Uncertainty in water resource model parameters used for climate change impact assessment

Abstract

Abstract:Despite their acknowledged limitations, lumped conceptual models continue to be used widely for climate-change impact assessments. Therefore, it is important to understand the relative magnitude of uncertainties in water resource projections arising from the choice of model calibration period, model structure, and non-uniqueness of model parameter sets. In addition, external sources of uncertainty linked to choice of emission scenario, climate model ensemble member, downscaling technique(s), and so on, should be acknowledged. To this end, the CATCHMOD conceptual water balance model was used to project changes in daily flows for the River Thames at Kingston using parameter sets derived from different subsets of training data, including the full record. Monte Carlo sampling was also used to explore parameter stability and identifiability in the context of historic climate variability. Parameters reflecting rainfall acceptance at the soil surface in simpler model structures were found to be highly sensitive to the training period, implying that climatic variability does lead to variability in the hydrologic behaviour of the Thames basin. Non-uniqueness of parameters for more complex model structures results in relatively small variations in projected annual mean flow quantiles for different training periods compared with the choice of emission scenario. However, this was not the case for subannual flow statistics, where uncertainty in flow changes due to equifinality was higher in winter than summer, and comparable in magnitude to the uncertainty of the emission scenario. Therefore, it is recommended that climate-change impact assessments using conceptual water balance models should routinely undertake sensitivity analyses to quantify uncertainties due to parameter instability, identifiability and non-uniqueness.

Are climate models “ready for prime time” in water resources management applications, or is more research needed?

“…Likewise, we should not apply watershed models uncritically as these will compound the uncertainty still further (Wilby, 2005). Such a negative result is useful, cautioning hydrologists against uncritically following this path; for without bias correction, model simulations cannot mimic reality.…”

Climate uncertainty in flood protection planning

“…Overall however, the error from the hydrological model is small, in particular for high flow indicators (Velázquez 5 et al, 2013). It is likely smaller than or comparable to forcing uncertainty (Wilby, 2005).…”

Exercice de scénarisation hydrologique en Afrique de l’Ouest—Bassin du Bani

“…Ils jugent qu'il faut donc utiliser avec précaution les jeux de paramètres calés dans un contexte de changement climatique, précisant que les paramètres sont dépendants de la période de calage. Wilby (2005) constate la même chose sur des bassins crayeux du Sud-Est de l'Angleterre : si on cale le modèle CATCHMOD en période de forts écoulements, les débits sont mieux reproduits sur des périodes de faibles écoulements que l'inverse. Il en attribue la raison à la plus grande gamme de conditions que contiendrait une période de forts écoulements.…”

“…Tout le long de cette chaine, de nombreuses incertitudes interviennent et il est important de les connaître (Wilby 2005, 2010, Harding et al 2012 Il n'y a bien souvent aucune justification aux différents choix qui sont faits, et il est donc plus judicieux d'utiliser le terme de projection ou de scénario (que nous avons employés ici) plutôt que de prévision, qu'elle soit climatique ou de ressource en eau, pour l'objectif qui est le nôtre : estimer la ressource en eau dans le futur, évaluer les impacts d'un changement global. L'objectif de ce travail est de s'intéresser plus précisément à la scénarisation hydrologique.…”