Quantification de l’impact de l’incertitude météorologique sur les prévisions de température de l’eau d’une rivière

Abstract

Producing river water temperature forecasts is crucial to guarantee water quality and to anticipate the threats that could impact ecosystems. For a managed watershed, forecasting this variable allows adapting the streamflow to respond to thermal threats, since it is an important influencing factor of the river thermal variations. In this context, it is the meteorological forecasts that become the main uncertainty source for water temperature forecasts. Thus, the goal of this project is to quantify the impact of meteorological uncertainty over the water temperature forecasts of a managed watershed. The Nechako River, a managed river system in British-Columbia, is used in this project as a study case. Probabilistic water temperature forecasts were produced by feeding meteorological forecast ensembles into the HEC-RAS model of the river. This process was then reproduced by introducing variations over the meteorological lead-time, in order to reduce their uncertainty and produce new water temperature forecasts. The uncertainty of the different ensembles was considered using three characteristics: their sharpness, their accuracy, and their reliability. The results of this study show that the thermal forecasts were indeed significantly improved in terms of their sharpness and their individual accuracy, but no significant impacts were noted over the probabilistic accuracy of the ensembles. Reliability was also investigated, and it was revealed that water temperature forecast ensembles were initially under-dispersed over the Nechako River, and that this issue was exacerbated when the HEC-RAS model was forced with more certain ensemble weather forecasts. The geographical disposition of the river and the meteorological forecast’s reliability are considered and discussed as causes of this issue. In the end, the analysis showed that acting on the meteorological uncertainty has a significant impact on the uncertainty of water temperature forecasts of a managed watershed. Looking into meteorological uncertainty reductions over other such managed watersheds could certainly help to reduce the thermal uncertainty. Nevertheless, it would then be necessary to identify the strengths and the weaknesses of the original forecasts prior to any adaptations, to aim for the specific needs of the studied watersheds.

Date	9 May 2022
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Richard Arsenault (Supervisor) & Annie Poulin (Co-supervisor)