Hydrology and erodibility of the soils and saprolite cover of the Swaziland Middleveld

Abstract

The weathering mantle of the Middleveld of Swaziland consists of thick soil-saprolite complexes. The isovolumetric chemical weathering of the saprolites has led to mass losses of more than 50%. Compared with saprolites 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 maximum (4.6–7.0%) in the central saprolite zone, corresponding to saturated hydraulic conductivities of 6.02–11.81 × 10⁻⁷ m/s. Similar to the saprolites, the overlying ferrallitic soils show total pore volumes ranging from 39 to 52%. Compared to the soils, the available water capacity of the underlying saprolites is two to four times higher and the saturated hydraulic conductivity is about two times higher due to the high portion of medium pores which amount to 70% of total pore space. In the areas affected by sheet erosion, most of the soil cover is denuded and the underlying saprolites essentially determine the site properties. The hydrological properties of the saprolites are therefore of great importance with respect to erosion during wet periods and plant growth during drought periods. The low structural stability of the saprolites, indicated by shear strength values < 5 kPa, results from a silty 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 expansion of deep incising erosion gullies in areas with magmatic rocks. In contrast, the clay-rich ferrallitic soils developed from saprolite are comparatively stable, indicated by shear strength values ranging from 7 to 12 kPa. The inherent stabilizing properties of the soil are altered by overgrazing and unwise land use leading to infiltration capacities below 65 cm/day and high overland flow potentials at low rainfall intensities.