A method for monitoring in situ the amount of water infiltration and the advance of the wetting front in soil

Abstract

The application discloses a method for monitoring soil water infiltration and wetting front advance in situ, and vertically inserts a TDR probe into soil; uses a time domain reflectometer to measure TDR reflection signals before and during the infiltration process; extracts apparent position information of the probe head, tail and wetting front in the TDR reflection signal waveform, and then calculates the apparent length of the TDR probe and the wetting zone; and uses electromagnetic wave transmission theory to calculate the total infiltration water amount and the wetting front position. The application is simple to operate, and can realize the monitoring of the infiltration process only by inserting the probe into the infiltration area; the method is flexible, does not need to prepare special infiltration devices, and is suitable for any form of field infiltration conditions; the application has high integration, does not need other measurements, and can realize the high-frequency monitoring of the cumulative infiltration amount and the wetting front advance simultaneously by using one probe; and the application has strong applicability, and can be used in different texture soils. The application provides a powerful tool for soil hydrological property investigation and multi-scale soil hydrological process research.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for monitoring soil water infiltration and wetting front advancement in situ, belonging to the technical field of farmland water conservancy and soil hydrology measurement. BACKGROUND

[0002] Infiltration is one of the most basic links of soil hydrological cycle, which not only determines how much water will form runoff into rivers and lakes and how much water will form net rain into soil in a rainfall process, but also determines how much water will enter the soil wetting root zone and how much water will flow out of the root zone to form seepage in a irrigation process. The measurement of infiltration and wetting front during the infiltration process helps to determine the soil infiltration capacity and the critical time for the wetting front to reach the entire root zone, providing reasonable parameters for the design of farmland irrigation system, and can also provide real-time reference data for farmland irrigation management. In addition, complete and accurate field infiltration process observation data can be used to predict multi-scale soil hydraulic properties in the field (Ma et al., 2017), providing parameters for accurate simulation of soil hydrological processes and effective tools for multi-scale soil hydrological process research.

[0003] Currently, there are two types of field soil infiltration process observation: one is to use special infiltration test devices such as disc infiltration meter, single / double ring infiltration meter, and Guifu instrument, to carry out separate infiltration tests before rainfall and irrigation, and to obtain soil cumulative infiltration by observing the change of water level in the water supply pipe of the infiltration device, but this method cannot obtain the wetting front advancement process. The other is to use pre-embedded soil profile water observation tubes or soil moisture sensors, and to use neutron probes or profile moisture instruments to observe natural rainfall or irrigation process and soil moisture changes before and after irrigation to obtain cumulative infiltration and rough wetting front position. The manual mode of observation has low frequency, which is difficult to meet the needs of soil hydraulic property prediction, and the automatic mode is too expensive and complex to install, which is not suitable for multi-point application.

[0004] Recently introduced geophysical observation methods, such as high-density electrical resistivity transilluminators (HDTs) and ground-penetrating radar (GPR), can obtain cumulative soil infiltration and wetting front advancement on a larger scale by non-destructively observing two-dimensional and even three-dimensional changes in resistivity and dielectric constant on soil profiles caused by infiltration. These methods offer large monitoring scales and strong representativeness. However, their monitoring accuracy is limited, they are expensive, and their application is restricted by high technical requirements. Other methods for determining the location of wetting fronts independently include conductivity methods (Rohit et al., 2000), temperature methods, and penetration methods (Wells et al., 2007), but they cannot simultaneously measure cumulative infiltration. Time-domain reflectometry (TDR) is currently one of the most reliable methods for automatically measuring soil moisture content. In fact, it was discovered long ago that TDR can monitor heterogeneous changes along the probe direction, a characteristic that gives it great potential for monitoring soil infiltration processes. However, current methods require waveform fitting, are overly complex to operate, and their effectiveness in different soil types is unknown, with no recorded application in fieldwork. Overall, there is still a lack of a simple, reliable, cost-effective method that can simultaneously measure the cumulative infiltration amount and the advance of the wetting front during the soil infiltration process in the field, as well as conduct in-situ multi-point simultaneous observations. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for in-situ monitoring of soil water infiltration and wetting front advance.

