摘要：Steeplands, when cleared from forests, are susceptible to erosion by rainfall and are prone to land degradation and desertification processes. The dominant factors affecting those erosion processes and hence the resulting runoff and soil losses are the aggressiveness of the rainfall during the successive plant growth stages, the soil cover-management, but also the topography (slope length and slope steepness). Depending on the type of (agro) climatological zone, the runoff water should either be limited and controlled (excess of water) or should be enhanced and collected from the slope on the downslope cropping area if water is short (negative soil water balance). Examples are given of practical applications in Ecuador where alternative soil conservation scenarios are proposed in maize cultivation in small fields on steep slopes. Adding peas and barley in the rotation of maize and beans resulted only in a slight decrease of the soil losses. Subdividing the fields into smaller parcels proved to give the best reduction in soil loss. Because the average slope steepness is high, erosion control measures such as contour ploughing and strip cropping have only small effects. Erosion and its effect on productivity of a sorghum -livestock farming system are assessed on four different areas in Venezuela with different levels of erosion. A Productivity Index (PI) and an Erosion Risk Index (ERI) were used to classify the lands for soil conservation priorities and for alternative land uses. Intensive agriculture can be applied on slightly eroded soil, whereas severely eroded soil can be used with special crops or agro-forestry. Semiintensive agriculture is possible on moderately eroded soil. Reforestation of drylands in Chili requires understanding of the infiltration/runoff process in order to determine dimensions of water harvesting systems. Infiltration processes in semi-arid regions of Chile were evaluated, using rainfall experiments and constant-head infiltration measurements. Correlations between infiltration parameters and locally variable characteristics as soil structure, field slope and stoniness were investigated for six different sites, aiming at improving the design and positioning of runoff collecting systems.
