Evapotranspiration partitioning based on underlying conductance in a complex tree-grass orchard ecosystem in the humid area of southern China

Partitioning of evapotranspiration (ET) into its components (vegetation transpiration (T), soil evaporation (ESoil) and evaporation from intercepted water (Ei)) in agroforestry systems can provide valuable assessments of plant water use. However, it is still challenging to quantify the contributions of different vegetation elements to T, especially when the underlying surface is complex, consisting of trees and heterogeneous grass. Based on the water-carbon coupling theory and flux conservation hypothesis, an ET partitioning method was proposed to partition total ET into T from the tree (TTree) and understory grass (TGrass), and ESoil in a humid region tree-grass orchard ecosystem during three growing seasons (from mid-March to end-October) of 2018-2020. The results demonstrated that the partitioned TTree and understory evapotranspiration (ETun=TGrrass+ESoil) agreed well with the values measured by the heat-pulse sap flow sensors (R2 of 0.744, and RMSE of 0.379 mm d-1) and microlysimeters (R2 of 0.650 and RMSE of 0.545 mm d-1). The derived underlying transpiration (TGs=TTree+TGrass) was strongly consistent with Zhou's underlying water use efficiency approach (R2 of 0.939 and RMSE of 0.382 mm d-1). The estimated TGs was 536, 433 and 383 mm during the three growing seasons, resulting in a higher TGs/ET ratio of 0.74 +/- 0.03 (+/- standard deviation). Among them, TGrass contributed to 0.32 +/- 0.05 ET, while the contribution was 0.42 +/- 0.02 for TTree. Esoil and residual evaporation (including Ei and estimated error) accounted for a small ET proportion, with the value of 0.13 +/- 0.02 and 0.14 +/- 0.01, respectively. The partitioned ET components (TTree/ET, TGs/ET and ESoil/ET) were well controlled by variations of atmospheric evaporation demand and vegetation factors, but were little affected by soil water content. Increased tree leaf area limited TGrass due to shading effects, however, TGrass tended to contribute more transpiration water under a higher atmospheric evaporation environment. The findings of this study enhance the knowledge about the contributions of individual ET components on total ET in a complex tree-grass ecosystem, and provide further insights on plant-plant interactions in terms of water use strategies and orchard water management.