This paper integrates recent research to improve, test, and verify the process based DeNtrification DeComposition (DNDC) model to estimate evapotranspiration (ET) through a revised FAO Penman–Monteith approach. ET estimates along with biomass and soil water content for spring wheat and corn were improved and found to be in good agreement with the observations at three field sites in eastern Canada (Ottawa). The statistical evaluations (RMSE (mm day−1) and R2) of ET for the newly revised model (0.88 and 0.76 for corn and 0.93 and 0.78 for wheat) showed improvements over DNDC93 (1.95 and 0.22 for corn and 1.62 and 0.09 for wheat) which used the Thornthwaite equation for estimating potential evapotranspiration. In addition, evaluations of water use efficiency for corn and wheat showed good agreement between observations and the modified model. The study demonstrates the inter-dependencies of ET with biomass production and soil/atmosphere water balance.