[0006] The technical solution adopted in this invention is: a method for in-situ monitoring of soil water infiltration and wetting front advance, the steps of which include:

[0007] (1) Before infiltration, insert a TDR probe of length L into the soil to be tested at a basically vertical position.

[0008] (2) Conduct infiltration experiments. Before water infiltration begins, use a time domain reflectometer to measure and collect the TDR probe reflection signal at a certain frequency. Continue collecting until the wetting front exceeds the probe length and the probe monitors the soil layer to reach saturation.

[0009] (3) There was no wetting front in the TDR measurement area before and after infiltration. The apparent positions of the probe head and tail before infiltration were extracted from the TDR reflection signal waveform, and then the apparent length L of the soil layer monitored by the initial TDR probe was obtained. ad The apparent positions of the probe head and tail at the end of infiltration are also obtained, thus acquiring the apparent length L of the soil layer monitored by the TDR probe at saturation. as The initial average dielectric constant K of the monitored soil layer was calculated using electromagnetic wave propagation theory. ad =(L ad / L) 2 and the average dielectric constant K at saturation as =(L as / L)2 Then, the K ad is converted into the initial soil water content θ as and the saturated water content θ i by using the relationship between the soil water content and the dielectric constant. s ;

[0010] (4) After the infiltration starts, the apparent positions Z h , Z w and Z e of the probe head, the wetting front and the probe tail corresponding to the TDR reflection signal waveform are extracted from the TDR reflection signal waveform before the wetting front reaches the probe tail, and the apparent length L a of the monitoring soil layer and the apparent length L aw of the wetting zone are calculated, respectively, and then the average dielectric constant K a of the whole monitoring soil layer and the average dielectric constant K a of the wetting zone are calculated. 2

[0011] K w = (L aw / L) 2 ;

[0012] (5) The apparent length L a of the monitoring soil layer is converted into the real-time average water content θ by using the relationship between the soil water content and the dielectric constant, and then the cumulative infiltration water content I = L*(θ-θ i ) is calculated.

[0013] (6) The actual length L w of the wetting zone is calculated by using the electromagnetic wave transmission theory and the soil water movement theory.

[0014] Preferably, the apparent positions of the wetting front and the probe tail in the TDR waveform in step (4) are extracted by using the second derivative method, and the specific steps are as follows: the first derivative and the second derivative of the waveform data are calculated, and after the position of the maximum value of the first derivative appears, the first zero point of the first derivative of the waveform data is determined as the apparent position Z h of the probe head; the second and third local maximum values of the second derivative after Z h appear, and the positions of the second and third local maximum values are the apparent positions Z w of the wetting front and the apparent position Z e of the probe tail, respectively; the apparent length L a of the monitoring soil layer is the distance between the apparent positions of the probe head and the tail Z e -Z h , and the apparent length L aw ​Z is the distance between the probe head and the apparent position of the wetting front w -Z h .

[0015] Preferably, the formula for calculating the actual length of the wetting zone in step (6) is: L w = L*(L ad -L a +L aw ) / L ad .

[0016] Preferably, the frequency of the time domain reflectometry measurement to collect the TDR probe reflection signal is 10s-2min.

[0017] The present application can be used in single-probe mode or multi-probe mode, and multiple points can be measured simultaneously. Steps (4)-(6) are used to simultaneously interpret the cumulative infiltration and wetting front position information from one TDR waveform, rather than measuring them independently.

[0018] The present application is based on electromagnetic wave transmission theory. The TDR conductive wave propagates in heterogeneous media, and the dielectric properties of the dielectric and the spatial heterogeneity of the dielectric properties will cause changes in the signal in the transmitted wave. Because water has a much larger dielectric constant than the solid phase of soil (water: 80; solid phase: 2-3), during the soil water infiltration process, water enters the soil, causing the water content of each layer of soil to increase, which in turn causes the soil dielectric constant to change, which is the main reason for the change in the TDR signal reflection wave shape. Therefore, changes in the signal can be used to obtain changes in soil water content and dry-wet zone position. The present application solves the problem of analyzing the TDR reflection waveform of heterogeneous media during the infiltration process, is simple to operate, has high flexibility, is convenient for multi-point field layout, can simultaneously measure the cumulative infiltration and wetting front advance, is a highly efficient and practical soil infiltration process measurement method with high accuracy, and can be used for the design and decision management of field irrigation systems, as well as the prediction of soil pore distribution characteristics or soil hydraulic property parameters based on the infiltration process. The present application can be used for soil water infiltration monitoring in any situation, including laboratory simulation experiments and field experiments, and the infiltration methods include rainfall, water accumulation, irrigation, and other forms.

