Dynamics of evapotranspiration partitioning for apple trees of different ages in a semiarid region of northwest China

Evapotranspiration in semiarid regions generally represents the greatest loss of water from an ecosystem and is sensitive to changes in the cover of vegetation. In this work, we compared the effects of apple tree of different ages by partitioning evapotranspiration into canopy interception (calculated), tree transpiration (thermal-dissipation probes) and soil evaporation (micro-lysimeter) in a semiarid region of northwest China from May to September in 2012, 2013 and 2014. Tree age had a clear influence on evapotranspiration partitioning. Daily transpiration and evapotranspiration were always higher for the 17- than the 7-year-old trees. Monthly canopy interception and transpiration of the 17-year-old trees were always higher, and monthly soil evaporation was always lower than the 7-year-old trees. Evapotranspiration was 339.1, 341.4 and 312.4 mm and 361.1, 367.2 and 336.3 mm for the 7- and 17-year-old trees in 2012, 2013 and 2014, respectively. Annual soil evaporation accounted for a large proportion of the evapotranspiration for the 7-year-old apple trees, ranging from 51.7 to 53.6%, and transpiration accounted for a large proportion of the evapotranspiration for the 17-year-old apple trees, ranging from 47.8 to 49.1%. Reference evapotranspiration was low during our experimental periods, and the relationship between actual evapotranspiration and reference evapotranspiration differed between the 7- and 17-year-old trees. Tree age was mostly responsible for the differences in evapotranspiration and its partitioning due to the morphologies of the trees. Tree age should therefore be taken into account when assessing the influence of evapotranspiration by apple trees on regional water budgets under our or similar climatic conditions.