation zones of farmland based on temperature according to claim 1, characterized in that: The meteorological parameters include plant canopy temperature, friction wind speed and net radiation at twice the canopy height, soil heat flux 8 cm underground, and soil temperature and humidity at g1 and g2 underground, and g1 and g2 are both less than 8 cm.

3. The method for estimating actual evapotranspiration of different irrigation zones of farmland based on temperature according to claim 1, characterized in that: The evapotranspiration of the irrigation zone to be tested is: Among them, the latent heat flux LE1 of the irrigation zone to be measured satisfies R n1 =H0+LE1+G,R n1 is the net radiation of the irrigation zone to be measured.

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