ANNUAL VARIABILITY IN SUMMER EVAPOTRANSPIRATION AND WATER-BALANCE AT A SUB-ARCTIC FOREST SITE

Evapotranspiration (ET) and precipitation were measured during five su mmers (1989-1993 inclusive) at a subarctic forest site near Churchill, Manitoba, canada. Mean daily ET varied from 2.14-3.18 mm d-1 during t he five summers, while mean daily precipitation (P) ranged from 1.46-3 .15 mm d-1. Yearly variability in summer ET was most influenced by ava ilability of surface moisture, then by atmospheric conditions (i.e. te mperature), and least of all by net radiation. In four of the five yea rs total summer ET exceeded P resulting in significant soil water defi cits and in the other year summer ET and P were similar in magnitude. The use of equilibrium evaporation (EE) as a predictor of ET was explo red. Separate relationships between ET and EE were computed for all fi ve years. Three statistically dissimilar groups of equations were foun d: 1989/1990, 1991/1992, and 1993. A single regression equation descri bing all years is presented.