Lake water levels are easy to measure, and can be seen as a transformation of discharge. Water level measurements from 42 lakes in Sweden were analysed, and eight of these lakes were modelled using the Swedish S-HYPE model. The objective was to test if a hydrological model can be calibrated using water level data instead of discharge. A semi-analytical lake routing was developed to resolve fast variations in water levels in small lakes. Seven tests were made in which data were sampled from real observations and the model was calibrated to each data set. It was found that water levels are useful for calibration of hydrological models, even without measuring discharge and establishing a traditional rating curve. The exponent in the rating curve equation could be set to 2 as a standard value. Approximate rating curves can be identified by model calibration. The results were improved considerably already when only four observed water levels were used (mean NSE = 0.88 for both water levels and discharge), compared to the general model (−1.44 and 0.76, respectively). Calibration of both the rating curve and the inflow gave nearly as good discharge simulations (mean NSE = 0.92) as traditional calibration using recorded discharge (0.93).

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