CN117609686B - Soil moisture content evaluation method based on frequency domain dielectric spectrum - Google Patents
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Abstract
The invention relates to the technical field of measurement. The invention provides a soil moisture content evaluation method based on a frequency domain dielectric spectrum, which comprises the following steps: adopting a soil moisture inversion model to obtain the soil moisture content:. Wherein,in order to achieve a volume water content,、、、is an empirical parameter;is the characteristic parameter of the apparent dielectric spectrum of the soil. The invention uses constructed soil waterThe soil moisture content is obtained by dividing the inversion model, the problem of frequency dependence of the dielectric constant is solved, and the problem that the traditional soil dielectric constant model can only predict the soil moisture content at one frequency is avoided.
Description
Technical Field
The invention relates to the technical field of measurement, in particular to a soil moisture content evaluation method based on a frequency domain dielectric spectrum.
Background
The water content of the soil is one of important parameters for determining the physical, chemical and crop growth of the soil, and has important influence on the fields of efficient water-saving irrigation of agricultural soil, agricultural crop production, soil drought degree and the like. Therefore, the accurate, rapid and efficient detection of the soil moisture content has important significance for agricultural development and application research in related fields thereof.
The traditional soil moisture content measuring method mainly comprises two types: direct methods and indirect methods. The direct method mainly measures the water content of soil by sampling, drying and weighing, and the method is accurate, but has large time and labor consumption, can only measure discrete points and can damage the soil structure. The indirect methods comprise a gravity method, a neutron scattering method, a resistance method and the like, and although the methods can continuously measure, the accuracy is low, and the method can be converted into the soil moisture content through an empirical model.
In recent years, with the rapid development of microwave remote sensing and ground penetrating radar technology, frequency domain dielectric spectrum measurement has become a novel nondestructive detection method for soil moisture content. However, in the process of frequency domain dielectric spectrum measurement, the soil moisture content can only be obtained by measuring at a specific frequency. For example: for the traditional soil dielectric constant model, taking the dielectric constant in the Topp empirical model as an example, the model is usually measured under the condition of fixed frequency, which leads to different coefficients of the Topp empirical model obtained by testing at different frequencies, that is, when the dielectric constant measurement is performed at different frequencies, the predicted water content is different, so that the accurate soil water content cannot be obtained once the frequency is changed. Therefore, the conventional dielectric constant model prediction accuracy is changed by the frequency change, and it is necessary to solve the frequency dependency problem.
Disclosure of Invention
The invention aims to solve the defects in the prior art and provides a soil moisture content evaluation method based on a frequency domain dielectric spectrum so as to solve the problem that the dielectric constant changes along with the change of frequency.
A soil moisture content evaluation method based on a frequency domain dielectric spectrum comprises the following steps: the following soil moisture inversion model is adopted to obtain the soil moisture content:
（1）
wherein,for volume water content, +.>、/>、/>、/>Is an empirical parameter; />Is the characteristic parameter of the apparent dielectric spectrum of the soil;
（7）；
is a dielectric loss factor characteristic parameter, < >>Is the real part of the dielectric constant.
