Simulating the water footprint of woodies in Aquacrop and Apex

As the crop cultivation sector is the largest human water consumer, models that simulate its water use are important in global water studies. Within this sector, herbaceous plants and woody plants can be discriminated. Aquacrop is a plant simulation model very capable of simulating herbaceous plants, but the carry-over effects from one year to another, the large number of plant varieties and the more complicated evaporation and transpiration behaviour make the relative simple model not suited for the simulation of woodies. Apex is a model capable of simulating both herbaceous and woody
plants, but the constant that drives biomass growth changes over the seasons and locations and loses
its linearity in stress conditions. This study compares the Aquacrop and Apex in the simulation of woody plants. For this the yield, the evapotranspiration and the water footprint resulting from these are important.
From the plants with the largest harvested areas, the apple tree, the grapevine, the olive tree and
the oil palm are selected as four important plants that will be simulated in this study. Each of these
plants is simulated on a field level in the region where their core production is located. To make a comparison between the two very different models possible, the input and the processes in Aquacrop and Apex are harmonized. To allow for a simulation of woody plants, Aquacrop only simulates the yearly foliage development of an already full-grown tree. Apex can simulate the plant development
in the first years that characterize woodies.
For a full-grown woody plant, Aquacrop and Apex show different yields and evapotranspiration rates because of differences in input, parametrization and model structure. Aquacrop and Apex show roughly the same yield patterns in irrigated conditions, but in rainfed conditions large differences can occur. The evapotranspiration rates are very similar in rainfed conditions, but in irrigated conditions they deviate a lot from each other. When we compare the yield with literature, both models in general overestimate the yield. The evapotranspiration is in accordance with literature values.
The climatic variability influences the yields and evapotranspiration rates. In both models the evapotranspiration responds very realistically to yearly climate fluctuations. The yield in Aquacrop also responds as expected, but the yield in Apex is dominated by a model processes that does not correspond to the climatic variability. The influence of the soil is limited in Apex, while it can have a large effect on especially the yield in Aquacrop. The development phase of woody plants is important for the lifelong average yields, because the
first years of a plants life are characterized by a rather low yield. The evapotranspiration rate also changes over the first years, but the effect of the development phase is negligible for the lifelong
average evapotranspiration. When we take the development of yield into account for the calculation of the water footprint, it becomes visible that the water footprints in irrigated conditions are quite similar between the models, while in rainfed conditions they can differ quite a lot because of the difference in yield underlying the water footprint. Compared to the literature also large differences can occur.
Both models show their limitations. Because of this, additional research is required to compare the models under a wider scope. A case study can help to find more reliable estimates for the parameter values in the models. From this study alone, it cannot be concluded that one model is better than another. When simulating woodies, Aquacrop does not seem to be inferior to Apex, despite the fact that Aquacrop model is not designed for these plants.