No abstract

Interception losses, rain and throughfall drop size spectra and kinetic energy were studied in four relatively undisturbed tropical forest ecosystems along a transect across the Central Andean Cordillera of Colombia at altitudes between 3000and lo00 m above sea level. Interception amounts ranged from 11 to more than 20 per cent of the total rainfall and fell within the normal range of interception figures observed in natural tropical forests.Drop size spectra were established using the filter paper method; the drop size distributions of the open field rainfall were unimodal while the throughfall had bimodal distributions, with a higher percentage of the volume of rain falling as large drops.The kinetic energy of the throughfall was higher than that of the open field rainfall (20-70 per cent), even after correcting for interception losses (4-30 per cent). Splash-cup experiments, conducted both in the field and in the laboratory, indicated that the kinetic energy is a good index of rainfall erosivity. Inside the forests the amounts of sand splashed from the splashcups was, after correction for interception losses, 2-16 per cent higher than outside the forests.Disturbance of the natural forests, for example by logging activities or cattle grazing, will further increase the throughfall kinetic energy and may lead to higher splash erosion rates inside the forests than in the open field.

Soil erodibility measurements for detachment by raindrop impact were made on samples collected at monthly intervals from forest, pasture and farmland sites. All sites exhibited strong seasonal variations in erodibility with maximum values occurring during the winter and minimum values in the summer. The relative erodibility of the different soils is described and the field and laboratory methods used are compared. The best time for sampling the soil for soil erodibility determinations is indicated.Erodibility is estimated from laboratory simulation experiments on undisturbed samples, from aggregate stability determinations and from field measurements with splash boards.

No abstract

The output of material from 11 small drainage basins in the Keuper region of central Luxembourg is considered. Attention is given to differences between forested and cultivated basins. Whilst the output of suspended solids from the cultivated Mosergriecht catchment may be as much as five times higher than from the forested Keiwelsbaach, the solute load is only 50 per cent and the runoff 20 per cent higher. Relationships between dispersed clay in suspension and the water chemistry observed for the forested drainage basins are not present in the cultivated catchments. Water having a milky appearance and containing dispersed clay is of more frequent occurrence in forested basins where subsurface runoff occurs in macropores and cracks in the undisturbed soil.