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Water vapor adsorption by the soil in Mediterranean ecosystems is very important to plant growth, We hypothesized that relative air humidity, soil texture, soil water content, and soil surface conditions affect water vapor adsorption. These factors were studied in four areas in Greece with semi-arid climatic conditions on four soils classified as Xerochrept. The study was conducted on hilly areas with various physiographic and microclimatic conditions greatly affecting daily fluctuations of air humidity. Time domain reflectometers (TDR) continuously monitored soil water content at depths 5, 10 and 25 cm. Rainfall, open pan water evaporation, air temperature, and relative air humidity were measured every 40 s, averaged. and recorded on data loggers every hour. Two weighing lysimeters were used to confirm soil water data measured by TDR-electrodes.

The obtained data indicated that night-time water vapor adsorption is very important for areas characterized by high oscillation of air humidity (>25%). Favored under certain physiographic and microclimatic conditions. Daily water vapor adsorption by the soil was inversely proportional related to the minimum air humidity, while adsorption increased proportionally as the daily amplitude of humidity increased. Cumulative water vapor adsorption decreased during the dry period as the amount of clay content increased. The depth of penetration of wetness increased as the amplitude of fluctuation in soil water content in the upper 5 cm soil layer increased. The presence of dense perennial shrubs growing in close contact with the soil surface greatly reduced water vapor adsorption by restricting the diurnal Fluctuations in air humidity in the soil-atmosphere interface. Rock fragments or plant residues on the soil surface slightly reduced water vapor adsorption but greatly favored soil water conservation. The amount of water vapor adsorbed by the soil may satisfy up to 70% of the loss of water through evaporation during the dry period under semi-arid climatic conditions. (C) 2001 Elsevier Science B.V. All rights reserved.