Impacts of anthropogenic forcing and internal variability at the watershed scale - going from the daily to the hourly time scale

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Climate change resulting from the anthropogenic impact on radiative forcing is now well established. In addition to increases in projected surface temperatures, the changes will affect all major components of the water cycle, including variability and intensity of rainfall at various spatial and temporal scales. The knowledge of the impacts of these changes comes largely from general circulation and earth system models. However, due to the limitations related to the available computing power, the outputs of these models have mostly been available at the daily time-step until recently. Changes at the sub-daily scales are therefore less well documented. Nevertheless, releases of higher-resolution regional climate models (convection-permitting or not) indicate that the impacts of anthropogenic forcing will be greater for short-duration and intense rainfall. We can thus predict a disproportionate impact on small watersheds whose hydrological response is rapid and commensurable with that of convective precipitation systems. Similarly, we can also predict a decrease in the impact of snowmelt related floods to those related to summer fall precipitation over small watersheds (<1000 km2). In hydrology, these results point to major impacts towards small rural and urbanized watersheds. The potential impacts of anthropogenic forcing change on the hydrology of these basins have been poorly studied for two main reasons: 1- There is no reliable hydrometeorological database at sub-daily time steps in North America (as well as in the majority of the world, except for a few European countries) to study these watersheds in sufficient numbers; 2- There are still few climate models whose main outputs (e.g. temperature and precipitation) are available at the sub-daily time steps. This research program specifically aims to fill this knowledge gap by using the tools and methodologies developed within our research laboratory and elsewhere to study the impacts of climate change at the daily time step on small to medium-size watersheds. These tools and methods will be combined with the use of new databases and outputs of climate models with high temporal and spatial resolution to thus be able to tackle the study of small to mdieum-size watersheds up to the hourly time step and below.