The effect of land parameters on vegetation performance and degree of erosion under Mediterranean conditions

Important land parameters such as soil texture, soil depth, topography, parent material, and climatic conditions were studied in a semi-detailed soil survey conducted in the island of Lesvos, and were related to the vegetation performance and degree of erosion. Rainfall data indicated that the island could be divided into two major climatic zones: a semi-acid (western part) and a dry sub-humid (eastern part). A 45% rainfall gradient has been recorded in the above two climatic zones. The great reduction in rainfall combined with the higher evapotranspiraton demands in the semi-arid zone has significantly affected vegetation performance and degree of erosion in the area. Due to the general lack of available water, the semi-arid part of the island is dominated by poor marquis vegetation, while olive trees, oak and pine forests prevail in the dry sub-humid part under similar topogaphical and geomorphological conditions. Vegetation cover increases with increasing rainfall and soil depth. Slope grade has a variable effect on soil erosion in the various climatic zones. Erosion is decreasing with increasing rainfall for the same slope classes. Severely eroded soils prevail in the semi-arid zone with slopes greater than 12%, while slightly to moderately eroded soils are found in the dry sub-humid zone under similar slope classes. The highest rates of land degradation have been measured in areas with soils formed on pyroclastic formations. Areas with soils formed on basic igneous rocks, shale and lava are well vegetated and protected from erosion under a broad range of slopes and weather conditions. (C) 2000 Published by Elsevier Science B.V. All rights reserved

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. (C) 1998 Elsevier Science B.V