Further, according to the soil moisture content evaluation method based on the frequency domain dielectric spectrum, the soil apparent dielectric spectrum characteristic parameterObtained according to the following way:
step 1: measuring the real part and the imaginary part of the dielectric constant of the soil by adopting different frequencies, and respectively and correspondingly obtaining a characteristic curve of the real part of the dielectric constant changing along with the frequency and a characteristic curve of the imaginary part of the dielectric constant changing along with the frequency;
step 2: fitting according to formula (2) by using the characteristic curve of the real part of the dielectric constant changing along with the frequency to obtain fitting parameters、/>、/>、/>；
（2）
Step 3: fitting according to formula (3) by utilizing the characteristic curve of the dielectric constant imaginary part changing along with the frequency to obtain fitting parameters、/>、/>、/>；
（3）
Wherein,is the real part of the dielectric constant, < >>Is the imaginary part of the dielectric constant;
step 4: according to the fitting parameters、/>、/>、/>、/>、/>、/>、/>Deriving any point of a characteristic curve of the real part of the dielectric constant along with the change of frequency, wherein the derivation relation is shown in a formula (4):
（4）
step 5: according to the fitting parameters、/>、/>、/>、/>、/>、/>、/>And deriving any point of a characteristic curve of the dielectric constant imaginary part changing along with the frequency, wherein the derivation relation is shown in a formula (5):
（5）
step 6: according to formulas (4) and (5), dielectric loss factor characteristic parameters are calculated：
（6）
Step 7: according to the dielectric loss factor characteristic parameterReal part of dielectric constant +.>Calculating to obtain the characteristic parameter +.>
（7）
The invention provides a soil moisture evaluation method based on frequency domain dielectric spectrum, which is characterized in that soil moisture is obtained through a constructed soil moisture inversion model, a dielectric constant real part-frequency characteristic curve and a dielectric constant imaginary part-frequency characteristic curve under different frequency conditions measured by an instrument are subjected to differential treatment, so that corresponding differential characteristic parameters are extracted, then the characteristics of soil dielectric characteristics can be described according to apparent dielectric constants, and the two differential parameters are utilized to define soil apparent dielectric spectrum characteristic parametersThe soil moisture inversion model can solve the problem that the dielectric constant changes along with the frequency change, namely the predicted soil moisture content under any frequency or frequency band condition through an instrument is not influenced by the dielectric constant frequency change, solves the problem of dielectric constant frequency dependence, and avoids the problem that the traditional soil dielectric constant model can only predict the soil moisture content under one frequency.
Drawings
FIG. 1 is a graph of the real part of dielectric constant under different volume water contents;
FIG. 2 is a graph of the imaginary part of dielectric constant under different volume water contents;
FIG. 3 is a graph of loss factor for different volume water fractions;
FIG. 4 shows the characteristic parameters of the real part dielectric spectrum of the soil under the condition of different volume water contentsIs a graph of (2);
FIG. 5 shows characteristic parameters of dielectric spectrum of imaginary part of soil under different volume water contentsIs a graph of (2);
FIG. 6 shows dielectric loss factor spectra of soil under different volume water contentsIs a graph of (2);
FIG. 7 is a graph of characteristic parameters of red soil dielectric spectrum and volume water content under different frequency point conditions obtained by a Topp conversion model;
FIG. 8 is a graph of red soil dielectric spectrum characteristic parameters and volume water content under different frequency point conditions obtained through a Herkelrath conversion model;
fig. 9 is a graph of predicted and measured soil volume moisture content.
Detailed Description
For the purpose of making the objects, technical solutions and advantages of the present invention more apparent, the technical solutions in the present invention will be clearly and completely described below, and it is apparent that the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
The invention provides a soil moisture content evaluation method based on a frequency domain dielectric spectrum, which adopts the following soil moisture inversion model to obtain the soil moisture content:
（1）
wherein,for volume water content, +.>、/>、/>、/>Is an empirical parameter; />Is a characteristic parameter of the apparent dielectric spectrum of soil (a parameter representing the variation of dielectric constant with frequency).
Specifically, most of traditional empirical models of soil dielectric constants only consider the relationship between dielectric constants and water contents at a single frequency, and an optimal prediction model for the soil dielectric constants at different frequencies is lacking. The method establishes a soil moisture content estimation model based on the relation between dielectric spectrum characteristic parameters and soil moisture content. The application carries out differential treatment on the dielectric constant real part-frequency characteristic curve and the dielectric constant imaginary part-frequency characteristic curve under different frequency conditions measured by an instrument so as to extract corresponding differential characteristic parameters, then can describe the characteristics of the dielectric property of the soil according to the apparent dielectric constant, and utilizes the two differential parameters to define the characteristic parameters of the apparent dielectric spectrum of the soilThe soil moisture inversion model is provided by the treatment of the methodThe model can solve the problem that the dielectric constant changes along with the change of frequency, namely the soil moisture content predicted by the instrument under any frequency or frequency band condition is not influenced by the change of the dielectric constant frequency.
