Hydrological processes in a small arid catchment: scale effects of rainfall and slope length

Many studies have been concerned with the scale issue in geomorphology and hydrology. Most studies focus on the possibility of data transfer from small to large watersheds. Less attention has been given to the scale problem within very small watersheds where differences in slope length may seriously affect the spatial distribution and extent of runoff-contributing areas to the channel. It is usually assumed that runoff and erosion increase as slope length and angle increase. However, field observations in the Negev Highlands show positive relationships between slope length and deposition rates, regardless of slope angles. Short hillslopes are devoid of colluvial mantle, while thick colluvial deposits, with well-developed soil profiles indicative of long-term stability, are found at the base of long hillslopes. The hypothesis advanced is that temporal variations in effective rainfall control the spatial structures of surface properties that further enhance the effects of temporal variations in rainfall. Long-term monitoring of rainfall and runoff (1982–1998), conducted at spatial scales varying from few hundreds of m2 up to 0.3 km2, show a decrease in runoff at the hillslope scale. The low efficiency of runoff and erosion processes on long hillslopes is because the concentration time required for continuous flow along such slopes is longer than the duration of most effective rain showers prevailing in the area. Field data lead to the notion that runoff and erosion models in which a positive relationship between slope length and angle and runoff and erosion rates is assumed should not be applied to arid and semiarid areas. More attention should be given in latter areas to the complex hydrological relationships between rainfall scales and spatial scales. The implications of data obtained regarding the spatial distribution of areas contributing to channel flow, at a geological time scale and under changing climatic conditions, are discussed.