1 Introduction
Soil is a loose porous body composed of three phases of solid phase, liquid phase and gas phase. Solid-phase substances include soil minerals, organic matter and microorganisms; liquid-phase substances mainly refer to soil moisture; gas phase is air that exists in soil pores. Under normal conditions, the appropriate soil three-phase ratio is: solid phase rate is about 50%, volumetric water content is 25-30%, and gas phase rate is 15-25%. Each of these three types of substances in the soil has its own physical and chemical properties, which are connected and constrained with each other, forming a complex and contradictory unity. The interrelationship between them makes the soil in constant changes, thus forming different soil structures, affecting the soil's physicochemical properties and fertility foundation, which in turn affects the soil's gas conduction and thermal conductivity properties, hydrodynamic properties and plant growth.
Soil permeability is an important feature to measure the existence status and volume ratio of three-phase substances in soil. Its quality is mainly determined by the total porosity, pore connectivity and aeration porosity of the soil (Shao Mingan, Wang Quanjiu, etc., 2006), It has a significant regulation effect on soil water, fertilizer, gas and heat, physical and chemical and biological processes, plant root penetration and plant use of water and nutrients.
(Wang Weihua, Wang Quanjiu, etc., 2009).
Compactness is one of the important physical properties of soil, and it is also an intuitive reflection of soil pore characteristics; soil gas permeability directly affects soil gas exchange capacity, which in turn affects soil moisture and nutrient availability. At the same time, soil gas conductivity can be used to analyze soil pores Because of its geometric distribution, structure, and soil stability, the characteristics of soil air conduction have received great attention (Tongyan'an, Wang Quanjiu, 2002; Wang Weihua, Wang Quanjiu, etc., 2009), but the research on soil air has not been widely carried out due to the relatively difficult test methods. However, the scope and potential of its application are very wide (Wang Weihua, Wang Quanjiu, etc., 2009; Jury et al., 2004). Soil hydraulic conductivity has always been a characteristic indicator of soil moisture dynamics. At the same time, it is also an important parameter that reflects soil permeability, and has been continuously paid attention by researchers. However, because the characteristics of multiphase flow in soil are controlled by pore characteristics, such as bending, connectivity, and shrinkage characteristics, experimental studies are often lacking to illustrate the effects of soil structure and porous system characteristics on soil air permeability and soil hydraulic conductivity (Wang Weihua, Wang Quanjiu et al., 2009; Hillel D, 1998).
2 Observation system design
2.1 Objective
With the development of electronic technology, the measurement of soil compactness has become more scientific and convenient. The soil compactness observation unit of the AZ-S0220 soil three-phase characteristic observation system is portable, which can display the measured depth, the insertion speed and the soil compaction degree, and can analyze the data and draw the variation of the compaction degree with the depth through the supporting professional software The graph of the curve can also be equipped with a GPS receiver for precise positioning of the measuring point.
The study of soil air conductivity has not been widely carried out, and the test method is relatively difficult. At present, there are few instruments dedicated to soil air conductivity measurement. The AZ-S0220 soil three-phase characteristic observation system adopts the soil air conduction characteristic measurement unit based on Darcy's law. It is currently the only instrument used to measure the soil air conduction characteristic in the field or field. It is aimed at homogeneous soil, ring blade soil samples, original The measurement room is configured for the soil and the deep soil, which can meet different scientific research needs. It can also measure soil water content and soil water potential simultaneously to study the relationship between soil moisture and soil air conductivity.
The relationship between soil water saturation and unsaturated water infiltration, the relationship between soil water suction and soil water content is an important hydraulic parameter that reflects soil pore conditions, infiltration characteristics and soil water holding capacity, and has always been one of the research hotspots. . However, due to the great variability of field and field soils, accurate measurement of soil hydraulic parameters is time-consuming and labor-intensive, and often requires more repetitions. Therefore, reliable, simple and fast measurement methods are urgently needed.
