Rainfall interception and the coupled surface water and energy balance

Evaporation from wet canopies (E) can return up to half of incident rainfall back into the atmosphere andis a major cause of the difference in water use between forests and short vegetation. Canopy water budgetmeasurements often suggest values of E during rainfall that are several times greater than those predictedfrom Penman–Monteith theory. Our literature review identified potential issues with both estimationapproaches, producing several hypotheses that were tested using micrometeorological observations from128 FLUXNET sites world-wide. The analysis shows that FLUXNET eddy-covariance measurements tend toprovide unreliable measurements of E during rainfall. However, the other micrometeorological FLUXNETobservations do provide clues as to why conventional Penman–Monteith applications underestimateE. Aerodynamic exchange rather than radiation often drives E during rainfall, and hence errors in airhumidity measurement and aerodynamic conductance calculation have considerable impact. Further-more, evaporative cooling promotes a downwards heat flux from the air aloft as well as from the biomassand soil; energy sources that are not always considered. Accounting for these factors leads to E estimatesand modelled interception losses that are considerably higher. On the other hand, canopy water budgetmeasurements can lead to overestimates of E due to spatial sampling errors in throughfall and stem-flow, underestimation of canopy rainfall storage capacity, and incorrect calculation of rainfall duration.There are remaining questions relating to horizontal advection from nearby dry areas, infrequent large-scale turbulence under stable atmospheric conditions, and the possible mechanical removal of splashdroplets by such eddies. These questions have implications for catchment hydrology, rainfall recycling,land surface modelling, and the interpretation of eddy-covariance measurements.JRC.E.1-Disaster Risk Managemen