The invention providesSoil moisture content evaluation method based on frequency domain dielectric spectrum and based on soil apparent dielectric spectrum characteristic parametersThe constructed soil moisture inversion model is used for obtaining the soil moisture content, so that the problem of frequency dependence of the dielectric constant is solved, and the problem that the soil moisture content can only be predicted at one frequency by the traditional dielectric technical means is avoided, and the soil moisture content can be calculated at any frequency by the method provided by the invention.
Further, according to the soil moisture content evaluation method based on the frequency domain dielectric spectrum, the soil apparent dielectric spectrum characteristic parameterObtained according to the following way:
step 1: measuring the real part and the imaginary part of the dielectric constant of the soil by adopting different frequencies, and respectively and correspondingly obtaining a characteristic curve of the real part of the dielectric constant changing along with the frequency and a characteristic curve of the imaginary part of the dielectric constant changing along with the frequency;
step 2: fitting according to formula (2) by using the characteristic curve of the real part of the dielectric constant changing along with the frequency to obtain fitting parameters、/>、/>、/>；
（2）
Step 3: fitting according to formula (3) by utilizing the characteristic curve of the dielectric constant imaginary part changing along with the frequency to obtain fitting parameters、/>、/>、/>；
（3）
Wherein,is the real part of the dielectric constant, < >>Is the imaginary part of the dielectric constant;
step 4: according to the fitting parameters、/>、/>、/>、/>、/>、/>、/>For the characteristic curve of the real part of dielectric constant changing with frequencyThe meaning points are subjected to derivation, and the derivation relation is shown in a formula (4):
（4）
step 5: according to the fitting parameters、/>、/>、/>、/>、/>、/>、/>And deriving any point of a characteristic curve of the dielectric constant imaginary part changing along with the frequency, wherein the derivation relation is shown in a formula (5):
（5）
step 6: according to formulas (4) and (5), dielectric loss factor characteristic parameters are calculated：
（6）
Step 7: according to the dielectric loss factor characteristic parameterReal part of dielectric constant +.>Calculating to obtain the characteristic parameter +.>
（7）
The following describes how to obtain the soil moisture inversion model provided by the invention:
experiment design:
representative soil samples were selected and dielectric spectroscopy measurements were performed at different water contents. And (3) obtaining a series of soil samples with different water contents by controlling the water content of the soil. Multiple replicates were set per water cut to improve the reliability of the experimental results.
The soil sample is subjected to frequency domain dielectric spectroscopy using a suitable dielectric spectroscopy measurement device such as a dielectric spectrometer, agilent 4294A meter or ground penetrating radar, or the like. The dielectric constant and dielectric dissipation factor of the soil sample are recorded over a range of frequencies. The Agilent 4294A instrument is used for transmitting electromagnetic waves within a certain frequency domain range to specific soil, and collecting feedback signals of the soil on the transmitted electromagnetic waves, wherein the feedback signals comprise real parts of dielectric constants of different frequencies) Imaginary part ()>) Loss factor (+)>). Wherein the real part increases with the increase of the water content of the soil. When the water content of the soil is low, the absorption of the soil to electromagnetic waves is small, and the real part is small; while the soil contains waterWhen the rate is high, the absorption of electromagnetic waves by the soil increases, and the real part increases. The imaginary part decreases with increasing water content of the soil. The imaginary part reflects the propagation speed and the phase delay of the electromagnetic wave in the soil, and when the water content of the soil is low, the propagation speed of the soil to the electromagnetic wave is high, and the imaginary part is large; and when the water content of the soil is higher, the propagation speed of the soil to electromagnetic waves is reduced, and the imaginary part is reduced. The loss factor reflects the energy loss degree of the electromagnetic wave in the soil, and when the water content of the soil is low, the energy loss of the electromagnetic wave in the soil is small, and the loss factor is small; when the water content of the soil is high, the energy loss of the electromagnetic wave in the soil is increased, and the loss factor is increased (the frequency domain range can be selected according to actual requirements).
