Hydrological modelling of fine sediments in the Odzi River, Zimbabwe

Abstract

Siltation of reservoirs is a major concern in Zimbabwe. Therefore, development of prediction tools is of great importance. In the present study a recently developed empirical sediment model (HBV-SED) based on a daily rainfall-runoff model was applied to simulate riverine fine sediment transport in a 2 486 km 2 catchment in eastern Zimbabwe. The model performance was evaluated and changes in the model structure were suggested. The modelling was, however, associated with many uncertainties due to the adopted simplification of transport processes. An analysis of the model structure and a comparison with the rating curve function was done. The required length of data for calibration purposes was evaluated and model validation through split sample and proxy basin comparison was performed. Furthermore, since the empirical model was dependent on monitored runoff and fine sediment concentrations for calibration purposes, a field measurement campaign was conducted to assess the accuracy of observed data at the station studied. The field measurements showed large errors in monitored runoff and fine sediment concentrations for the 1998/ 99 wet season, which illustrated the uncertainty in predictions of fine sediment transport based on observed data. The HBV-SED model, which was applied over a period when data were believed to be fairly accurate, simulated the fine sediment transport volume well for the validation period if it was calibrated for a minimum of four years. A shorter calibration period led to a significant increase in prediction uncertainty. The model failed to simulate individual high fine sediment peaks accurately mainly due to poor performance of the rainfall-runoff model on a daily time-scale even if the seasonal flow dynamics were described properly. In the studied catchment the HBV-SED model application resulted in equally poor R 2 -values as the rating curve technique, while the estimated fine sediment volume was more accurate.

Estimating runoff from ungauged catchments for reservoir water balance in the Lower Middle Zambezi Basin

“…Pan evaporation (cumulated to monthly time step from available daily values) from HCB for the same time period was used, and was multiplied by a factor of 0.8 so that it closely reflected plant water use (Lidén et al 2001;Gumindoga et al 2016). The average monthly evaporation rates as required in the model were calculated from the daily pan evaporation obtained.…”

Impact of limited streamflow data on the efficiency and the parameters of rainfall—runoff models

“…In the context of sediment flux modelling, Lidén et al (2001) tested the HBV-SED model on a basin in Zimbabwe. The hydrological component of the model was calibrated using 1-to 5-year periods.…”

Assessing the Presence or Absence of Climate Change Signatures in the Odzi Sub-Catchment of Zimbabwe

“…Population density of whole catchment is above 60 persons per km 2 in large parts of the catchment; land use is a mixture of communal lands, commercial farms and forests [30]. On the commercial areas, large farms are found, which run extensive farming.…”

“…It is a perennial river that rises in the Eastern Highlands and flows to the south before feeding the Save River. Total catchment area is 2486 km 2 ; altitude difference is large, 950 to 2160 m a.s.l [30]. However this study shall focus on the upper sections of the sub catchment, as shown in Figure Figure 2 hereafter termed micro catchment.…”

“…The geology of the area is dominated by granitic bedrock and in-situ weathered sandy soils [30]. The landscape is hilly, specially in the upper parts where the mountains reach heights up to 2000 m but also the western spur of Chimanimani mountains where some tributaries rise [31].…”