Soil moisture content is the most direct indicator of drought. At home and abroad, soil moisture monitoring has been carried out since the middle of the 20th century. Various measurement methods have been studied at home and abroad. At present, drying weighing, tensiometer, neutron moisture meter and time domain reflectometer, frequency domain are mainly used. Measurement methods such as transmitters. Although these methods can measure soil moisture, the principles and characteristics are different. Looking at the status quo of automatic monitoring of domestic public opinion, there is currently no product widely used in the field, and there is no leading product to achieve automatic collection and transmission of large-scale public opinion information.
With the planning and implementation of the national drought control command system, automatic monitoring of public opinion in all provinces and municipalities is about to start in full swing. Especially in the spring of 2010, some provinces (regions) in southwest China have emerged with drought, and there is an urgent need for automated soil moisture monitoring instruments and information transmission systems. To obtain continuous and reliable soil moisture information and provide basic data for regional drought analysis. In this paper, according to the national drought monitoring system construction needs, the investigation and research work of soil moisture monitoring and sensing instruments at home and abroad is carried out, and the research products are classified and analyzed.
1 Classification and characteristics analysis of soil moisture monitoring instruments
According to the measurement principle, soil moisture monitoring instruments can be divided into the following types:
1) Time domain reflectometry (TDR);
2) Time domain transmission type instrument (TDT);
3) Frequency Domain Reflective Instrument (FDR);
4) Neutron Probe (Neutron Probe);
5) Tension meter (Tension meter);
6) Resister Method.
Although the traditional drying method requires some equipment, it is not a category of soil moisture monitoring equipment, it is only a method. The content and method of the drying method are clearly defined in Section 5.2 of the SL364-2006 Soil Moisture Monitoring Regulations. The current drying method is still the only method to verify the accuracy of the instrument [1].
1.1 Time Domain Reflectometer Instrument (TDR)
TDR is an important instrument for measuring soil water content in recent years. It is an instrument developed by measuring the difference between the dielectric constant of water and other media in the soil, and using time domain reflectometry technology. It is fast, convenient and The advantage of continuous observation of soil water content.
Since the dielectric constants of air, dry soil and water are relatively fixed, if the relationship between specific soil and dielectric constant is known, the effective dielectric constant of soil moisture can be measured indirectly. According to the basic principle that the propagation speed of electromagnetic waves in the medium is related to the dielectric constant of the substance surrounding the transport body, the dielectric constant between the dry soil and the water is very different, so the technique theoretically establishes the soil moisture. The measurement has good response and sensitivity.
The apparent dielectric constant K a of the soil can be converted as follows (Dirksen, 1999):
Where: T is the time of propagation of the electromagnetic wave propagation waveguide head, ns; L is the length of the waveguide head for measurement, cm; C is the propagation speed of electromagnetic wave in vacuum, cm/ns.
Or directly express the transmission time T and the dielectric constant Ka as:
The time required for the TDR signal pulse to propagate from the start point to the end of the waveguide head can be determined by measurement.
The TDR characteristics are analyzed as follows:
1) Time domain reflection method The soil moisture monitoring instrument emits high frequency waves along the waveguide head buried in the soil. The transmission speed (or transmission time) of the high frequency wave in the soil is related to the dielectric constant of the soil, and the dielectric constant is related to the water content of the soil. In this way, measuring the transmission time or speed of the high-frequency wave can directly measure the water content of the soil. In theory, this is the technique for measuring the highest accuracy of soil moisture monitoring.
2) Because the electromagnetic wave transmission speed is very fast, the accuracy of the TDR measurement time needs to be 0.1 ns, so the time circuit cost of the TDR is high, and the measurement result is less affected by the temperature.
3) The emission and measurement of the high frequency wave of the TDR moisture sensor is completed in the sensor body. When working, a high frequency electromagnetic wave of 1 GHz or higher is generated. The transmission time is picosecond. The output signal is generally an analog voltage signal, which can accurately express the soil at the insertion point. Moisture. The TDR soil moisture sensor can also output 4 to 20 mA, or 232 serial interface data, depending on the signal acquisition requirements. The above output of the TDR is easily connected to a conventional data collector to form an automatic measurement system.
