Modeling the coupling processes of evapotranspiration and soil water balance in agroforestry systems

How evapotranspiration (ET) and its partitioning in agroforestry systems are influenced by canopy structure as well as soil water availability is not well explained. Modeling work was combined with field experiment in this study to investigate the interaction of ET and soil water conditions in an apple tree (Malus pumila M.) and cocksfoot (Dactylis glomerata L.) agroforestry. The experiment was conducted in 2016–2018 in an apple orchard at a spacing of 4 m × 4 m on the Loess Plateau of China. The three tested planting patterns were: monoculture apple tree with clean tillage (CT), agroforestry with 2.4-meter-wide cocksfoot strips between tree rows that harvested frequently to maintain a low coverage (LC), and agroforestry with cocksfoot of a greater coverage (GC). The multi-source ET model developed for intercropping system was improved by considering the effects of large height difference of different species in agroforestry on water vapor transporting resistances, then the improved ET model was combined with a soil water balance model to simulate ET partitioning. Soil water content in 0–200 cm layer, seasonal ET calculated with soil water balance model, and directly measured understorey ET were used to validate the model. Results showed that the root mean square error (NRMSE) for simulated soil water content ranged from 7.3% to 10.1%, and NRMSE for simulated total ET and understorey ET were only 3.9% and 4.2% respectively. Transpiration of apple tree in GC and LC was reduced by 21.3−32.5% and 12.3−24.2% after applying agroforestry, the reduction was mainly attributed to the decrease in soil water content. Planting cocksfoot also reduced soil evaporation by 21.5−27.6%, total ET in the agroforestry was only changed by − 4.9–6.7% compared to CT. Scenario simulation indicated that the negative effects of cocksfoot on apple tree transpiration linearly decreased with increase of the maximum LAI of apple tree and exponentially increased with the increase of strip width of cocksfoot. An 18-year simulation showed that GC and LC reduced the transpiration of apple tree by 7.3% and 2.1% respectively, but did not accelerate soil water depletion in the deep soil layers. Therefore, agroforestry with well managed understorey grain or cover crops is encouraged to be applied in our study area to increase the production and provide ecological services. Water transportation model developed in this study could also be used to evaluate water use and improve the design and management of other agroforestry systems.