And exploring the relation between the dielectric spectrum characteristic parameter and the soil water content by a statistical analysis method. The characteristic parameters of the soil dielectric spectrum represent the law of the dielectric constant along with the frequency change, and the magnitude of the characteristic parameters is equal to the value of the dielectric constant deriving the frequency. FIG. 1 is a graph of the real part of dielectric constant under different volume water contents; FIG. 2 is a graph of the imaginary part of dielectric constant under different volume water contents; FIG. 3 is a graph of loss factor for different volume water fractions; 1-3, the real part and the imaginary part of the dielectric constant of the soil in 1-7 MHz have stronger correlation, and the frequency spectrum characteristic curve measured by the Agilent 4294A instrument is quantized to obtain the following relationship:
（2）
（3）
in the method, in the process of the invention,and->The real part and the imaginary part of the dielectric constant respectively; />For frequency (quantifying a spectrum curve measured by Agilent 4294A instrument, thereby extracting characteristic parameters and preparing for inversion of water content); />、/>、/>、/>、/>、/>、/>、/>Is a fitting parameter. And (3) fitting the dielectric spectrum by adopting the formula (2) and (3) to obtain the characteristic function relationship of the dielectric spectrum of the soil (see table 1 and table 2).
TABLE 1 Red soil dielectric Spectroscopy characterization Curve (real part)
TABLE 2 Red soil dielectric Spectroscopy characterization Curve (imaginary part)
As can be seen from tables 1 and 2, the fitting function relationship between the real part and the imaginary part of the soil dielectric spectrum and the frequency has strong correlation. In order to describe the soil frequency domain dielectric spectrum, the invention introduces the characteristic parameters of the real part dielectric spectrumAnd imaginary dielectric spectral characteristic parameter->It is defined as:
（4）
（5）
also introduced is dielectric loss factor spectrumThe following relationships are satisfied:
（6）
besides the real part and the imaginary part of the dielectric constant can describe the dielectric property of the soil, the apparent dielectric constant can also describe the dielectric response property of the soil, so that the characteristic parameters of the apparent dielectric spectrum of the soil are provided according to the definition of the apparent dielectric constant:
（7）
the dielectric spectrum characteristic curves of the soil meet the relation of cubic functions, and dielectric spectrum characteristic parameters are obtained by derivative calculation of the dielectric spectrum characteristic curves. The dielectric spectrum characteristic parameter L itself characterizes the rate of change of the dielectric constant of the soil with frequency, which rate of change generates a fluctuation characteristic with increasing frequency. Wherein FIG. 4 shows the characteristic parameters of the real part dielectric spectrum of the soil under the condition of different volume water contentsIs a graph of (2); FIG. 5 shows characteristic parameters of dielectric spectrum of imaginary part of soil under different volume water contents>Is a graph of (2); FIG. 6 shows the dielectric loss factor spectrum of soil under the condition of different volume water contents>Is a graph of (2); as shown in fig. 4-6, when the frequency is less than 4.5MHz, the characteristic parameters are gradually reduced as the frequency increases. At this time, the water molecules in the soil generate larger steering polarization under the action of an external electric field, dielectric relaxation and loss increase are reduced along with the increase of frequency, and the dielectric spectrum is in a negative change rate, so that the characteristic parameters of the dielectric spectrum are rapidly reduced. When the frequency is greater than 4.5MHz, the characteristic parameters are gradually increased with the increase of the frequency.
