In the summer of 1987 a detailed field experiment was conducted at the Petawawa National Forestry Institute at Chalk River, Ontario, to evaluate the performance of the water balance technique for the estimation of evapotranspiration. Bowen ratio – energy balance estimates of evapotranspiration, available over a portion of an intensive measurement period, provided an independent check on the water balance estimates. The daily totals of evapotranspiration were on average 1% less than the Bowen ratio – energy balance estimate. The length of time required to maintain the evapotranspiration estimates below a 10% level of uncertainty using the water balance technique is 13 days. The lowest average daily percent probable error is obtained for an integration period with (i) a minimum number of precipitation events, (ii) a large number of days, and (iii) a high evapotranspiration rate.