Macrohydrology — The new challenge for process hydrology

Abstract The paper describes an important, new area of hydrological interest, Macrohydrology, and highlights research into large-scale hydrological processes with emphasis on evaporation. It reviews three recent and relevant international experiments, ARME, HAPEX and FIFE, each illustrating different perspectives but all addressing the need to provide a large-scale description of land surface hydrology. Observations from these experiments and the results of recent theoretical studies are used to achieve a better definition of the large-scale aggregation process. The use of a one-dimensional model format is supported, providing this includes a vegetative canopy, and providing model parameters no longer necessarily possess local physical or physiological relevance. The role of mesoscale meteorological processes and of the atmospheric boundary layer (ABL) is discussed around classes of land surface defined with respect to scale and order of their vegetation cover. The existence of an ABL is shown often to simplify, but sometimes to complicate, the synthesis of the aggregate description. In the course of the paper it becomes apparent that remote sensing skills and mesoscale meteorological modelling expertise must now be considered necessary components of process hydrology.