The water balance of a first order catchment in the montane grasslands of South Africa

Measurements of daily actual evaporation, streamflow, rainfall and soil water storage were measured for a two-year period in a 97 ha grassland catchment located in the Natal Drakensberg. Actual evaporation was measured using the Bowen ratio energy balance technique. Annual precipitation and discharge data were also used to estimate evaporation over a number of years. Results show that in normal years precipitation is equally split between evaporation and streamflow. In dry years evaporation is the dominant component of the water balance. The data were used to develop simple expressions to calculate annual streamflow and evaporation. Good agreements between actual and modelled trends of streamflow using the ACRU hydrological model were found (r=0.80). In general ACRU underestimated streamflow by 15%.