Moisture recycling on the Canadian Prairies for summer droughts and pluvials from 1997 to 2003

This study investigated the contribution of land surface evapotranspiration to the atmospheric water balance for the agricultural region of the Canadian Prairies by estimating the recycling ratios, including the moistening and precipitation efficiencies, for drought areas, for areas which received normal rainfall, and for pluvial areas for the summers of 1997-2003. It asked the question: are dry and wet summers perpetuated by moisture and energy feedbacks from the surface water budget to the atmospheric water balance? A comparison of the study period mean moistening efficiencies for the drought areas, for the areas which received normal rainfall, and for the pluvial areas, along with the relatively high correlation between the area-average moistening efficiencies and the area-average rainfall amounts for the Census Agricultural Regions (CARs), indicated an intra-summer feedback between evapotranspiration and rainfall. The study period mean precipitation efficiencies, for the summer when precipitation on the Canadian Prairies is predominately convective, indicated that moisture, convective available potential energy (CAPE) and lift came together, and converted water vapour into rain, less frequently, on average, for the drought areas than for the areas which received normal rainfall, and more frequently, on average, for the pluvial areas. The CAR area-average moisture influx values demonstrated that there was generally sufficient moisture advected into the Canadian Prairies to produce the observed range of rainfall amounts. Thus, it was the other ingredients that are required for convective rain, that is CAPE and lift, which were relatively scarce during summer droughts, but plentiful during pluvials. Clear differences in the study period mean summer recycling ratios for the drought areas, for the areas which received normal rainfall, and for the pluvial areas indicated that recycled moisture, attributable to evapotranspiration within the region, was a factor in determining whether an area had a dry, normal or wet summer. Therefore, it can be concluded that, for the agricultural region of the Canadian Prairies, dry and wet areas during the summers of 1997-2003 were perpetuated by moisture and energy feedbacks from the surface water budget to the atmospheric water balance.