| As a basic attribute of soil, soil conductivity is closely related to soil properties (electrolyte composition, concentration, soil colloid type, soil texture, soil structure, soil moisture content and soil temperature) and meteorological factors (rainfall, temperature, moisture, evaporation, wind speed, air pressure and sunshine), etc. Soil conductivity index is usually expressed as soil conductivity or its reciprocal-soil resistivity, while soil resistivity is the fundamental to lightning grounding, being an important evaluation standard for determining soil corrosivity, and a basic indicator of soil fertility. Therefore, soil conductivity is widely applied in lightning disaster risk assessment, lightning protection, anticorrosive engineering of underground metal facilities and fine agriculture, etc. Therefore, the study on the coupling effect between meteorological factors and soil properties on soil conductivity is to study the coupling effect between meteorological factors, soil temperature and soil moisture content on soil resistivity. However, currently the study on soil conductivity is concentrated on the impact of the physical and chemical properties of soil on soil conductivity, and research measures primarily include experiments or field measuring of soil conductivity and physical and chemical properties of soil to analyze the relations between soil conductivity and its physical and chemical properties, without such reports as continuous and automatic field observation of soil conductivity, soil moisture content, soil temperature and meteorological factors, and the use of mass data of long-term field observation to analyze and study the impact of soil moisture content, soil temperature and meteorological factors on soil conductivity. Therefore, this study primarily develops the automatic measuring device of soil resistivity, based on which the continuous monitoring of soil conductivity, soil moisture content, soil temperature and meteorological factors will be possible, thus eventually interpreting the coupling effect between meteorological factors and soil properties on soil conductivity.This study selects the observation field of the Meteorological Bureau of Hechuan District, Chongqing as the field test site for the, study on the coupling effect between meteorological factors, soil moisture content and soil temperature on soil conductivity. By using existing observation instruments and devices on the observation field and a independently developed automatic soil resistivity measuring device, field observation tests of rainfall, temperature, moisture, wind, air pressure, evaporation, vapor pressure, sunshine and soil resistivity, soil temperature, soil moisture content, rainfall pH value and rainfall conductivity (K value) are conducted, and through analysis and study of the observation test data in a year, the following major findings are obtained:(1) A multichannel soil resistivity automatic measuring device is developed. Remote control software of automatic soil resistivity measuring system is developed for the remote control of the automatic soil resistivity measuring system. Characteristic parameter database of continuous grounding for soil resistivity with an interval of one hour is established. Remote automatic test devices and methods for soil resistivity, soil conductivity and grounding resistance are provided. Unattended remote automatic monitoring is achieved to replace traditional filed soil resistivity, soil conductivity and grounding resistance measuring instruments and devices that require site tests by engineers (which was very difficult before), achieving multichannel, long-term, continuous, stable and automatic measuring of soil resistivity, soil conductivity and grounding resistance. Meanwhile, the automatic soil resistivity measuring device provides continuous automatic monitoring of the resistance reduction function and corrosiveness of resistance reducers, facilitating meteorological departments to manage resistance reducers as per Grounding Resistance Reducer (QX/T 104-2009).(2) The study finds that meteorological factors affect soil resistivity by affecting soil temperature and soil moisture content, and are indirect acting factors, while soil temperature and soil moisture content are factors directly affecting soil resistivity, of which moisture contents of soil of different depths are the major factor affecting the resistivity of soil layers of different depths, and are the most sensitive factor. Regarding meteorological factors, rainfall, temperature and sunshine are the major factors affecting the resistivity of soil layers of different depths, while other meteorological factors are covariant factors affecting soil resistivity. Rainfall is the most sensitive factor affecting soil resistivity. In particular, the rainfall of the current hour has most remarkable impact,on soil resistivity variation. With the increase of rainfall by day/ hour, the resistivity of soil layers of different depths will decrease, but the decrease trend of its soil resistivity will weaken, and even an increase trend will appear. Only a proper rainfall can reduce the soil resistivity to its minimum. In raining seasons (May to September), monthly rainfall is the major factor affecting the resistivity of soil layers of different depths. In non-raining seasons (January to April, October to December), monthly average temperature is the major factor affecting the resistivity of soil layers of different depths. Monthly rainfall affects the monthly average resistivity of soil layers of different depths with a remarkable hysteretic nature, being one month for the shallow layer of soil, and two months for the deep layer of soil. The monthly average resistivity variation trend of soil layers of different depths remarkably lag behind soil temperature variation. The monthly average resistivity variation trend of soil layers of 0-15cm,0-30cm and 0-80cm deep is one month later than temperature variation; the monthly average resistivity variation trend of soil layers of 0-160cm and 0-320cm deep is four months later than temperature variation.