[0019] The beneficial effects of the present application are as follows:

[0020] 1. Lightness. The present application requires fewer devices, does not require any infiltration device, is convenient to carry in the field, has fewer operation steps, and the process is simple and easy to master. It can be conveniently applied anywhere with infiltration, and only needs to insert the probe into the infiltration area to realize the monitoring of indoor water accumulation and rainfall infiltration test, field flooding, sprinkling irrigation, drip irrigation, and precipitation infiltration process.

[0021] 2. Integrated Functionality. Traditional field infiltration observation methods can only measure cumulative infiltration or wetting front, and the measurement accuracy of the wetting front during the infiltration process is poor and the frequency is low. Furthermore, it is impossible to simultaneously achieve high-frequency measurement of both. This invention uses a single probe to automatically and simultaneously observe initial moisture content, saturated moisture content, infiltration start and end times, cumulative infiltration, and the advance of the wetting front. Its highly integrated functionality requires no additional measurements and is far superior to any known infiltration measurement method.

[0022] 3. It has scalability. This invention only requires a single probe to measure infiltration at one location. Figure 1 a) This can easily be expanded to a single infiltration process, with simultaneous measurements at multiple points using multiple probes. Figure 1 b) Facilitates repeated observations of spatially variable soils.

[0023] 4. It is economical and efficient. Compared with the current disc infiltrator, single-ring and double-ring infiltrator methods, this invention has no special requirements for soil characteristics, underlying surface conditions and infiltration methods. It can be used not only on flat land, but also on slopes. It can simultaneously measure infiltration at multiple points and simultaneously measure the advance of the moist front. It requires less preparation time, has low labor costs, and greatly improves efficiency, making it more suitable for large-scale field measurements.

[0024] 5. High potential application value. This invention not only solves the problem of accurately observing infiltration volume and wetting front during irrigation infiltration processes, which are directly related to irrigation design, but also solves the problem of observing infiltration processes under various underlying surface conditions in the field. Because accurate in-situ observation of multi-scale infiltration processes is an important prerequisite for studying multi-scale soil pore structure characteristics and predicting soil hydrological process parameters, this invention provides a powerful tool for investigating soil pore structure characteristics in the field and for studying multi-scale soil hydrological processes. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the measurement modes of the present invention, wherein (a) is the single-probe measurement mode and (b) is the multi-probe measurement mode.

[0026] Figure 2 The diagram shows the TDR signal characteristics with and without a moistening peak, where (a) shows the TDR signal characteristics without a moistening peak and (b) shows the TDR signal characteristics with a moistening peak.

[0027] Figure 3 This is a schematic diagram of the TDR waveform analysis method with wetting front proposed in this invention.

[0028] Figure 4Comparison of cumulative infiltration and wetting front obtained by the method of the present application with other single methods, wherein (a) is comparison of cumulative infiltration obtained by the method of the present application with the weighing method, and (b) is comparison of wetting front advancing process obtained by the method of the present application with the manual observation method;

[0029] Figure 5 Field application scenarios of the method of the present application and obtained cumulative infiltration and wetting front advancing, wherein (a) is a field application scenario of multi-probe observation mode, (b) is an example of TDR waveform change observed by one probe, (c) is an example of cumulative infiltration process obtained by one probe, and (d) is an example of wetting front advancing process obtained by one probe.

[0030] The present application will be further described below in conjunction with specific examples. DETAILED DESCRIPTION

[0031] The content of the present application will be further described below in conjunction with soil infiltration tests, but should not be understood as a limitation on the present application. Modifications and replacements made to the method, steps or conditions of the present application without departing from the spirit and essence of the present application shall all fall within the scope of the present application. If not specifically indicated, the technical means used in the following examples are conventional means familiar to those skilled in the art.

