The TDR method has been widely used abroad, and some research institutions in the country have begun to introduce and develop TDR. The TDR system is similar to a short-wave radar system, which can monitor soil water and salt conditions directly, quickly, conveniently and in the field. Compared with other methods, TDR has strong independence, and the measurement results are almost independent of soil type, density and temperature.
Method for determining soil moisture
(1) Drying method (weight loss method)
Drying is the most common method for measuring soil moisture and is the standard method used to determine soil water content. Wet soil samples of known weight are usually weighed out of the undisturbed soil column taken from the field, dried in an oven at 105 ° C and weighed. The moisture lost by heating represents the soil moisture in the wet sample.
(2) Electrical resistance method
The electric resistance method is a method which utilizes the fact that the resistance of some porous substances such as gypsum, nylon, glass fiber, etc., is related to their water content. When these electrode-embedded block components are placed in moist soil, they absorb the soil moisture and reach equilibrium. The electrical resistance of the block components is determined by their water content and is in turn determined by the soil moisture tension or suction in the vicinity. The relationship between the resistance reading and the percentage of soil moisture can be determined using a calibration method. These block components are used over a period of time to determine the water content at selected locations in the field. They give fairly accurate moisture readings in the 1 to 15 atmosphere suction range.
(3) Neutron scattering (neutron scattering)
Neutron scattering is a unique method for determining soil moisture in the field. The effectiveness of neutron moisture meters is based on the principle that hydrogen is unique in its ability to drastically reduce the speed of fast neutrons and scatter them. The principle of the neutron moisture meter is illustrated in Figure 6-3. Although expensive, the neutron moisture meter has many advantages and can accurately determine the amount of water in the mineral soil as the main source of hydrogen. This method has significant limitations on organic soils because many of the hydrogen in organic matter is present in forms other than water. In addition, it is not suitable for determining the soil water content of 0-15 cm in the surface layer.
(4) TDR method
The TDR method was developed in the early 1980s. It was first found to be useful for the determination of soil volumetric water content, and it was found to be useful for the determination of soil salinity. The full name of TDR is Time-Domain-Reflectometry. Abbreviated as TDR, translated into Chinese as a time domain reflectometer. The TDR method has been widely used abroad, and some research institutions in the country have begun to introduce and develop TDR. The TDR system is similar to a short-wave radar system, which can monitor soil water and salt conditions directly, quickly, conveniently and in the field. Compared with other methods, TDR has strong independence, and the measurement results are almost independent of soil type, density and temperature. The application of TDR technology to the determination of soil moisture conditions under icing conditions can yield satisfactory results, while other methods are more difficult. Another feature of TDR is that it can simultaneously monitor soil water and salt content, and simultaneously measure at the same place, and the results are consistent. The two measurements are completely independent and do not affect each other.
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