Impacts of Sloped Terrain on Evapotranspiration from a Boreal Forested Catchment

The boreal forest is one of the most common types of landscape in the world, covering roughly 10% or the earth's emerged surface. There are a large number of studies on the exchanges of water and energy between the atmosphere and forested surfaces, but few have focused on the influence of topography on evapotranspiration (ET). Many physical processes typically found on slopes can affect ET to a significant degree. The orientation of a slope determines the local surface radiation budget. As a well-known driver of ET, it most certainly would influence water transfers to the atmosphere. Hypothetically, slope flows could provide additional turbulence and hence affect local ET. The uncertainties regarding these effects are in part due to the scarcity of ET measurements over mountainous terrain. In the past, slopes were deemed unfit for the eddy covariance approach. However, many recent studies have shown that the eddy covariance method can be employed over complex terrain when proper experimental and post-processing precautions are taken. The goal of this study is to quantify the impacts of sloped terrain on ET from various types of slopes and forest stand maturity. In summer 2015, we heavily instrumented a watershed of the Montmorency Forest (47°17'N; 71°10'W), located in the Laurentian Mountains, Quebec, Canada. Two flux towers were installed: a 20-m high tower over a juvenile forest (8 to 10-m trees) and a 10-m high tower over saplings (4 to 5-m trees). Both towers are equipped to measure eddy fluxes and net radiation on a significantly sloped terrain (about 20%). With several complementary meteorological instruments, we are measuring every major components of the surface energy budget. Since both study sites have different slope orientations and surrounding topography, we are developing the relationships between solar exposition, topographic shading and ET rates; these relationships should in all likelihood be transposable to other mountainous catchments. We also are characterizing the impacts of slope flows on local turbulence and hence ET, highlighting the distinctions between two stages of maturity of the forest. The knowledge collected throughout this study will advance our understanding of land-atmosphere interactions in sloped and forested terrains. This in turn will provide more useful information to scientists interested in modeling water flows and climate in homologous complex environments.