[0032] Experimental purposes: In Example 1, four representative soils in the black soil region of Northeast China, oasis of Xinjiang, red soil region in the south, and Loess Plateau are selected, i.e. Harbin black soil, Xinjiang sandy soil, Yingtan red soil, and Yan'an loess soil. The accuracy and applicability of the method of the present application in determining cumulative infiltration and wetting front advancing are verified by comparison with the traditional weighing method and manual observation method. In Example 2, a field irrigation test of typical fluvo-aquic soil in the Huang-Huai-Hai Plain is selected to verify the stability of the method of the present application in monitoring cumulative infiltration and wetting front advancing under field conditions, and to provide a reference example for field application of the method of the present application.

[0033] Example 1

[0034] 1. Materials and methods

[0035] The tested soils are selected to be typical farmland soils with large differences in north and south properties: loess soil, collected in Baota District of Yan'an City, Shaanxi Province (36°44'N, 109°35'E); sandy soil, collected in Xini'er Town of Korla City, Xinjiang (41°35'N, 81°10'E); black soil, collected in Daowai District of Harbin City, Heilongjiang Province (45°49'N, 126°50'E); red soil, collected in Yujiang District of Yingtan City, Jiangxi Province (28°20'N, 116°95'E). After the soil samples are collected, they are brought back to the laboratory, dried, crushed, sieved through a 2mm sieve, and uniformly mixed as test soil. The particle composition of the four soils is shown in Table 1.

[0036] Table 1. Main physicochemical properties of the tested soils

[0037]

[0038] The infiltration test system consisted of a Marvin bottle and a soil column, both made of plexiglass. The soil column was 35cm long with an inner diameter of 19cm, and the base was a porous flange plate. The Marvin bottle had an inner diameter of 19cm and a length of 50cm. The monitoring system consisted of a platform scale (30kg capacity, 2g accuracy) capable of automatically acquiring data, a 30cm long TDR probe, a CR1000 data logger, a TDR200 time domain reflectometer, and a 12V battery. Before filling the soil column, filter paper was placed on the porous plate at the bottom to allow air to pass through without soil leakage. Four types of soil were weighed and placed into the plexiglass column in six layers according to the bulk density in Table 1, roughly consistent with the bulk density measured during field sampling. After each layer of soil was filled, it was compacted to the corresponding mark using a special tool, and then the surface was roughened before the next layer was filled to prevent physical stratification. The soil was filled to a depth of 30cm, the same length as the TDR probe, with a 5cm space left at the top of the soil column to create conditions for water accumulation and infiltration. After the soil loading is completed, the TDR probe is vertically inserted into the soil from the surface, and the soil column is placed on a platform scale. The system monitoring time interval is set to 10 seconds, the TDR200 reflection waveform monitoring parameters are set, and the initial water level in the Marble bottle is recorded before infiltration. Monitoring is then initiated to obtain the reflection waveform information of the initial soil state and the initial system weight. After the infiltration test begins, in addition to automatically monitoring the changes in soil column weight and TDR reflection waveform information, the change in the wetting front advancement distance over time is manually recorded until the soil column becomes saturated with water, ending the test. The cumulative infiltration data can be accurately obtained by recording the changes in soil column weight before and after infiltration. According to steps 3-6 of the method of this invention, the process of extracting the cumulative infiltration and wetting front from the TDR waveform data is as follows:

[0039] Step 3: Since there was no wetting front in the TDR measurement area before and after infiltration, the apparent positions of the probe head and tail were extracted from the TDR reflected signal waveform using the tangent method proposed by Or, et al. (2004). Figure 2 a) thereby obtaining the apparent length L of the soil layer monitored by the TDR probe at initial and saturation. ad and L as The initial average dielectric constant K of the monitored soil layer was calculated using electromagnetic wave propagation theory. ad =(L ad / L) 2 and the average dielectric constant K at saturation as =(L as / L) 2 Then, using the formula from Topp et al. (1980), θ = 0.53 * 0.1 + 2.92 * 0.01 * K. a -5.5*0.0001*K a2 +4.3*0.000001*K a 3 Convert K ad and K as to the initial and saturated water content of soil θ i and θ s ;