4) The TDR soil moisture sensor currently on the market is a typical point type soil moisture measuring instrument. It is small in size, light in weight, and can be replaced by a single sensor. It is easy to operate and maintain.
The TDR soil moisture sensor body is a sealed probe with a probe. When the probe is fully inserted into the soil, the measurement output signal is output through a wired cable, and can be connected to a telemetry terminal or a handheld meter. A schematic diagram of moisture monitoring of TDR products is shown in Figure 1.
1.2 Time Domain Transmission Instrument (TDT)
TDT technology is another kind of soil moisture measurement technology. The characteristic of TDT technology is that electromagnetic wave is one-way propagation in the medium. It is not required to obtain the reflected signal after detecting the signal after one-way transmission of electromagnetic wave. The technique is also based on the difference in soil dielectric constant to determine soil moisture content [2].
The TDT characteristics are analyzed as follows:
1) The moisture analyzer developed by the TDT principle has a lower operating frequency, the circuit design is simpler, and the cost is lower than that of the TDR instrument;
2) The typical product is a strip-shaped soil moisture sensor, which has potential for popularization and application in some soil type heterogeneous soil types;
3) The output signal of the moisture analyzer developed based on the TDT principle is generally analog, and can be connected to a conventional data collector to form an automatic measurement system.
Figure 2 shows an application example of a strip-shaped TDT moisture sensor with a length of 3 m. The soil moisture at the root of the plant measured in the soil is the mean value of the soil moisture around the cylinder of the strip sensor. The measurement range is a cylindrical soil belt. The volume of the soil is large, which can effectively avoid the contingency of the point measurement and obtain the spatial average of the soil moisture in the measurement zone. Above the picture are some external acquisition and display instruments.
1.3 Frequency Domain Reflective Instrument (FDR)
The FDR soil moisture monitoring sensor measures the capacitance between the electrode inserted into the soil and the soil (the soil is treated as a dielectric) and forms a loop with the high frequency oscillator. The high frequency signal is generated by a specially designed transmission probe, and the impedance of the transmission line probe changes as the impedance of the soil changes. Impedance includes apparent dielectric constant and ionic conductivity. The application of frequency sweep technology, the selection of the appropriate electrical signal frequency to minimize the impact of ion conductivity, transmission probe impedance changes rely almost exclusively on changes in soil dielectric constant. These changes produce a voltage standing wave. The variation of the dielectric constant of the standing wave with the medium surrounding the probe increases or decreases the voltage generated by the crystal oscillator. The difference in voltage corresponds to the apparent dielectric constant of the soil.
The characteristics of FDR are analyzed as follows:
1) The frequency domain measurement technique used in soil science has only been applied in recent years. It has been carried out for nearly half a century to measure the soil moisture content by measuring the relative capacitance at a certain frequency, that is, the dielectric constant (Halbertsma et al. 1987). The frequency domain method is simpler than the time domain method and the measurement is more convenient. However, in the past, it was often difficult to obtain accurate dielectric constant measurements. Reliable soil moisture content must be obtained for subsequent calibration by each application. In recent years, with the development of electronic technology and components, the frequency domain moisture sensor for measuring the dielectric constant has been successfully developed. Since the frequency domain method uses an operating frequency lower than TDR, it is easy to implement on the measurement circuit and has a low cost.
2) The frequency domain method generally works in the frequency range of 20 to 150 MHz. Various circuits can convert the change of dielectric constant into DC voltage or other analog output form. The output DC voltage is within a wide working range. Soil water content is directly related. There are no strict requirements for transmission cables.
3) The original FDR sensor developed in China was a high-frequency capacitive sensor, which was later updated to a standing wave FDR sensor. The first wave-type FDR soil moisture monitoring sensor developed in China has no temperature compensation due to the design idea of ​​the foreign first-generation FDR sensor, and the measurement result is highly variable. The foreign standing wave FDR soil moisture monitoring sensor is also constantly innovating, gradually increasing the functions of temperature compensation and the corresponding measurement accuracy. However, the FDR soil moisture monitoring sensor uses electromagnetic waves of about 100 MHz. Therefore, when the wave is greatly affected by soil temperature and conductivity (salt) during transmission, the measurement accuracy is better than the TDR and TDT soil moisture monitoring sensors. Be lower.
