From the perspective of geomorphology, three important aspects of climate should be considered if conditions become more arid: (a) any decrease that might occur in the annual rainfall amount; (b) the duration of rainfall events; and (c) any increase in the intervals between rainfall events. These, together with increasing temperature, lead to less available water, less biomass and soil organic matter content and hence to a decrease in aggregate size and stability. As a consequence, the soil permeability decreases, soils develop surface crusts and in®ltration rates decrease dramatically. Such changes in vegetation cover and soil structure lead to an increase in overland ¯ow and in the erosion of the fertile topsoil layer. Positive feedback mechanisms may reinforce these eects and lead to deserti®cation. This paper considers the results of ®eld investigations into the spatial variability of a number of `quick response' variables at two scales: the regional and the plot scales. Concerning the regional scale spatial variability, results of experimental ®eld work conducted along a climatic transect, from the Mediterranean climate to the arid zone in Israel, show that: (1) organic matter content, and aggregate size and stability decrease with aridity, while the sodium adsorption ratio and the runo coecient increase; and (2) the rate of change of these variables along the climatic transect is non-linear. A steplike threshold exists at the semiarid area, which sharply separates the Mediterranean climate and arid ecogeomorphic systems. This means that only a relatively small climatic change would be needed to shift the borders between these two systems. As many regions of Mediterranean climate lie adjacent to semiarid areas, they are threatened by deserti®cation in the event of climate change.
Concerning spatial variability at the plot scale, dierent patterns of overland ¯ow generation and continuity characterize hillslopes under dierent climatic conditions. While in the Mediterranean climate area in®ltration is the dominant process all over the hillslope, in the arid area overland ¯ow predominates. In contrast to the uniform distribution of processes in these two zones, a mosaic-like pattern, consisting of locally arid' water contributing and moist' water accepting patches is typical of the transitional semiarid area. Such pattern is strengthened by ®res or grazing which are characteristic of this area. The development of such mosaic pattern enables most rainfall to be retained on hillslopes.
Changes in the spatial pattern of contributing versus accepting water areas can be used as an indicator of deserti®cation and applied to developing rehabilitation strategies.