Numerical Simulation of Soil Temperature Distribution in Controlling Soil Temperature in a Ridge by Circulating Water in Buried Pipes

For a long time in the protected cultivation of fruit vegetables, there has been a practice of circulating warm water in pipes buried in soil ridges to control soil temperature in the root zone. Recently this technique has been applied to soil cooling in the cultivation of strawberry or ornamental plants such as Alstroemeria L. However there is little research on temperature distribution in the soil during the temperature control process or on the effects of ridge geometry, physical properties of the soil, climate conditions, pipe arrangement, water temperature and other factors on soil temperature. The authors have conducted numerical simulations of temperature distribution in ridge soil using the finite difference method to clarify the problems mentioned above and to optimize the system. In this paper, numerical solutions were conducted to explore the effects of free-convection heat transfer between the ridge surface and ambient air and the absorption of direct solar radiation at ridge surface, assuming that the effect of moisture in ridge soil on heat transfer is negligible. Results are shown for the cases of soil cooling during the high temperature period and soil heating during the low temperature period in Japan. In addition, the effects of physical properties of the soil, ambient air temperature, direct solar radiation absorbed at ridge surface, and the surface temperature of pipes on soil temperature distribution were examined.In conclusion, (1) the effect of solar radiation absorbed at the south side of ridges with E-W orientation on soil temperature is slight during the high temperature period, but significant during the low temperature period because of low solar altitude; (2) the effects of soil physical properties and ambient air temperature are small for both processes; (3) the effect of the surface temperature of pipes is comparatively large in both processes.