Author Topic: Why does a conceptual hydrological model fail to predict discharge changes in re  (Read 180 times)

Pankaj Dey

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  • Institute : Indian Institute of Science
  • Programming language : MATLAB, R
Several studies have shown that hydrological models do not perform well when applied to periods with climate conditions that differ from those during model calibration. This has important implications for the application of these models in climate change impact studies. The causes of the low transferability to changed climate conditions have, however, only been investigated in a few studies. Here we revisit a study in Austria that demonstrated the inability of a conceptual model to simulate the discharge response to increases in precipitation and air temperature. The aim of the paper is to shed light on the reasons of these model problems. We set up hypotheses for the possible causes of the mismatch between the observed and simulated changes in discharge and evaluate these using simulations with modifications of the model. In the baseline model, trends of simulated and observed discharge over 1978–2013 differ, on average over all 156 catchments, by 92 ± 50 mm yr−1 per 35 yrs. Accounting for variations in vegetation dynamics, as derived from a satellite-based vegetation index, in the calculation of reference evaporation explains 35 ± 9 mm yr−1 per 35 yrs of the differences between the trends in simulated and observed discharge. Inhomogeneities in the precipitation data, caused by a variable number of stations explain 44 ± 28 mm yr−1 per 35 yrs of this difference. Extending the calibration period from 5 to 25 yrs, varying the objective function by including annually aggregated discharge data, or estimating evaporation with the Penman–Monteith instead of the Blaney–Criddle approach has little influence on the simulated discharge trends. The precipitation data problem highlights the importance of using precipitation data based on a stationary input station network when studying hydrologic changes. The model structure problem with respect to vegetation dynamics is likely relevant for a wide spectrum of regions in a transient climate and has important implications for climate change impact studies.