[0040] Step 4, after the start of infiltration, the wetting front in the TDR measured interval, using the second derivative method from the TDR reflection signal waveform before the wetting front reaches the tail of the probe to extract the apparent position of the probe head, tail and wetting front (Z Figure 3 ): first remove the noise of the original signal with 2 times standard deviation, and then use the sliding average method to smooth the data, and then take the first and second derivatives of the waveform data, after the position of the first maximum value P1 of the first derivative appears, determine the position of the first zero point O1 of the first derivative as the apparent position Z h of the probe head corresponding to the waveform data, the positions of the second and third local maximum values K2 and K3 of the second derivative between the first derivative zero point O1 and the second maximum value P2 are the apparent positions Z w and Z e of the TDR probe tail corresponding to the wetting front, monitor the apparent length L a of the soil layer, which is the distance between the apparent positions of the probe head and tail Z e -Z h , the apparent length L aw of the wetting zone is the distance between the apparent positions of the probe head and the wetting front Z w -Z h , the average dielectric constant K a of the entire monitoring soil layer is (L a / L) 2 , the average dielectric constant K w of the wetting zone is (L aw / L) 2 ;

[0041] Step 5, use Topp et al. (1980) formula θ = 0.53*0.1 + 2.92*0.01*K a -5.5*0.0001*K a 2 +4.3*0.000001*K a 3 Convert the average dielectric constant K a of the TDR probe monitoring soil layer to the real-time average water content θ, and then calculate the cumulative infiltration water content I = L*(θ-θ i );

[0042] Step 6: Calculate the actual location L of the wetting front using electromagnetic wave propagation theory and soil moisture movement theory. w =L*(L ad -L a +L aw ) / L ad .

[0043] 2. Experimental Results

[0044] Soil properties, such as texture and organic matter, are important factors affecting the TDR measurement results. Harbin black soil, Yan'an loess, Korla sandy soil, and Yingtan red soil represent the main soil types in our major grain-producing areas. These regions have a large geographical span and significant differences in soil occurrence environments. Soil types, clay minerals, texture, and organic matter also vary considerably (Table 1). Using these soil types to verify the accuracy and applicability of this invention has universal significance.

[0045] The cumulative infiltration and wetting front of the four soil types measured by the weighing method and the method of this invention are plotted separately. Figure 4 a and 4b. (By...) Figure 4 It can be seen that the cumulative infiltration amounts of Harbin black soil, Yan'an loess soil, Korla sandy soil, and Yingtan red soil obtained by the method of this invention are basically consistent with the measurement results of the traditional standard method (weighing method). Compared with the weighing method, the root mean square error (RMSE) of the method of this invention is 0.41 cm, 1.07 cm, 0.42 cm, and 0.83 cm, respectively. Except for Yan'an loess soil, the average relative error of the cumulative infiltration amount is less than 10%. The wetting front of Yan'an loess soil and Korla sandy soil measured by the method of this invention has the best agreement with the results of manual observation, with relative errors of less than 5%. The prediction error of Yingtan red soil is also generally within 10%. The measurement error of the cumulative infiltration amount of Yan'an loess soil may be due to the generation of preferential flow, which causes the water flow to escape from the TDR monitoring area locally. Harbin black soil is heavy and clayey with high organic matter content, resulting in rapid TDR signal attenuation. This leads to an indistinct apparent position of the probe tail on the waveform, causing errors in the cumulative infiltration measurement. However, the wetting front position is very clear even on the black soil TDR waveform, and the apparent position of the wetting front determined by the method of this invention is accurate. The accurate determination of TDR waveform analysis and moisture content for clayey organic soils can be corrected through specialized calibration experiments using the relationship between soil dielectric constant and soil moisture content, in addition to the Topp formula. There is ample information available in the literature on this, which will not be elaborated here. Therefore, the cumulative infiltration and wetting front positions obtained by this invention are relatively reliable and accurate, and have strong adaptability to differences in soil type, texture, and characteristics, especially sandy and loamy soils.