Establishing a soil moisture content estimation model: and establishing a soil moisture content estimation model based on the relation between the dielectric spectrum characteristic parameters and the soil moisture content. The method can adopt regression analysis, a support vector machine, an artificial neural network and other methods, takes the characteristic parameters as independent variables, takes the soil moisture content as dependent variables, establishes a model and carries out parameter fitting and verification. Most traditional soil dielectric constant empirical models only consider the relation between dielectric constant and water content at a single frequency, and lack research on optimal prediction models of soil dielectric constants at different frequencies. In order to quantify the relation between the characteristic parameters of the dielectric spectrum of the soil and the volume water content, the water content state of the soil is estimated. 7 frequency points are selected in the frequency range of 1-7 MHz, and system analysis is carried out at 1MHz, 2MHz, 3MHz, 4MHz, 5MHz, 6MHz and 7MHz respectively. Based on a typical Topp model and a Herkelath model, the apparent dielectric spectrum characteristic parameters are used for representing the dielectric constants of the frequency domain, and then the following relation is obtained after the Topp conversion model:
（1）
wherein,、/>、/>、/>is an empirical parameter;
the herkelath conversion model yields the following relationship:
（8）
in the method, in the process of the invention,for volume water content, +.>、/>Is an empirical parameter (inversion of soil moisture content in order to quantify the relationship between dielectric spectrum characteristics and volume moisture content). Therefore, after the selected 7 frequency domain dielectric spectrum characteristic parameters are brought into the formulas (1) and (7) to be fitted, the prediction accuracy of the Topp model conversion and the Herkelrath conversion model under different frequency points can be obtained (see Table 3).
TABLE 3 relationship of soil dielectric spectrum model under different frequency points
FIG. 7 is a graph of characteristic parameters of red soil dielectric spectrum and volume water content under different frequency point conditions obtained by a Topp conversion model; FIG. 8 is a graph of red soil dielectric spectrum characteristic parameters and volume water content under different frequency point conditions obtained through a Herkelrath conversion model; as shown in fig. 7 and 8, the relationship between the characteristic parameters of the apparent dielectric spectrum of the soil and the volume water content under the condition of different frequency points has a good fitting effect. As can be seen from Table 3, the fitting accuracy R of the Topp conversion model under different frequency points 2 The fitting precision R of the Herkelath conversion model is between 0.94822 and 0.98781 2 And the accuracy of the two fitting models gradually decreases with the increase of frequency between 0.84129 and 0.98319. In contrast, the Topp conversion model considering a section of frequency (1-7 MHz) can be used for fitting the relation between the characteristic parameter of the soil apparent dielectric spectrum and the volume water content more accurately, namely, when the Topp conversion model is used for soil moisture inversion, the coefficient of the soil dielectric constant model under each frequency is not required to be calibrated one by one, the problem that one frequency point corresponds to one dielectric constant is solved, and the soil moisture model inversion can be directly carried out through any section of frequency spectrum (namely a plurality of frequency points).
Authentication and application: and verifying by using an independent soil sample, and evaluating the accuracy and stability of the formulated soil moisture content estimation method. After the dielectric spectrums of the soil under the conditions of 6 groups of different volume water contents are actually measured, the characteristic parameters of the apparent dielectric spectrums of the soil under the conditions of different volume water contents are extracted. Under the condition of the optimal predicted frequency point of 1MHz, the apparent dielectric spectrum characteristic parameters are converted based on a Topp conversion model, and the soil moisture content estimation evaluation is carried out. The volume water content of 6 groups of soil samples is actually measured by adopting a drying method, and the estimated effect of the apparent dielectric spectrum characteristic parameters on the soil water content is verified by comparison. As can be seen from table 4, the relative errors between the predicted and measured values of the volume water contents of the 6 groups of soil samples were smaller than 0.012.
Table 4 6 actual measurement results of soil samples
Fig. 9 is a graph of the predicted value and the measured value of the soil volume moisture content, and as shown in fig. 9, the abscissa represents the measured value of the soil volume moisture content, the ordinate represents the predicted value of the soil volume moisture content obtained based on the dielectric spectrum characteristic parameter, and the solid line represents the contour line of the measured value and the predicted value. It can be seen that the data points are very close to the contour lines, the predicted value of the water content of the soil volume obtained through inversion calculation of the dielectric spectrum characteristic parameters is basically consistent with the measured value, and the error is relatively small. Therefore, the soil apparent dielectric spectrum characteristic parameters provided by the application can be used for accurately predicting the soil moisture content, namely in actual engineering, the soil dielectric spectrum can be directly measured through the electromagnetic wave emission and receiver, then the characteristic parameters corresponding to the dielectric constant real part-frequency characteristic curve and the dielectric constant imaginary part-frequency characteristic curve are extracted, and the characteristic parameters are brought into a dielectric constant conversion model, so that the moisture content of the soil in a certain frequency spectrum range can be represented.
