Abstract
The choice of data periods for calibrating and evaluating conceptual hydrological models often seems ad-hoc, with no objective guidance on choosing calibration periods that produce the most reliable predictions. We therefore propose to systematically investigate the effects of calibration and validation data choices on parameter identification and predictive performance. We demonstrate our analysis on the Deggendorf/Kollbach catchment in Bavaria, Germany, for its long series of continuous hydrological and meteorological records. After classifying these data into three hydrological conditions (wet, dry and mixed) and combining them into periods of varied data length (2, 4, 8, 15, 20 and 25 years), we repeatedly calibrate a conceptual rainfall runoff hydrological model – Hydrologiska Byråns Vattenbalansavdelning (HBV) to these distinct data sets via Bayesian updating in a Monte Carlo setting. Then, we analyze predictive performance and posterior parameter statistics in various validation periods of distinct hydrological condition and time-series length. We call this the Comprehensive Differential Split-Sample Test (CDSST). Our results suggest that hydrological conditions in calibration tend to have a stronger impact than time-series length, and that calibrating on dry conditions might be a robust choice when aiming at predicting arbitrary future conditions (wet, dry or mixed). Furthermore, we found that posterior parameter estimates converged to a common optimum range with increasing data size under all investigated calibration scenarios, indicating that compensation of model structural errors by parameter fitting is independent of the chosen calibration condition. However, calibrating on time-series 8 years or longer led to overconfident predictions that failed to reliably envelope future data. While these findings are specific to our case study, we recommend using the CDSST to stress-test conceptual hydrological models to identify robust model parameters and/or deficiencies in the model structure. In general, we expect our proposed approach to be a valuable basis for model error diagnosis in any type of dynamic environmental system model, because it answers the following three questions: (1) what is the importance of physical processes not explicitly covered by the model? (2) How much overconfidence is present in the model? And (3), what are case-specific recommendations for appropriate calibration and validation setups?
Description
The comprehensive differential split-sample test: A stress-test for hydrological model robustness under climate variability - ScienceDirect
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