4) The FDR soil moisture monitoring sensor generally outputs DC voltage, which is easy to access the conventional data collector for continuous and dynamic public opinion monitoring. It can set up a public opinion monitoring network, and the system construction cost is lower than the first two.
1.4 neutron meter, vacuum gauge, resistance meter
The neutron meter is a long-established instrument for measuring soil volumetric water content. The neutron moisture meter consists of a high-energy radioactive neutron source and a thermal neutron detector. The neutron source emits fast neutron rays with energies in the range of 0.1 to 10.0 M electron volts in all directions. In the soil, fast neutrons are rapidly decelerated by the surrounding medium, mainly by hydrogen atoms in the water, into slow neutrons, and a slow neutron "cloud" with density and moisture content is formed around the detector. The slow neutrons scattered to the detector produce electrical pulses that are counted; the number of slow neutrons counted in a given time is related to the volumetric water content of the soil, and the larger the neutron count, the greater the soil moisture content.
The neutron instrument is suitable for manual measurement of soil moisture. When using a neutron moisture meter to monitor the soil moisture content, the instrument should be determined based on the soil drying method before each catheter is buried [3].
Because neutron instruments have radioactive sources, the use of equipment management is limited by the environment.
A tensiometer is an instrument that measures the tension in soil in an unsaturated state. Commonly used tensiometers measure from 0 to 100 kPa (Dirksen, 1999). Water always flows from a high water potential to a low water potential, and the water transport in the soil is based on the soil water potential gradient. The water potential reflects the water holding capacity of the soil. Moisture is affected by multiple forces in the soil, which reduces its free energy. This change in potential energy is called soil water potential (soil suction). The application principle of the tensiometer is similar to the way in which the roots of the plant take moisture from the soil. It measures the force exerted by the crop to extract water from the soil.
Because the tensiometer is inexpensive, it can be deployed in a large number of applied research fields to study soil water distribution. The pressure value display can be an analog meter and a pressure sensor, which can be used for automatic measurement by electrical modification.
The resistance method is commonly used to measure soil moisture by a porous medium block gypsum resistor block. Due to its low sensitivity, it is currently used less.
2 Soil moisture monitoring instrument technical indicators
At the heart of the soil moisture monitoring instrument is a soil moisture monitoring sensor that converts the physical properties of the water in the soil into electricity that can be recognized by the electronic device. For the purpose of constructing an automatic moisture monitoring station, the following technical requirements are proposed for the soil moisture sensor in accordance with the relevant provisions of the Soil Moisture Monitoring Regulations.
1) Working environment temperature: -25 ~ +55 °C;
2) Working environment humidity: 100% RH (no condensation);
3) The error does not exceed 2% (when the absolute moisture content is in the range of 5% to 50%);
4) Measurement range: generally 0-50%;
5) Stabilization time: under normal circumstances, it should be no more than 10 s;
2.1 Typical TDR Instrument Specifications
A typical TDR soil moisture monitoring sensor is an example of a foreign Trime-EZ portable soil moisture meter.
The Trime-EZ TDR soil moisture sensor is used in conjunction with a hand-held meter to measure soil moisture in a portable manner, as well as an automatic metering device with an analog interface.
Measuring range: 0 to 100% volumetric water content;
Water content 0～40% Range: Accuracy ±2%;
Water content 40% to 70% Range: Accuracy ± 3 %;
Repeated measurement accuracy: ± 0.5 %;
Operating temperature: -15 ~ 50 °C;
Power supply: 7 ~ 15 VDC;
Power consumption: static 8 mA, 250 mA when measuring;
Output: 0 to 1 V or 4 to 20 mA or RS-232 digital interface;
Shell protection rating: IP68;
The cable length is 5 m (special length can be ordered).
