Microclimate and Water Vapor Exchange at the Soil Surface

Estimates were obtained on the annual water vapor exchange through adsorption, condensation, and evaporation at the ground surface under field conditions. The investigation was coordinated with measurements of environmental microclimatic conditions. For a 2-month period data were obtained also on nocturnal water accretions on corn and soybean leaves. A small area of land on the Purdue University dairy farm near West Lafayette, Ind., was chosen as site for the field experiments. Five sampling conditions were employed, in which 20 small metal dishes served as containers for various soil and water samples. Each sampling set consisted of four filled dishes, two of the dishes to be placed on the ground surface of bare soil and the other two on meadow, during experimental periods. The annual moisture gains from adsorption and condensation of atmospheric water vapor are estimated at 1.22 inches for bare soil, and 0.99 inch for soil having grass-clover vegetation. The water vapor transfer from the subsoil to the soil surface, not including evaporational losses, attained 1.34 inches of water on bare ground and 0.77 inch on meadow. This results in total moisture gains of 2.56 inches and 1.76 inches, respectively. The field tests indicated that 0.59 inch of water per month per acre of ground collected on corn leaves in summer. Similarly, soybeans yielded 1.32 inches in the same period. This accumulated moisture on the plant leaves in clear nights consists of condensational water from the atmosphere and of guttation. Water vapor transfer from the subsoil toward the top layer normally remains confined to night hours during the period from March to October. Under the same conditions, a two-directional moisture loss from the ground surface layer takes place in the daytime: a part of the moisture can be driven to greater soil depths, while perhaps a larger part is subject to evaporation. Water vapor transfer from the subsoil to the surface may be a continuous process in winter, unless the soil is water saturated.