Lab lysimeter disaggregated ET data for the validation of a two-source model

One of the main issues with obtaining accurate Evapotranspiration (ET) measurements for heterogeneous crops is managing the partition between the contribution of bare soil / cover crop and that of the main crop. Mostly, ET estimates are obtained as an aggregate of the two components, as direct measurements of distinct Evaporation (E) and Transpiration (T) are possible only with high-accuracy and time-costly field lysimeters. Hydrological modelling can provide these kinds of estimates, with the dichotomy between single-source (one energy balance equation for the whole pixel) and two-source (one balance equation each for the vegetated and the non-vegetated pixel fraction) models approaching the problem from different perspectives. In this work, a laboratory lysimeter was employed to obtain disaggregated fluxes from a global ET value and use them to validate the partitioned estimates from a two-source version (FEST-2-EWB) of the single-source FEST-EWB distributed hydrological model, which was also included in the validation as a reference. The lysimeter was sown with grass distributed in three rows, alternated with similar rows of bare soil, with irrigation being provided to the former and not to the latter. Thermal imagery from proximal sensing observations was used to calibrate the models. Two boxes were placed on the lysimeter, one completely vegetated and the other left bare. These boxes were periodically weighted separately from the lysimeter, obtaining accurate measurements of their ET, that were then scaled back to the correspondent areas in the main lysimeter. The model runs, provided similar calibration performances, showed similar global ET values, close to those measured over the lysimeter, but diverged when looking ad transpiration alone. The two-source model offered estimates much closer to those derived from the lysimeter than the single-source model.