ARIDISATION PROCESSES INDUCED BY INCREASED RAINFALL AT A DESERT FRINGE

Dry areas are regarded as sensistive to climatic changes. A positive relationship between average annual rainfall; geomorphic processes and environmental factors is often assumed for areas with average annual rainfall of 100-300 mm. This assumption disregards the fact that climatic changes in such areas are often accompanied by the alteration of surface properties, related to the deposition of aeolian material (Loess during wet phases and sand during dry phases). This raises an important issue: Does loess deposition enhance the expected positive effect of rainfall increase or limit it through the negative effect of changing surface properties.

The study was conducted along two transects in the Northern Negev desert. The first transect is located in the Negev Highlands. The southern dry area is characterized by extensive rocky outcrops; while the northern wetter area is loess covered. Hydrological,pedological, botanical and zoological data clearly show that the climatologically wetter area is environmentally more arid than the southern drier area. The results are explained by the negative effect that loess deposition had on runoff, infiltration depth and salt accumulation. Under the rainfall regime prevaling in the area an increase in the loess cover reduced the frequency of runoff generation as well as the depth of water infiltration leading to a salinization process. A better water regime prevails in rocky areas. The high frequency and rate of runoff generation allow here a process of water concentration at specific sites where water availability is beyond that allowed by the direct rainfall.

The second transect is located along the Israeli-Egyptian border.In this case the lithological substratum (sand) is uniform along the whole area. Data on rainfall, runoff, soil moisture and vegetation cover were collected at five monitoring sites.Data obtained show that the wetter area is ecologically more arid than the southern, climatologically drier area.The results are explained by the fact that under wetter conditions a very thick biological topsoil crust develops. The crust absorbs large rain amounts, limiting thus the depth of water penetration and soil moisture. A better water regime was found in the dry area where a thin crust, with a lower water holding capacity, allows deeper infiltration and water concentration by runoff.