Duration of water saturation in selected soils of Weatherley, South Africa

Abstract

Soil water contents were measured weekly for six years (1997–2005) in the Weatherley catchment in the northern Eastern Cape Province of South Africa, and used to calculate average duration of water saturation above 70% of porosity (ADs>0.7). Data were used to determine wetness in soils, representative of midslopes, foot-slopes and valley bottoms. Hutton soils (Typic Ustorthents), representative of midslopes, had ADs>07 = 17 days year−1 in the subsoil. Westleigh soils (Aerie Endoaquents), representative of footslopes, had ADs>07 = 175 days year−1 in the subsoil. Katspruit soils (Typic Endoaqualfs), representative of valley bottoms, had ADs>07 = 334 days year−1 in the subsoil. These differences were highly significant. It was hypothesised that Hutton soils drained fastest (within half a month), and would contribute to interflow; Westleigh and Katspruit soils would drain slower (over a period of 6 and 11 months respectively) and would not contribute to interflow, but would rather contribute to peak flow during rainfall events. This hypothesis was tested against flow data from the Weatherley and Cathedral Peak VI catchments, during a rain-free period following prolonged rain. Total outflow during the rain-free periods was 4 362 m3 (2.7 mm) for Weatherley and 52 093 m3 (76.9 mm) for Cathedral Peak VI. The difference in outflow was attributed to the larger water storage capacity, from a larger area of midslope soils in Cathedral Peak catchment VI, compared to the relatively small area of midslope soils in the Weatherley catchment.