Anthropogenic vegetation transformation and the potential for deep convection on the Canadian prairies

Abstract

Listen

Translate article

Widespread agricultural cultivation has converted nearly 60% of the Canadian Prairie grasslands eco-climatic zone from native mixed perennial grasses to annual field crops, predominantly spring wheat. This study examined the effect this anthropogenic landscape transformation has had on the partitioning of latent and sensible heat. For each type of vegetation, a representative growing-season Bowen ratio curve, based on modelled evapotranspiration values, was determined for sample sites — Winnipeg, MB, in the transitional grassland, and Swift Current, SK, in the arid grassland. A relationship between the Bowen ratio, the noon global radiation and the change in the Lifted Index between morning and afternoon was used to asses how modifying the apportionment of surface heat flux may have changed the seasonal frequency and severity of thunderstorms. Prior to the emergence of annual crops, and again during their senescence and in the post-harvest period, Bowen ratios are generally higher (i.e., evapotranspiration rates are lower) than they would be if the Prairie grassland eco-climatic zone had remained a sea of perennial grasses. Thus, the available buoyant energy or the potential for deep convection has been reduced in these periods. Given similar atmospheric dynamic forcing and advection patterns, this change has likely reduced the frequency of thunderstorms during the early and the late portions of the growing season. In contrast, during rapid foliage expansion and seed production for spring wheat, and for similar annual field crops, Bowen ratios are generally lower (i.e., evapotranspiration rates are higher) than they would be without agriculture. Thus, the potential for deep convection has been enhanced in these periods. With similar dynamic and advection patterns, it is probable that thunderstorms are now more frequent in the middle of the growing season and increased available buoyant energy may have made them, on average, more severe. Key words: Agrometeorology, land-use change, thunderstorms