FACILITATING INTERPRETATION OF THE CATHEDRAL PEAK VI CATCHMENT HYDROGRAPH USING SOIL DRAINAGE CURVES

Abstract

It was hypothesized that, with regard to subsurface flow from the cathedral Peak VI catchment: (a) a defined change in the slope of the hydrograph during a prolonged almost rain free period reflects the onset of outflow from a particular water storage zone of the catchment; (b) reliable soil (upper vadose zone) drainage curve data can help to interpret outflow from that particular water storage zone. To test the hypothesis, measured hydrograph and soil water data from previous studies in the Cathedral Peak VI catchment were used. Drainage curves were derived for the main soils during the rain free autumn periods of four years 1991 to 1994 and the relevant hydrograph were subdivided into appropriate logical regions. The contribution of the soils (upper vadose zone) to streamflow was estimated using the soil drainage curves from selected sites together with the areas of the catchment that they represented. The estimations were compared to estimations done using data from measured streamflow values for the equivalent periods of the hydrographs. The results obtained by the two estimates were as follows, with measurement year, then hydrograph and soil drainage curve estimates (m 3 ), respectively: 1991, 24841, 20061; 1992, 39048, 16528; 1993, 29967, 28448; 1994, 15703, 20809. Apart from the results for 1992, for which a logical explanation is provided, it is concluded that the hypothesis is valid.