Evaluating Evaporation Components in Flux Data and Model Output

The total observed evaporation from a vegetated surface with full cover is the sum of two components: a transpiration component coming through the plant stomata and an interception component from water on the surface of the leaves. These two components have different temporal evolution, with interception reducing quickly in the hours immediately after rainfall and transpiration continuing for much longer. Land surface models explicitly calculate the two components as their relative sizes impact the evolution of the vegetation as well as affect the land-atmosphere interactions. To evaluate such a model therefore, we must not only assess the total evaporation flux, but also the relative contributions of the interception and transpiration components. In this study, we fit hourly flux data in dry-down periods between rainfall events to a simple two-store model, and use this to identify the signatures of the two components and their relative contributions to the total evaporation flux. We use data from a variety of flux sites to identify the differences between biomes, due to both vegetation type and climate and use this to evaluate the UK land surface model: JULES.