Evapotranspiration partitioning and its implications for plant water use strategy: Evidence from a black locust plantation in the semi-arid Loess Plateau, China

Evapotranspiration (ET) partitioning is crucial for understanding the impacts of ecophysiological and physical processes on water balance and plant water use strategy. Black locust (Robinia pseudoacacia) is one of the most widely planted species in the semi-arid Loess Plateau, China. The effects of its water use on stand soil water storage (S) and its water use strategy raised much concern due to soil water depletion and its growth degradation. In this study, meteorological factors, soil water content (SWC), ET components and net primary productivity (NPP) were observed in a black locust plantation during a drier (2015) and a wetter (2016) growing season. ET was 160.0 mm in 2015 and 255.0 mm in 2016, which accounted for 104.1% and 68.6% of the precipitation (P), respectively. Soil evaporation (E) accounted for the majority of ET (45.4-69.4% at monthly scale), while transpiration (T) was the smallest component of ET (9.7-28.8%). These results showed that it is E that consumed more soil water rather than T in the black locust plantation. E should be carefully considered and improved in predicting or modeling water budgets (e.g. soil water, ET partitioning and runoff) in the reforested catchments in this area. Additionally, the monthly T/ET decreased with P increasing, suggesting that black locusts could transpire water more effectively to survive under drier conditions with less P. Our study can not only improve the understanding of water budget in the reforested catchment in the semi-arid Loess Plateau but also provide the significant evidences of investigating the plant water use strategy from the point of ET partitioning.