Problems of estimating hydrological characteristics for small catchments based on information from the South African national surface water resource database

Rapid assessments of water resource availability in South Africa have been facilitated by the availability for a number of years of a national data set of naturalised monthly flow time series. However, these data are only available for moderate to large catchments (referred to as quaternary catchments). In the absence of further information it has often been the practice to apply a simple catchment area-based scaling factor to estimate subquaternary scale flow characteristics. This has proved to be problematic in many studies. The paper presents a comparison of quaternary and subquaternary flow data using 41 gauged catchments and develops a simple approach to scaling based on estimates of the mean annual rainfall characteristics for the two areas. The use of the scaling method in a model designed to provide preliminary, low-confidence, estimates of environmental flow requirements suggests that it represents an improvement. However, there is still a need for a method that allows flows of different magnitudes and frequencies of exceedance to be scaled differentially.