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. Hydrological models play an important role in water resources management.These models generally rely on discharge data for calibration. Discharge timeseries are normally derived from observed water levels by using a ratingcurve. However, this method suffers from many uncertainties due toinsufficient observations, inadequate rating curve fitting procedures, ratingcurve extrapolation, and temporal changes in the river geometry.Unfortunately, this problem is prominent in many African river basins. Inthis study, an alternative calibration method is presented using water-leveltime series instead of discharge, applied to a semi-distributed rainfall-runoff model for the semi-arid and poorly gauged Mara River basin in Kenya.The modelled discharges were converted into water levels using theStrickler–Manning formula. This method produces an additional model output; this is a“geometric rating curve equation” that relates the modelled discharge tothe observed water level using the Strickler–Manning formula and a calibratedslope-roughness parameter. This procedure resulted in good and consistentmodel results during calibration and validation. The hydrological model wasable to reproduce the water levels for the entire basin as well as for theNyangores sub-catchment in the north. The newly derived geometric ratingcurves were subsequently compared to the existing rating curves. At thecatchment outlet of the Mara, these differed significantly, most likely dueto uncertainties in the recorded discharge time series. However, at the“Nyangores” sub-catchment, the geometric and recorded discharge were almostidentical. In conclusion, the results obtained for the Mara River basinillustrate that with the proposed calibration method, the water-level timeseries can be simulated well, and that the discharge-water-levelrelation can also be derived, even in catchments with uncertain or lacking ratingcurve information.

hydrology and earth system sciences

Rainfall-runoff modelling using river-stage time series in the absence of reliable discharge information: a case study in the semi-arid Mara River basin