The AZ-S0220 soil three-phase characteristic observation system adopts a soil saturation hydraulic conductivity observation unit that does not require special contact layer and soil surface treatment to perform in-situ non-destructive measurement. The measurement of unsaturated hydraulic conductivity usually goes through the tedious process of soil sampling, saturation, weighing, evaporation, weighing, etc. The measurement efficiency is low. The AZ-S0220 soil three-phase characteristic observation system adopts the principle based on saturated evaporation The automatic measurement system of soil unsaturated hydraulic conductivity is controlled by a computer program and is equipped with an electronic balance with high-precision automatic lifting and weighing to achieve unattended, automatic sequential measurement and recording of water evaporation and weight changes on the surface of the original saturated soil, and then The analysis of the measured data by the software has fully improved the efficiency and accuracy of the measurement; for field observation stations with less complete experimental conditions, a stand-alone version of the portable soil unsaturated hydraulic conductivity observation unit can be applied, usually consisting of three measurement units and one Precise electronic balance and related software can realize the soil unsaturated hydraulic conductivity observation of three samples; in order to meet the expansion of the later experimental requirements, an automatic observation system can also be equipped with up to 20 measuring units and electronic balance connected in series.
The traditional soil moisture characteristic curve measurement technology has obvious problems of low efficiency, but under low water suction conditions, the accuracy of the measurement of soil dehydration and water absorption process is more serious, even the pressure membrane instrument that was once regarded as a classic cannot be accurately measured The process of soil moisture changes below 1 Bar. AZ-S0220 soil three-phase characteristic observation system adopts the principle based on traditional pressure plate method
The automatic soil moisture characteristic curve measurement unit fully considers this process and fills the gap. The measurement unit can set up to 12 pressure gradients within the range of 0-1Bar and can complete up to 4 dry and wet cycle measurements under the premise of ensuring accuracy and automation. It is to study soil moisture under low water suction conditions. The dynamic process of desorption and its hysteresis effect, and the establishment of a scientific water dynamics model provide advanced technical means.
2.2 Observation content
Soil tightness, soil air permeability, soil saturated hydraulic conductivity, soil unsaturated hydraulic conductivity, soil moisture characteristic curve under low water suction and its hysteresis effect.
2.3 System composition
The AZ-S0220 soil three-phase characteristic observation system consists of a soil compactness measurement unit, a soil air conduction characteristic observation unit, a soil saturated hydraulic conductivity measurement unit, and an automatic measurement system for soil unsaturated hydraulic conductivity, which can realize soil release under low suction conditions. The automatic measuring unit composed of soil moisture characteristic curve of water and water absorption dynamic accurate measurement is composed.
3 Data processing
3.1 Soil air conductivity
Where PL: soil air conductivity; vL: air flow rate; l: measurement chamber height; Dh: pressure difference in the sample chamber height, expressed in centimeters of water column.
3.2 Soil saturated hydraulic conductivity
Where Kf: saturated hydraulic conductivity; Ku: unsaturated hydraulic conductivity; Q: steady-state infiltration flow rate; r: infiltration radius; α: infiltration coefficient.
3.3 Soil saturated hydraulic conductivity
Where Ku: unsaturated hydraulic conductivity; v: infiltration water flow rate; Gr: pressure gradient of water flow; △ m: weight difference between different samples; A: cross-sectional area of ​​the sample ring knife; T: each sample Measurement time; 10: number of samples; △ Tens: corrected pressure difference.
4 Application case
4.1 Application of AZ-S0220 soil three-phase characteristic observation system in the study of the relationship between gas conductivity, water conductivity and water content of undisturbed soil and disturbed soil.
In order to analyze the relationship between soil air conductivity and soil water conductivity, this paper studied the characteristics of water conductivity and air conductivity of soil samples in the Yangling wheat experimental field in Shaanxi Province with water content, and compared the air conductivity and water conductivity of undisturbed soil and disturbed soil. Characteristics, analysis of the relationship between relative hydraulic conductivity and relative air conductivity and saturation, it was found that the water conductivity decreased with the increase of water content, and both the water conductivity and air conductivity of the original soil were larger than the disturbance soil, confirmed The soil structure and pore characteristics have a huge impact on the transmission of water and gas. Although the change trends of the disturbed soil and the undisturbed soil are basically the same, the curves do not coincide, indicating that the disturbed soil and the undisturbed soil have different pore connectivity and curvature. (Wang Weihua, Wang Quanjiu, Fan Hua, 2009)
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