Comment on hess-2021-597

doi:10.5194/hess-2021-597-rc1

Abstract

Various methods are available for assessing uncertainties in climate impact studies. Among such methods, model weighting by expert elicitation is a practical way to provide a weighted ensemble of models for specific real-world impacts. The aim is to decrease the influence of improbable models in the results and easing the decision-making process. In this study both climate and hydrological models are analyzed and the result of a research experiment is presented using model weighting with the participation of 6 climate model experts and 6 hydrological model experts. For the experiment, seven climate models are a-priori selected from a larger Euro-CORDEX ensemble of climate models and three different hydrological models are chosen for each of the three European river basins. The model weighting is based on qualitative evaluation by the experts for each of the selected models based on a training material that describes the overall model structure and literature about climate models and the performance of hydrological models for the present period. The expert elicitation process follows a three-stage approach, with two individual elicitations of probabilities and a final group consensus, where the experts are separated into two different community groups: a climate and a hydrological modeller group. The dialogue reveals that under the conditions of the study, most climate modellers prefer the equal weighting of ensemble members, whereas hydrological impact modellers in general are more open for assigning weights to different models in a multi model ensemble, based on model performance and model structure. Climate experts are more open to exclude models, if obviously flawed, than to put weights on selected models in a relatively small ensemble. The study shows that expert elicitation can be an efficient way to assign weights to different hydrological models, and thereby reduce the uncertainty in climate impact. However, for the climate model ensemble, comprising seven models, the elicitation in the format of this study could only reestablish a uniform weight between climate models.

Highlights

Uncertainty of future climate projections is a key aspect in any impact assessment, such as hydrological impacts (Kiesel et al, 2020; Krysanova et al 2017)
Probabilities for the three models in the French case study had little spread compared to the other case studies (Fig. 3a)
Our study suggests that expert elicitation can be a suitable methodology to assign probabilities to hydrological models applied for climate change impact assessments as these probabilities are variable and robust across the expert panel and across the different elicitation steps and discussions (Table 4)

Summary

Introduction

Uncertainty of future climate projections is a key aspect in any impact assessment, such as hydrological impacts (Kiesel et al, 2020; Krysanova et al 2017). 60 Information on the confidence of the climate change projection and impacts result is often not sufficiently transparent for end-users (Schmitt and Well, 2016). If the models that are discarded or given low weight have projections furthest away from the ensemble mean, potentially as a consequence of missing process descriptions, model weighting may in addition result in a reduced uncertainty. In this respect there are different traditions in 70 the climate and hydrological modelling communities

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Discussion

Conclusion