| The urban heat island effect (UHIE) causes many environmental and ecological problems. Now, most of the researches on UHIE pay attention to its influences on the atmospheric environment, while few works on the shallow soil. In this paper, though the actual observation, the time and space variation of the soil heat and dry island effects were studied systematically. The research and corresponding findings are presented as follows:1. By analysing the temperature data of atmosphere from Weather station of Nanjing in the last 50 years and the temperature data of the shallow soil within 40cm depth in the last 30 years, the variation of temperature is obtained. The variation of atmospheric temperature in Nanjing can be divided into three stages:from 1961 to 1967 is the stage that atmospheric temperature declined, the mean annual temperature dropped by 1.05 ℃. From1968 to 1985 is the stage that atmospheric temperature fluctuated. From 1985 to 2007 is the stage that the atmospheric temperature fluctuations increased, the mean annual temperature increased by 2.33 ℃. In the last 30 years, the temperature of the surface (0cm) was higher than that of shallow soil and atmosphere, the temperature increased by about 2.8 ℃. Soil temperature is on the rise, the shallow soil above 20cm depth increased by 2.0 ℃ while 1.75 ℃ at depth of 40cm.2. The effect of urban ground cover on urban atmosphere island effect is very remarkable. Through the observation of air temperature that 80cm above the ground and the surface temperature of concrete and grass from July 27 to July 28,2008 in the urban and rural areas of Nanjing, some important data and conclusions are obtained. The mean daily temperature difference between the urban and suburban areas: Cement is 3.1 ℃, Atmosphere is 1.9 ℃, Grass is 1.2 ℃. The surface temperature of concrete is 3.8 ℃ over the temperature of air,4.7 ℃ higher than that of Grass. The surface temperature of grass ground is 0.9℃ lower than the air. It means that Grass has a remarkable effect on reducing the UHIE.3. Two long-term observation station and 600 short-term monitoring points were established respectively within the 300cm depth of shallow soil in Nanjing urban and rural areas respectively. From June 2009 to June 2010, there is a one year observation on the temperature and moisture field of soil. The results showed that:(1) As the same in the atmosphere, the urban heat island exists in the shallow soil of urban area. Based on the soil temperature data of Non PVC monitoring system, within 300cm depth, the mean annual soil heat island intensity (SHII) of urban soil is 2.14"C, the ranges of mean daily SHII are 0.37℃-3.83℃. The ranges of mean monthly SHII are 1.34℃-2.9℃, at maximum in July and minimum in November. The mean quarterly SHII are Winter (2.57℃), Summer (2.34℃), Spring (1.63℃), Autumn (1.53℃).(2) Based on the soil temperature data of PVC monitoring system, within 150cm depth, the mean annual SHII of urban soil is 1.27℃, the ranges of mean daily SHII are 0.28℃-3.03℃. The ranges of mean monthly SHII are 0.74℃-2.00℃, at maximum in July and minimum in Octomber. The ranges of mean quarterly SHII are 0.83℃-1.51℃, at maximum in Summer and minimum in Autumn.(3) Based on the soil temperature data of cover monitoring system, within 300cm, the mean annual SHII of urban soil:Bare soil is 1.82℃, Grass is 2.20℃, Concrete is 2.22℃. The mean monthly SHII, Bare soil are at maximum in June (3.54℃) and minimum in November (0.16℃). Grass are at maximum in July (3.02℃) and minimum in November (1.28℃). Concrete are at maximum in January (4.06℃) and minimum in May (0.92℃).(4) Based on the soil moisture data of Non PVC monitoring system, within 100cm depth, the mean annual soil dry island intensity (SDII) of urban soil is-7.2%, the ranges of mean daily SDII are -28.0--4.5%. The ranges of mean monthly SDII are-19.1%--2.0%, at minimum in July and maximum in January. The mean quarterly SDII are Winter (-3.7%), Summer (-13.8%), Spring (-6.3%), Autumn (-5.2%).(5) Based on the soil moisture data of cover monitoring system, within 100cm depth, the mean annual SDII of urban soil:Bare soil is-6.03%, Grass is-15.04%, Concrete is-7.00%. The mean monthly SDII, Bare soil are at maximum in February (0.06%) and minimum in May (-14.81%). Grass are at maximum in May (-10.21%) and minimum in July (-26.30%). Concrete are at minimum in July (-23.95%) and maximum in May (-2.54%).(6) Based on the soil moisture and temperature data of 600 monitoring points from August 11,2010 to August 13,2010 in urban and rural areas, at 25cm depth, the totally mean SHII of urban soil is 3.30℃:the mean SHII of urban soil, Bare soil is 3.24℃, Grass is 3.34℃, Wood is 3.36℃. The totally mean SDII of urban soil is-2.40%:the mean SDII of urban soil, Bare soil is -2.32%, Grass is -2.26%, Wood is-2.05%.4. In order to study the heat transfer in the soil, heat transfer characteristics of cover, soil permeability, soil swelling and so on. A series of indoor physical model test were made, the results show:(1) Homemade physical model of soil temperature test system has good sensitivity and precision. It can be used to simulate the variation of soil temperature with time.(2) Cover has some effects on the heat transfer process in soil. With the coverage of concrete, the equilibrium soil temperature is higher than that of bare soil because of the variation of the thermal properties. The initial heat increasing rate of the concrete soil is also higher than that of the bare soil. But with depth increasing, the influence of concrete on the initial heating rate gradually decreased.(3) The temperature transfer process of saturated soil is similar with that of air-dried soil. But compared with the air-dried soil, the specific heat of the saturated soil is higher, thermal conductivity and thermal diffusivity is smaller, so its equilibrium temperature is lower than dry soil.(4) The process of water infiltration is restricted by the water holding capacity of the soil, infiltration distance, water supply and time, and other factors. During the soil expansion process, the thickness of the soil bond water layer around changes with the water supply. It makes the strength of the connection between soil particles reduced or increased and leads to expansion or contraction of the air-dried soil.5. In order to improve the moisture measurement accuracy of the shallow soil, a large number of tests were made, and the techniques of soil moisture measurement based on FDR were studied. The results showed:An obvious temperature effect exists on the soil moisture sensor that based on FDR. The measured value increases linearly with the temperature. The ranges of temperature correction factor, TDR-3 are 0.2%-0.3%(m3/m3)/℃ and PR2 are 0.1%-0.2%(m3/m3)/℃.6. Based on a large number of observations outside and model test in the lab. The formation mechanism of the urban heat island effect and the dry island effect in the shallow soil in Nanjing is analyzed systematically. The urban atmosphere heat island effect transfers heat to the soil through different kinds of cover, which increases soil temperature. The evaporation of moisture in the soil increased at the same time. The two factors above cause the urban soil heat and dry island effect.In addition, the shallow soil and dry island heat island effect are also associated with the thermal properties of the cover in the land/air interface, material composition, physical properties and structure state of the soil, water supply and other factors. |
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