THE EFFECT OF WATER-VAPOR 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 evalu ated. Time domain reflectometers were used for continuously monitoring soil moisture content at various depths. Water vapour adsorption by t he soil was measured using weighing lysimeters. The obtained data indi cated that nighttime water vapour adsorption is very important for are as 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 o f 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 b e responsible for the diurnal fluctuations of the volumetric soil mois ture content. This fluctuation was related to the clay content, the de pth and the initial moisture content of the soil, Water vapour adsorpt ion could be related to the soil moisture tension of the uppermost soi l layer, the daily minimum value, and the amplitude of air humidity. R ock fragments restricted water vapour adsorption, but their contributi on to the reduction in evaporation was greater under certain climatic conditions. (C) 1998 Elsevier Science B.V.