[0046] Example 2

[0047] 1. Materials and Methods

[0048] The field experiment was conducted at the Fengqiu Agroecological Experimental Station of the Chinese Academy of Sciences (35°00′N, 114°24′E) in Fengqiu County, Xinxiang City, Henan Province, as a long-term water-nitrogen coupling experimental site. Fengqiu County is located in northeastern Henan Province, under the jurisdiction of Xinxiang City, in the central part of the Huang-Huai Plain. It has a warm temperate continental monsoon climate with an average annual temperature of 13.9℃ and an average annual precipitation of 615.1 mm, which is unevenly distributed throughout the year. The main crop is winter wheat-summer maize rotation. Water is one of the most important limiting factors for grain production in this region, with an annual water deficit of approximately 311 mm. Irrigation is a key measure to ensure grain production. The experimental soil is typical alluvial soil, and its main physical properties are shown in Table 2. This field experiment was conducted on September 17, 2019, during the irrigation period at the long-term water-nitrogen coupling experimental site, located in plot 21. The experimental treatment was a nitrogen fertilizer application of 190 kg N / ha per season, and the irrigation amount during the experiment was 100 mm.

[0049] Table 2. Main physicochemical properties of the tested soils

[0050]

[0051] Test monitoring system such as Figure 1 As shown in b, a multi-probe mode is adopted (here, multi-probe mode means that each probe measures, calculates, and reflects the seepage status at each measurement point separately), including 14 TDR probes (30cm long), a CR1000 data acquisition unit, a TDR200 time domain reflectometer, and a 220V AC to 12V DC converter. Before irrigation, the probes are vertically inserted into different positions within the irrigation area. Figure 5 a) The TDR200 reflection waveform monitoring parameters were set, with a time interval of 2 minutes. Five minutes before the experiment, the TDR system was started to measure the initial soil reflection waveform before irrigation. After irrigation began, the location of surface water reaching each probe was observed until irrigation ended and there was no surface water, at which point the experiment was terminated. According to steps 3-6 of the method of this invention, the cumulative infiltration rate was extracted using the TDR waveform data. Figure 5 c) and moist fronts ( Figure 5 The process for d) is as follows:

[0052] Step 3: Since there is no wetting front in the TDR measurement interval before and after infiltration, the apparent positions of the probe head and tail are extracted from the TDR reflection signal waveform using the tangent method proposed by Or, et al. (2004), thereby obtaining the apparent length L of the soil layer monitored by the TDR probe at the initial and saturation stages. ad and L as The initial average dielectric constant K of the monitored soil layer was calculated using electromagnetic wave propagation theory. ad =(L ad / L) 2 and the average dielectric constant K at saturation as =(Las / L) 2 Then, the Topp et al. (1980) formula θ = 0.53*0.1 + 2.92*0.01*K a -5.5*0.0001*K a 2 +4.3*0.000001*K a 3 Convert K ad and K as to the initial and saturated water content of the soil θ i and θ s ;

[0053] Step 4, after the start of infiltration, the wetting front in the TDR measured interval, using the second derivative method from the TDR reflection signal waveform before the wetting front reaches the tail of the probe to extract the apparent position Z h , Z e and Z w of the probe head, tail and wetting front. Monitor the apparent length L a = Z e -Z h of the soil layer, the apparent length L aw = Z w -Z h of the wetting zone, the average dielectric constant K a = (L a / L) 2 of the entire monitoring soil layer, and the average dielectric constant K w = (L aw / L) 2 of the wetting zone;

[0054] Step 5, using the Topp et al. (1980) formula θ = 0.53*0.1 + 2.92*0.01*K a -5.5*0.0001*K a 2 +4.3*0.000001*K a 3 Convert the average dielectric constant K a of the TDR probe monitoring soil layer to the real-time average water content θ, and then calculate the cumulative infiltration water content I = L*(θ-θ i );

[0055] Step 6, using electromagnetic wave transmission theory and soil water movement theory to calculate the actual position of the wetting front L w = L*(L ad -L a +L aw ) / L ad .

