A key flux in the hydrological cycle, evapotranspiration (ET) recycles terrestrial precipitation to the atmosphere. Its energetic equivalent, latent heat flux, is central to the surface energy balance. Evaporative fraction (EF) therein quantifies the proportion of available surface energy dissipated as latent rather than sensible heat. Previous studies point to high mean ET from tropical old-growth forests and mature oil palm plantations, but fine-scale assessments of ET and EF across heterogeneous land-cover mosaics are lacking. Here, UAV (uncrewed aerial vehicle) thermography coupled with energy balance modelling is particularly suitable, covering sizable areas at high spatial resolution (~10 cm). In our study, an international team affiliated at the Universities of Göttingen, Twente, Bogor and Bonn applied the method across tropical lowland landscapes with secondary forest, oil palm and shrubland, and used a one-source energy balance approach to model ET and EF. The objective was to assess variability among and within land-cover classes, including fine-scale correlations and clustering. Our findings indicate low and variable EF in systems with limited canopy cover, i.e. (young) oil palm and shrubland, and high EF at high spatial continuity in structurally more complex and closed secondary forest, thus adding spatially coherent evaporative cooling and microclimate regulation as further arguments for preserving and promoting secondary forests in tropical mosaic landscapes, aside from well-documented benefits for biodiversity and other ecosystem functions.
Reference: Bulusu, M., Ellsäßer, F., Hendrayanto, Hölscher, D., Röll, A. (2026). Fine-scale spatial variation of evapotranspiration across tropical land-cover mosaics revealed by UAV thermography. Hydrological Processes (accepted for publication), DOI: https://doi.org/10.1002/hyp.70611
