The effect of water vapour adsorption on soil moisture content under Mediterranean climatic conditions

The effect of water vapour adsorption on the soil moisture content of different soils under semi-arid climatic conditions (Greece) was evaluated. Time domain reflectometers were used for continuously monitoring soil moisture content at various depths. Water vapour adsorption by the soil was measured using weighing lysimeters. The obtained data indicated that nighttime water vapour adsorption is very important for areas characterised by high oscillation of air humidity. In a period of 8 months, viz. from February to August 1996, a total amount of 226 mm of water vapour was adsorbed by the soil, while the total rainfall was only 179 mm in the same period. Water vapour adsorption was found to be responsible for the diurnal fluctuations of the volumetric soil moisture content. This fluctuation was related to the clay content, the depth and the initial moisture content of the soil. Water vapour adsorption could be related to the soil moisture tension of the uppermost soil layer, the daily minimum value, and the amplitude of air humidity. Rock fragments restricted water vapour adsorption, but their contribution to the reduction in evaporation waS greater under certain climatic conditions.