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Water-carbon relationships have been widely recognized in previous studies but rarely included in the Budyko framework. This study improves the Budyko-Fu model by considering the relationship of the underlying surface parameter with vegetation dynamics. Then the evapotranspiration (ET) was estimated using the improved Budyko model and the main driving factors of ET change were identified. The results show that the improved Budyko model considering the gross primary production in the equation could capture the annual ET changes quite well. ET tends to increase with an increase rate of 6.89 mm/a in the study area. Vegetation changes is the most important factor influencing the ET changes contributing 69.87 %, while climate changes in precipitation and potential evapotranspiration contribute 31.23 % and −1.10 % respectively. The main contributors to the estimated ET change differed in the subregion. With vegetation changes being the dominant factor for ET change in the southwestern part, while ET increase was mainly due to the increase in precipitation in the northeastern part of the study area. The contribution of vegetation to the estimated ET change shows a spatially increasing trend from northeast to southwest, while the contribution of precipitation shows a decreasing trend from northeast to southwest. This study proposed a new method for the estimation of ET changes based on the water-carbon coupling relationships and highlight the different contributions of vegetation dynamics to ET changes.