Experimental Study On Effects Of Open Hole Ratios On The Movement Of Soil Water, Soil Salt, And Soil Heat Transefer

Owing to the neccecity and emergency of reclamation of salinized soils and prevention of secondary salinization in recent years, study on the coupled movement of soil water, soil salt and soil heat under plastic mulch in arid and semi-arid areas had been paid attention to. Rsearch on soil mass transfer process under plastic mulch has been conducted limitedly in the world. This work was supported by National Natural Science Fundation. We combined laboratory tests with theoretical analysis, used laboratory soil column test systems to study the effect of plastic film hole ratios on the movement of soil water, soik salt and soil heat during soil water infiltration and evaporation process to analyze the mechanism. The main results are as follows:(1) Power functions were both found for relationship between capillary water rise velocity with time, and between capillary water supply rate with time. There was linear function relationship between the capillary water rise velocity and the capillary water supply rate for uniform soil.(2) The supply of capillary water impacted evaporation at different plastic film hole ratios, the cumulative evaporation of different open hole ratios increased with time gradually, there was power relationship between cumulative evaporation and time as well as relative open hole ratios with Rose empirical formula. Unit film hole area of evaporation decreased when open hole ratios increased, so did the rate of evaporation at different plastic film hole ratios. Soil temperature on soil surface was large and was small when soi depths increased, the upper soil temperature gradient was large, the deeper of soil depth, the smaller of the temperature gradients, soil temperature at depths depper than 30cm varied not so obviously, so 30cm can be defined as influence depth for soil temperature changes. There were power functions between soil temperatures and soil depths.(3) The variations between cumulative evaporation at unit depth and evaporation time fit Gardner's theory for all the soil column heights, there were 2-variable linear functions between cumulative evaporation at unit depth, evaporation time and total soil height. There was power relationship between cumulative evaporation at unit depth by unit film hole area and total soil height as well as open hole ratios. The smaller the total soil height, the larger the variation ranges of soil salt content at unit depth and soil salt concentration. Plastic mulch with open holes restrained soil water evaporation rates and also affected accumulation of soil salt on soil surface, it was effective to decrease soil salt accumulation on soil surface by decreasing open-hole ratios of plastic.(4) The initial soil water contents affected soil water supply capacity. During soil evaporation, the cumulative evaporation changes at any time at different film hole ratios were calculated according to evaporation coefficients at known initial soil water contents, cumulative evaporation per unit hole area film(ER)decreased rapidly when the open hole ratios increased, the function of ER and the open hole ratios of different soil moisture were established; the greater of the soil moistures the greater of the salt concentrations on surface under perforated plastic mulch. The salt concentration distributions in the profiles were similar at different initial soil moisture, there were power functions between soil salt concentration at different initial soil moistures and vertical positions under perforated plastic mulch.(5) Soil texture affected soil evaporation. Hhydraulic conductivity of sandy soil were smaller than loam when soil water suction was large under the stability evaporation, when the loamy soil layer position changeed, the changing point of soil water movement was accelerated or was restrained when the relative size of soil hydraulic conductivity changed for sandy and loamy soils. The trends of cumulative evaporation were similarly for bare soils at different loamy soil layer positions, it was clear that the restrain effects of soil layer on cumulative evaporation were more obvious when layer position were closer to soil surface. There were quantititative functions between cumulative evaporation, relative cumulative evaporation, unit film hole area of evaporation and open hole ratios for different loamy soil layer positions.(6) The soil water contents in the loamy soil layers were higher significantly than the upper and lower sand layers during evaporation, and soil water content of sand changed less than loamy soil, soil water content in the sand soil layers on the top of loamy soil changed significantly, which was similar with situations of capillary water rise process; increase of soil temperature was caused by the formation of dry layer, and it significantly reduced intensity of soil water evaporation. The higher of the layer positions, the greater of its impacts on the distributions of soil temperature. The trend of temperature changed and was similar under different open hole ratios. The smaller the open hole ratios, the larger the soil temperature in the profiles, which showed that plastic mulch can increase the soil temperature effectively.