(3) Through the study on the coupling effect between rainfall process time and rainfall on the recovery time of soil resistivity to its value before the rainfall, the following findings are obtained:"When the rainfall is less than 0.1 mm, its recovery time is 0 hour. When the rainfall process time is less than one hour or the rainfall is less than 2.0 mm, its recovery time will not be greater than 24 hours. When the rainfall process time is 2-10 hours or the rainfall is 2.1-5.0 mm, its recovery time will not be greater than 72 hours. When the rainfall process time is greater than 11 hours or the rainfall is greater than 5.1mm, its recovery time will be greater than 72 hours and less than 232 hours", tackling the problem that for soil resistivity measuring of lightning disaster risk assessment, lightning protection and anticorrosive engineering of underground metal facility, how long after the rainfall that soil resistivity, soil conductivity and grounding resistance measuring can be conducted to eliminate rainfall influence and obtain reliable and valid measured value of soil resistivity, soil conductivity and grounding resistance, scientifically supporting the security assessment of lightning protection and grounding devices for buildings (structures) as well as grounding devices for facilities and instruments.(4) Through the study and analysis of the impact of rainfall particle charges on soil resistivity by using the observation data of daily rainfall pH value and its conductivity (K value) and the automatic monitoring data of the resistivity of soil layers of different depths, the following findings are obtained:"At a fixed place of the same area, the remarkable positive correlation between the average daily resistivity of soil layers of different depths and the K value of daily rainfall is a result of the combined action of the remarkable negative correlation between daily rainfall and average daily resistivity of soil layers of different depths, and the remarkable negative correlation between daily rainfall and K value of rainfall. Therefore, the concentration of charged ions in daily rainfall improves far less conductivity that daily rainfall does, and the impact of pH value of daily rainfall and its conductivity on soil resistivity, as compared with the impact of dailyrainfall on soil resistivity, can be neglected", thus finding the problem that "currently in the R&D of resistance reducers, only resistance reducers are used to provide soil with limited charged ions to improve soil,resistivity, thus reducing grounding resistance", and proposing three major resistance reduction principles of "low resistivity, water absorption and water retention" for the R&D of resistance reducers.(5) Through the analysis of the coupling effect between meteorological factors, soil temperature and soil moisture content on soil resistivity, it is found that the regression model of such coupling effect on the resistivity of soil layers of different depth contributes far more than the regression model of the individual impact of any of meteorological factors, soil temperature and soil moisture content on the resistivity of soil layers of different depths does, and contributes more remarkably than the coupling effect between soil temperature and soil moisture content, than the coupling effect between meteorological factors and soil temperature, and than the individual impact of rainfall on the resistivity of soil layers of different depths. And, the following findings are obtained:"The different best regression model should be taken when evaluates soil resistivity according to different data.That means for average daily resistivity of soil layer of 0-15cm, average daily resistivity of soil layers shall be estimated by maximally using the regression model of the coupling effect between average daily (daily value) meteorological factors and the average daily temperature of soil of different depths on average daily resistivity of soil layers of different depths. The daily and hourly resistivity of soil layers of 0-30cm,0-80cm,0-160cm and 0-320cm shall be estimated by maximally using the regression model of the coupling effect between daily and hourly meteorological factors, and the coupling effect between daily and hourly temperature of soil of different depths and daily and hourly soil moisture content on the daily and hourly resistivity of soil layers of different depths. The average daily resistivity variation of soil layer of 0-80cm can be estimated by using the regression model of the coupling effect between average daily moisture content variation or average daily moisture content variation and soil temperature variation on the average daily resistivity variation of soil layers of different depths. The average daily resistivity variation of soil layer of 0-30cm shall be estimated by maximally using the regression model of the coupling effect of average daily (daily value) meteorological factors variation, and the coupling effect between average daily soil temperature variation in different depths and average daily soil moisture content variation on the average daily resistivity variation of soil layers of different depths." The regression model to estimate the resistivity of soil layers of different depths by using the observation data of meteorological factors from weather stations, soil temperature in different depths and the observation data of moisture contents is established, providing a reliable computational method for the estimation of the resistivity of soil layers of different depths for lightning disaster risk assessment, lightning protection, anticorrosive engineering of large underground metal facilities and the study on soil fertility, thus providing historical observation data of soil resistivity and soil conductivity.
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