Experimental Study on Evaporation in a Bare Land

Research Project

Project/Area Number	59550341
Research Category	Grant-in-Aid for General Scientific Research (C)
Allocation Type	Single-year Grants
Research Field	Hydraulic engineering
Research Institution	Kyoto University
Principal Investigator	TOMOSUGI Kunio (1985) Disaster Prevention Research Inst., Kyoto Univ., 防災研究所, 助教授 (50027265) 下島栄一京都大学, 防災研究所, 助手
Co-Investigator(Kenkyū-buntansha)	SHIMOJIMA Eiichi Disaster Prevention Research Inst., Kyoto Univ., 防災研究所, 助手 (80027276)
Project Period (FY)	1984 – 1985
Project Status	Completed (Fiscal Year 1985)
Budget Amount *help	¥1,300,000 (Direct Cost: ¥1,300,000) Fiscal Year 1985: ¥300,000 (Direct Cost: ¥300,000) Fiscal Year 1984: ¥1,000,000 (Direct Cost: ¥1,000,000)
Keywords	Evaporation / Bare Land / Wind Tunnel / 地下水面 / 水蒸気輸送
Research Abstract	In order to clarify the evaporation process in a bare land affected by a meteorological and a land conditions in a complicated manner, a laboratory experiment was undertaken by continuously applying the airflow of a constant speed, temperature and humidity through a wind tunnel for a long time on a uniform porous material layer (pML) with unrestricted water table downwards. The PML was initially saturated. The employed porous materials are two kinds of sand (mean particle-diameter (d); 0.24 mm, 0.45 mm), the Akadama-soil (d = 3 mm) with a double structure of pores, macropore and micropore, and a uniform glass bead (d = 12 mm). Only the speed was variously changed for the condition of airflow. The following has been elucidated through the experiment. 1) The evaporation rate and the falling speed of eater table at the relatively early stage become large as the wind becomes strong and the pore-size of PML becomes small. However, the effect of wind speed on such quantites becomes very small with time except in the case of glass bead with large pores. 2) The evaporation rate divided by the friction velocity, when the water table is relatively high, is uniquely determined against the depth of water table. 3) The water content profile in the unsaturated zone above the water table can be approximated by the boundary drying soil-water retention curve except in a very dry place. 4) The evaporating front moves downwards from the surface of PML with time. 5) The water vapour in the PML above the evaporating front is transported by the molecular diffusion and by the turbulent diffusion. The movement near the surface of PML is mainly caused by the latter. The mixing length in the case of glass bead is a half magnitude of the particle-diameter.