A hydrological model that incorporates the energy balance (EVAP-1)

The ability of hydrologists to provide reliable assessments of the impact of climate change on the hydrological regime is affected not only by their choice of hydrometeorological tools but also, and more importantly, by their choice of hydrological model and evapotranspiration formulation. The latter was a focus of recent debates within the hydrologic and atmospheric modeling communities because, even though both climate and hydrological models calculate evapotranspiration, they often result in strongly divergent values. It has been shown that the current recourse to the agronomic concept of potential evapotranspiration in hydrological models may be their weak point, given its lack of robustness in a changing climate.

This project thus aims at developing more physical approaches for calculating evapotranspiration within the hydrological models that are grounded on the conservation of energy at the surface. To do this, an integrative approach, combining both field measurements and modeling results, is proposed. It will allow the improvement of the tools currently available for assessing the potential impacts of climate change on water resources and hydrological extremes, such as severe floods and droughts

Photo : D. Nadeau

Improve the representation of evapotranspiration in hydrological models in order to provide more reliable assessments of the impact of climate change on the hydrological regime of river watersheds.

This will allow the provision of credible information for various vulnerability and adaptation studies, which in turn will ensure better water resources management in Quebec.

• Hydrometeorological sampling campaign to measure the exchange of water and energy as well as the accumulation and snow melt throughout the watershed on two sites in southern Quebec;

• Observation and modeling of the effects of a persistent snow cover under forest cover on hydrometeorological processes;

• Post-processing of climate simulations used for hydrological projections;

• Coupling of a surface-vegetation-atmosphere transfer model, known as a land-surface scheme, with an operational semi-distributed hydrological model;

• Inclusion in global hydrological models of an evapotranspiration module based on the maximum entropy production principle.

This project performed an in-depth study of the evolution of a set of hydroclimate variables in order to better understand the transfer of water and energy across the land-vegetation-atmosphere interface, in which evapotranspiration plays an important role. A vast hydroclimate dataset from a multitude of sites around the world (including northern sites) was compiled to enable the quantification of water and energy cycles under a wide variety of climate conditions. These data proved to be very useful for model validation.

First, special attention was given to the installation of equipment for measuring water and energy budgets in a forest watershed in a humid boreal environment: the Ruisseau des Eaux volées experimental watershed (BEREV) in Forêt Montmorency (FM). This site is now home to state-of-the-art micrometeorological and hydrometeorological equipment, such as 3D ultrasonic anemometers, hygrometers, radiometers, sap flow sensors, piezometers, gauging stations, etc. (Fig. 1). 

Figure 1: Micrometeorological and hydrometeorological equipment in the forest

The hydrometeorological observations collected over several years have led to a better understanding of the hydrological regime of humid boreal forests. Figure 2 (Isabelle et al. 2020) shows the cumulative curves of the BEREV water budget components for two hydrological years.

Figure 2: Cumulative curves of BEREV water budget components over the 2016-2017 and 2017-2018 hydrological years  (Isabelle et al., 2020).

The hydroclimate data show that more than 60% of precipitation leaves this watershed through its water system, which is twice the global average. This explains why so many regions in Canada have such abundant access to water. Most evapotranspiration takes place from June to September: 67% and 61% in the 2016-2017 and 2017-2018 hydrological years, respectively. It should also be noted that 12 to 16% of annual water vapour transfer to the atmosphere occurs via sublimation when air temperatures are below 0 °C.

The EVAP project also refined our knowledge of how to best model evapotranspiration and other land surface fluxes. Two modelling approaches based on ground surface energy conservation were explored: a purely physical approach using meteorological surface models, and the Maximum Entropy Production (MEP) approach, based on Bayesian probability theory and nonequilibrium thermodynamics.

Figure 3 compares the latent and sensible heat fluxes simulated by the CLASS surface model and the MEP model. Initially, simulations were performed for sites characterized by a year-round absence of snow, for a variety of ecosystems, and both CLASS and MEP models showed great promise (Hajji et al. 2018, Alves et al. 2019, 2020). The CLASS and SVS surface models were also compared (Leonardini et al. 2020). Meanwhile, an efficient coupling of the different MEP modules was developed to combine in a flexible and continuous manner the three existing MEP models, each of which is specialized for a specific surface type: bare soil, vegetation and snow (Hajji et al. 2018). The study was then expanded to snowy sites, with the inclusion of sublimation.

Figure 3: Latent and sensible heat flux as simulated by CLASS and MEP for a variety of ecosystems (Alves et al., 2019)

• Ministère de l’Environnement et de la Lutte aux Changements Climatiques (MELCC)

• Environment and Climate Change Canada (ECCC)

• Hydro-Québec