2.2 Typical FDR Technical Specifications
1) Foreign FDR products take the UK ML2x as an example
The FDR instrument can measure the moisture of various soils and various media, and can be used as the basic tool for moisture fixed point monitoring or mobile measurement. The indicators of the soil moisture probe are as follows:
Measuring range: 0 to 100% volumetric water content;
Accuracy: ± 5 % (0 ~ 70 °C, instrument default soil type);
Working temperature: -10～+ 70 °C;
Dimensions: probe, 60 mm long, total length, 207 mm;
Standard cable length: 5 m (up to 100 m)
2) Domestic FDR products take moisture sensor MP-406 as an example
The indicators of the soil moisture probe are as follows:
Measurement parameters: 0 to 100% volumetric water content;
Measurement accuracy: ± 5 % after given soil calibration;
Measurement area: 90% of the effect is in a cylinder with a diameter of 2.5 cm and a length of 6 cm around the central probe;
Stabilization time: about 10 s after power-on;
Response time: responds to 99% of changes within 0.5 s;
Working voltage: 7 ~ 15 V;
Working current: 20 mA;
Output signal: 0~1 V;
Sealing material: PVC;
Probe material: stainless steel;
Cable length: standard length 5 m;
Maximum transmission length: 100 m.
3 Application and selection of soil moisture monitoring instruments
3.1 Requirements for use
The soil moisture automatic monitoring system includes the collection, transmission, storage, processing and automatic reporting of soil moisture information. The collection is the basis of the drought monitoring system. The automated moisture collection instrument must be accurate and reliable. Therefore, the monitoring equipment should have the following requirements in terms of use:
1) After the calibration or calibration, the working characteristics are stable and need not be calibrated again;
2) The characteristics of batch products are consistent;
3) The small size is convenient for long-term automatic work after the site is buried in the measurement site;
4) No need for routine maintenance, suitable for large-scale construction of unmanned monitoring stations;
5) The price is moderate, and the application cost is low.
3.2 Experimental application examples
In an experimental station, according to the instrument layout depth requirements in SL364-2006 "Soil moisture monitoring specification", three soil moisture monitoring sensors are used, which are vertically arranged at a depth of 10, 20, 40 cm [1], and connected to the data acquisition and transmission terminal ( YDH-1 type), a lyric automatic station was set up, and the transmission mode includes GPRS, SMS and other communication options. Figure 4 is a line diagram showing the process of soil moisture change in the experimental station. The curve shows the change of water at 10, 20, 40 cm soil depth from thin to thick. The vertical line in the figure represents the daily rainfall, and the vertical line indicates the daily rainfall. value. Additional daily rainfall information is used to respond to changes in the response to soil moisture in response to soil moisture.
4 Conclusion
The TDR soil moisture sensor can be buried horizontally or vertically at different depths of the soil. According to the SL364-2006 Soil Moisture Monitoring Specification, the multi-point method is used to measure the soil moisture distribution under different soil depths. Due to the high transmission frequency, the TDR instrument has high measurement accuracy and high instrument cost. The TDT instrument is suitable for areas with large spatial variability of soil moisture or soil texture. When the point-like sensation does not have the representativeness of the measuring point, the strip TDT is more suitable. The FDR method is the same as the TDR. The instrument transmission frequency is lower than the TDR, the accuracy is slightly lower, and the instrument cost is low.
In principle, the soil moisture sensor has two continuous transformation processes: soil water content→voltage/current→soil volume water content. The former process is converted into a voltage/current amount by the electromagnetic wave of the instrument propagating in the medium, and the latter process is a conversion formula for determining the volumetric water content of the soil water according to the measured voltage/current amount. There are different degrees of conversion errors in the two processes. The first process is affected by product quality factors, and the second process is affected by human factors, which will lead to certain errors in the accuracy of the instrument.
At present, there are many varieties of soil moisture sensors at home and abroad, but there is still no instrument in China to form mainstream products. There are no commercial TDR sensing instruments in China, and soil moisture monitoring and sensing instruments are not mature enough. Technical research on soil moisture monitoring instruments, and strengthen the comparative testing, optimization screening and application assessment of instruments in practical applications.
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