In situ evaluation of internal drainage in duplex soils of the Tukulu, Sepane and Swartland forms

Abstract

Internal drainage is a critical hydrological process in soil water conservation. Pedological and physical properties of the South African Tukulu, Sepane and Swartland soil types were studied in respect to soil water storage and hydraulic characteristics during internal drainage. Soil profiles were excavated and classified in situ alongside instantaneous drainage plots and saturated hydraulic conductivity experiments. Laboratory desorption experiments were used to establish the soil water characteristic curves (SWCC) of diagnostic horizons. Each SWCC was then fitted with the van Genuchten 1980 model and matric suction of soil water content corresponding to the 1 200 hours drainage curves were estimated. The pedocutanic B and prismatic C horizons in the Tukulu and Sepane soils had clay plus silt content of more than 40% and were characterised by abrupt transitions, signs of wetness and a sharp drop in hydraulic conductivity at higher soil water content. Consequently, deep drainage from the Tukulu and Sepane was about 40% less than that of the Swartland. The drained upper limit was located when drainage flux rate approached 0.001 mm h⁻¹ irrespective of soil type. At this rate deep drainage losses over the seven-month fallow period are equivalent to 1% annual rainfall.

Keywords: clay soils, drainage flux, drained upper limit, hydraulic conductivity, pedological properties