The technology solves the problem that the traditional dielectric constant model can only carry out soil moisture inversion under one frequency condition, and can directly measure a section of dielectric spectrum or invert under any frequency condition to obtain the soil moisture content.
The invention has the following beneficial effects:
1. the frequency domain dielectric spectrum is adopted to estimate the soil moisture content, so that the estimation precision and efficiency can be effectively improved, and real-time monitoring can be realized;
2. the dielectric spectrum data can be directly converted into soil moisture content by a frequency domain dielectric spectrum integration method, so that the problems of frequency dependence of frequency domain dielectric spectrum model parameters and complex data processing and analysis are avoided;
3. by optimizing the equipment and the signal processing method, the measurement precision and stability can be improved, and meanwhile, the complexity of the equipment and the operation is reduced;
4. the invention utilizes the pre-established frequency domain dielectric spectrum models (1) and (7) to convert the frequency domain dielectric spectrum data into the soil moisture content, thereby establishing an accurate and effective model and being capable of accurately converting the dielectric spectrum data into the moisture content.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solution of the present invention, and are not limiting; although the invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical scheme described in the foregoing embodiments can be modified or some technical features thereof can be replaced by equivalents; such modifications and substitutions do not depart from the spirit and scope of the technical solutions of the embodiments of the present invention.
Claims (1)
1. The soil moisture content evaluation method based on the frequency domain dielectric spectrum is characterized by comprising the following steps of: the following soil moisture inversion model is adopted to obtain the soil moisture content:
（1）
wherein,for volume water content, +.>、/>、/>、/>Is an empirical parameter; />Is the characteristic parameter of the apparent dielectric spectrum of the soil;
（7）
is a dielectric loss factor characteristic parameter, < >>Is the real part of the dielectric constant;
the characteristic parameters of the apparent dielectric spectrum of the soilObtained according to the following way:
step 1: measuring the real part and the imaginary part of the dielectric constant of the soil by adopting different frequencies, and respectively and correspondingly obtaining a characteristic curve of the real part of the dielectric constant changing along with the frequency and a characteristic curve of the imaginary part of the dielectric constant changing along with the frequency;
step 2: fitting according to formula (2) by using the characteristic curve of the real part of the dielectric constant changing along with the frequency to obtain fitting parameters、/>、/>、/>；
（2）
Wherein,is frequency;
step 3: fitting according to formula (3) by utilizing the characteristic curve of the dielectric constant imaginary part changing along with the frequency to obtain fitting parameters、/>、/>、/>；
（3）
Wherein,is the real part of the dielectric constant, < >>Is the imaginary part of the dielectric constant; />Is frequency;
step 4: according to the fitting parameters、/>、/>、/>、/>、/>、/>、/>Deriving any point of a characteristic curve of the real part of the dielectric constant along with the change of frequency, wherein the derivation relation is shown in a formula (4):
（4）
step 5: according to the fitting parameters、/>、/>、/>、/>、/>、/>、/>And deriving any point of a characteristic curve of the dielectric constant imaginary part changing along with the frequency, wherein the derivation relation is shown in a formula (5):
（5）
wherein,for the real part dielectric spectrum characteristic parameter, < >>Is an imaginary dielectric spectrum characteristic parameter;
step 6: according to formulas (4) and (5), calculating to obtain the dielectric loss factor characteristic parameter：
（6）
Step 7: according to the dielectric loss factor characteristic parameterThe real part of the dielectric constant +.>Calculating to obtain the characteristic parameter +.>：/>（7）。
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| CN1719245A (en) * | 2005-08-02 | 2006-01-11 | 中国农业大学 | Method and sensor for synchronous real time measuring water content and conductivity of soil |
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