HYDROLOGY AND ERODIBILITY OF THE SOILS AND SAPROLITE COVER OF THE SWAZILAND MIDDLEVELD

The weathering mantle of the Middleveld of Swaziland consists of thick soil-saprolite complexes. The isovolumetric chemical weathering of th e saprolites has led to mass losses of more than 50%. Compared with sa prolites from quartz-diorite and granodiorite, those from diorite have higher portions of easily weatherable plagioclases and amphiboles and 20-30% greater total pore space. The macro pore space reaches a maxim um (4.6-7.0%) in the central saprolite zone, corresponding to saturate d hydraulic conductivities of 6.02-11.81 X 10(-7) m/s. Similar to the saprolites, the overlying ferrallitic soils show total pore Volumes ra nging from 39 to 52%. Compared to the soils, the available water capac ity of the underlying saprolites is two to four times higher and the s aturated hydraulic conductivity is about two times higher due to the h igh portion of medium pores which amount to 70% of total pore space. I n the areas affected by sheet erosion, most of the soil cover is denud ed and the underlying saprolites essentially determine the site proper ties. The hydrological properties of the saprolites are therefore of g reat importance with respect to erosion during wet periods and plant g rowth during drought periods. The low structural stability of the sapr olites, indicated by shear strength values < 5 kPa, results from a sil ty texture, absence of organic matter, and low contents of Fe- and Al- oxides. As a result, saprolites are highly susceptible to erosion and represent an essential precondition for the development and rapid expa nsion of deep incising erosion gullies in areas with magmatic rocks. I n contrast, the clay-rich ferrallitic soils developed from saprolite a re comparatively stable, indicated by shear strength values ranging fr om 7 to 12 kPa. The inherent stabilizing properties of the soil are al tered by overgrazing and unwise land use leading to infiltration capac ities below 65 cm/day and high overland flow potentials at low rainfal l intensities. (C) 1997 Elsevier Science B.V.