The complementary relationship evapotranspiration theory and models are discussed and ex plored.Based on the long-term observed solar radiation, evaporation and meteorological data in Hubei Province, the formulas for calculating potential and wet surface evapotranspiration are proposed and established.Morton models(CRAE,CRWE and CRLE) are modified and used to estimate catchment evapotranspiretion and lake evaporation.It is shown thaqt the model estimaters are very close to the Pan data and water balance estimator.The modified models provide a fairly reliable estimate of monthly and annual evaporation.The complementary relationship theory provides another way for the resea ch and the estimation of evapotranspiration.