Summer rainfall recycling for an agricultural region of the Canadian prairies

Abstract

Water vapour from regional evapotranspiration that falls again as rain within the same region is referred to as recycled rainfall. This study estimated the summer rainfall recycling ratios (i.e., the relative contribution of recycled rain to total rain) for an agricultural region of the Canadian Prairies by examining the soil water balance component of the land surface branch of the water cycle along with the atmospheric branch of the cycle for June-July-August of 1997, 1998 and 1999. Recycled rain accounted for 24 to 35% of the total rainfall. For these three summers, this study also estimated the horizontal water vapour flux divergence for the region, and compared the areal average summer evapotranspiration totals determined from land-use weighted soil water balance modelling and as residuals in the atmospheric water balance equation. The horizontal water vapour flux divergence per unit area averaged 36 mm. The mean difference between the evapotranspiration estimates was about 15%. It can be concluded, for the summers of 1997, 1998 and 1999, that: (1) regional evapotranspiration was secondary to advection, yet significant, as a source of water vapour for rainfall on the Canadian Prairies, (2) the agricultural area of the Canadian Prairies was a source region of water vapour - consistent with the arid character of the climate, and (3) the land-use weighted soil water balance estimates of areal average evapotranspiration were reasonably accurate.