Inferring mean rates of sediment yield and catchment erosion from reservoir siltation in the Kruger National Park, South Africa: An uncertainty assessment

Reservoir siltation surveys facilitate the quantification of the mean area-specific sediment yield (SSY) of catchments for decadal and longer time spans. This requires information on the volume (VS) and dry bulk density (dBD) of reservoir deposits, the period of time (TR) during which they were accumulated, the reservoir sediment trap efficiency (TE) and the catchment area (A). For the calculation of the catchment-wide average rate of erosion by water (E), the sediment delivery ratio (SDR) of the catchment needs to be estimated. Each step in this workflow represents a potential source of uncertainty. Here, we explore the extent to which individual error sources contribute to uncertainties in SSY and E values. Mean SSY and E values are inferred for small to medium-sized catchments (≤100 km2) of 15 small (≤350 × 103 m3) intermittently dry reservoirs located in the southern Kruger National Park and observation periods of 30 to 65 yr. Mean relative uncertainties of resulting SSY and E values amount to ±21% and ±46% at the 95% confidence level, respectively. Uncertainties in SSY values arise mainly from the TE estimation (mean fractional uncertainty contribution of 64%), while the SDR estimation is the major cause (79%) for uncertain E values. Uncertainties in the determination of VS and dBD values are rather unimportant, contributing together <30% and <6% to the uncertainty of SSY and E values on average, respectively. Conclusions are drawn regarding reasonable surveying and sampling strategies.