[0056] 2. Experimental results

[0057] Compared with the indoor packed soil column, there are two problems in monitoring the field soil infiltration process during the precipitation or irrigation process. One is that the texture and water content distribution of the soil profile may not be so uniform, and the other is that the field soil may not be so flat, and the water depth may also fluctuate. Whether the TDR signal reflection waveform monitored can still be extracted by the method of the present application, and whether the extracted results have stability and continuity need to be determined. As an example, Figure 5 shows the waveform monitored by a 30cm TDR probe in a field test of a meadow soil irrigation farmland over time Figure 5 b), and the cumulative infiltration Figure 5 c) and the wetting front Figure 5 d) change conditions extracted by the method of the present application. The results show that the fluctuation of the TDR signal waveform observed in the field soil irrigation is slightly larger than that of the indoor soil column observation, but the overall shape does not change fundamentally, and the apparent position of the wetting front and the TDR probe head and tail is still clear and variable. The smooth and continuous cumulative infiltration and wetting front change curve can be obtained by using the method of the present application, which shows that the method of the present application has sufficient stability and continuity in field application. In addition, Figure 5 The results of b show that when the wetting front reaches near the end of the probe, it is difficult to distinguish the apparent position of the wetting front and the tail of the probe, and the error will increase. The effective interval of the wetting front extraction in the actual measurement should be slightly smaller than the probe length. ​ The results of d show that the field soil structure is good, and the infiltration process is very fast. It is difficult to grasp the wetting front process if the monitoring frequency is too low. A relatively high monitoring frequency should be maintained in actual monitoring. In terms of this case, a monitoring frequency of 2min can basically reflect the change of the wetting front. If a monitoring interval of less than 1min is used, a better monitoring effect is expected.

Claims

1. A method of monitoring in situ the amount of water infiltration and the advancement of the wetting front in soil, characterized in that The steps include: (1) before water infiltration, the TDR probe with length L is vertically inserted into the soil to be measured until the head of the probe is flush with the soil; (2) before water infiltration, the TDR probe reflection signal is measured by the time domain reflectometer at a certain frequency, and the collection is continued until the wetting front exceeds the probe length, and the soil layer monitored by the probe reaches the saturated state; (3) There was no wetting front in the TDR measurement section before and after infiltration. The apparent positions of the probe head and tail before infiltration were extracted from the TDR reflection signal waveform, and then the apparent length L of the soil layer monitored by the initial TDR probe was obtained. ad The apparent positions of the probe head and tail at the end of infiltration are also obtained, thus acquiring the apparent length L of the soil layer monitored by the TDR probe at saturation. as The initial average dielectric constant K of the monitored soil layer was calculated using electromagnetic wave propagation theory. ad =( L ad / L) 2 and the average dielectric constant K at saturation as =(L as / L) 2 Then, using the relationship between soil moisture content and dielectric constant, K ad and K as Converted to initial soil moisture content θ i and saturated water content θ s ; (4) After infiltration begins, the apparent positions Z corresponding to the probe head, the wetted front, and the probe tail are extracted from the TDR reflected signal waveform before the wetted front reaches the probe tail at each time point within the TDR measurement range. h Z w and Z e Calculate the apparent length L of the soil layer monitored by the TDR probe. a and the apparent length L of the wetted area aw Then, the average dielectric constant K of the entire monitored soil layer was calculated. a =( L a / L) 2 and the average dielectric constant K of the wetted region w =( L aw / L) 2 ; (5) Using the relationship between soil water content and dielectric constant, the average dielectric constant K of the soil layer is monitored by the TDR probe a converted into real-time average water content θ, and then the cumulative soil infiltration water I = L*(θ - θ i ) is calculated. (6) The actual length L of the wetting zone is calculated by using the electromagnetic wave transmission theory and the soil moisture movement theory w , and the specific calculation formula is: L w = L*( L ad - L a + L aw ) / L ad .

2. The method for monitoring soil water infiltration and wetting front advancement in-situ according to claim 1, wherein: The apparent positions of the wetting front and probe tail in the TDR waveform in step (4) are extracted using the second derivative method, as follows: The first and second derivatives of the waveform data are calculated respectively. After the location where the maximum value of the first derivative occurs, the first zero point of the first derivative of the waveform data is determined to be the apparent position Z corresponding to the head of the TDR probe. h The second derivative in Z h The locations where the second and third local maxima occur are the apparent locations Z corresponding to the wetting front, respectively. w The apparent position Z corresponding to the tail of the TDR probe e Monitoring the apparent length L of the soil layer a Z is the distance Z between the apparent positions of the probe head and tail. e -Z h The apparent length L of the moist front aw Z is the distance between the probe tip and the apparent position of the wetted front. w -Z h .

3. The method for monitoring soil water infiltration and wetting front advancement in situ according to claim 1 or 2, characterized in that: The frequency of the time domain reflectometer for measuring and collecting the TDR probe reflection signal is 10s-2min.

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