Climatic trends associated with summerfallow in the Canadian Prairies

The Canadian Prairies have undergone important land use changes over the past 150 years. Beginning in the early 20th century, a significant portion of agricultural land was under summerfallow primarily to conserve soil moisture. The area under fallow grew to over 11 Mha, which constitute about 25% of Canadaʹs cultivated land, and mostly remained at that level until 1975, subsequent to which improved land management practices led to significant reductions in areas under summerfallow. By 2001 summerfallow area had been reduced to 5.4 Mha, and future projections expect it to fall to 3.5 Mha. Numerous modeling studies and observations have shown that land use change can have a significant impact on regional and local climate. In the Canadian Prairies, these effects would likely be seen during the mid-June to mid-July period, when agricultural crops undergo rapid foliar expansion and substantial transpiration, thus contributing to significantly higher latent heat fluxes. Observations of 1976–2000 climate trends in the black, dark brown and brown soil zones of the Canadian Prairies showed that there have been substantial reductions in maximum temperature (1.7 °C decade−1), diurnal temperature range (1.1 °C decade−1) and solar radiation (1.2 MJ m−2 decade−1), as well as a corresponding increase in precipitation (10.3 mm decade−1) during the mid-June to July period. These findings are in opposition to trends that would be expected from climate change from an enhanced greenhouse effect, and suggest that there is substantial correspondence between reductions in summerfallow and changes in climate in the agricultural regions of the Canadian Prairies.