Chapter 3 patterns and thresholds of runoff generation and sediment transport on some Mediterranean hillslopes

Abstract

Runoff and sediment transport data obtained from hillslopes in two limestone areas in southeast Spain are analysed in order to define spatial and temporal thresholds for sediment movement at the patch scale under Mediterranean semiarid and subhumid climatological conditions. The data discussed in this paper include a 7-year series of runoff and sediment collection in open Gerlach plots. The 136 events are analysed in relation to characteristics of rainfall, soil and soil surface components. In both cases slopes behave as a patchwork of runoff and runon areas and the size of the runoff or runon patches being dependent on the climatological conditions. These control the hydrological disconnection between different parts of the slopes. Hortonian and saturation runoff can both be generated and infiltrated downslope. The differences in rainfall characteristics lead to the differences in transport rates between sites. But soil properties and surface components characteristics are also very relevant and do not always behave in the same way. In these conditions runoff rates are similar in both sites, but sediment yield is much higher in the dryer site. The role of the biological factor in controlling runoff and sediment transport is higher in subhumid site. When we move to the drier site, abiotic factors become more important. Some thresholds are defined for the changes in runoff generation mechanisms and rates of sediment transport.