Plastic film mulching not only has the advantages of heat preservation, moisture conservation and increasing crop yield, but also can solve the adverse factors such as drought, water shortage and low ground temperature in early spring, and increase the drought resistance of crops. At the same time, it can inhibit the occurrence of saline-alkali damage in arid areas and ensure the sustainable and stable growth of agricultural production.
Under the condition of wide plastic film coverage, plastic film becomes the physical barrier of water vapor exchange between soil and atmosphere, thus changing the upper boundary conditions of infiltration and evaporation, and obviously increasing the ground temperature. Therefore, the influence of temperature field on soil water movement and solute transport has become an important issue that can not be ignored.
1. Analysis of Diurnal Variation of Land Temperature in Vertical Section of Soil Layer after Wide Plastic Film Covering
Plastic film mulching can collect solar energy into soil, and solar radiation is the only heat source for surface warming. Although there are many factors affecting the ground temperature in the field, and the change of the natural conditions in the field is uncertain, the diurnal variation of the ground temperature in the vertical soil profile still has some girls to follow.
2. Geothermal changes with depth
The diurnal and annual variations of soil temperature at different depths are different. In the daytime, the surface absorbed heat by radiation, but its heat dissipation was smaller than that of heat absorption, so the surface soil temperature increased; in the night, the soil lost more heat than the absorbed heat, so the surface soil temperature decreased. The annual variation of soil temperature varies in different depths. The variation of surface soil temperature is larger than that of each depth, and along the direction of soil depth, the change of ground temperature tends to be gentle with the increase of depth.
1. The ground temperature in the inner membrane is greater than that at the edge of the film at all times in a day; the soil temperature in the inner membrane, the edge of the film, the surface layer of the film and the surface layer of the film can be fitted as a sinusoidal function of time.
2. In a planting season, the maximum temperature, maximum margin and average temperature of topsoil are all in the film, followed by the film margin, inter-film and uncovering film; the maximum temperature and maximum amplitude of topsoil occur at 16:00, the maximum of average ground temperature of topsoil occurs in the film; the minimum temperature of topsoil occurs at 8:00.
3. In the film, the ground temperature at different depths between the open field and the film has a change process in one day and in the planting season. In one day, the ground temperature in the film increases from 8:00 to 25 cm, and decreases from 16:00 to 25 cm; at 12:00 and 16:00, the ground temperature decreases according to the order of the film, the open field and the film edge; in the planting season, the average ground temperature, the temperature variation range and the maximum ground temperature are along the film. The temperature gradient above 20 cm is about 20℃, while the temperature gradient below 20 cm does not change much. Therefore, it can be concluded that the diurnal variation of temperature under wide plastic film cover in the planting season in this area mainly affects the depth above 20 cm.
4. The variation range of soil temperature in 10:00-20:00 soil profile is larger in a day, and the maximum variation occurs in the film, with a value of 3.0-18.5℃. The variation range of inter-film and uncovered plots is relatively small. The relationship between the maximum variation range of soil temperature and depth in a day can be fitted as an exponential function.