W. James Shuttleworth

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.

Stomatal and surface conductance of tropical rainforest

Abstract Although the absolute values of stomatal conductance of tropical rainforest vary greatly, there is some similarity in the response to humidity deficit and radiation. Stomatal conductance decreases downward through the canopy of Amazonian rainforest. Using a multi-layer approach and measured profiles of stomatal conductance and weather variables through the canopy, good agreement can be obtained between calculated and observed values of dry canopy evaporation. The relationship between the biological response of stomata to radiation at the leaf level and the response of surface conductance to radiation above the canopy is derived by relating the profile of stomatal conductance through the canopy to the attenuation of radiation. Simple use of responses derived at leaf level will greatly overestimate surface conductance if used with above-canopy radiation measurements. Three models of surface conductance of the same Amazonian forest, varying in their degree of complexity, were tested against measured evaporation data for the Reserva Ducke forest in Brazil. A simple model with surface conductance varying only with time of day was found to model the observed data slightly better than a more complex environmental model. Using a constant value of surface conductance gave a poorer fit to the data, although the average evaporation can be calculated accurately. It is recommended that the more complex environmental model be used when estimates of evaporation are required under any conditions substantially different from those of the central Amazonian forest where the data were collected.

Post-deforestation Amazonian climate: Anglo-Brazilian research to improve prediction

Abstract This paper reviews the experimental activity and primary results of the Amazon Region Micrometeorology Experiment (ARME), and the subsequent impact of these data in improving the prediction of post-deforestation Amazonian climate using general circulation models. It previews the new 5 year programme of collaborative Anglo-Brazilian research which will extend these studies by providing calibration of the land-atmosphere interaction for cleared forest areas, and measurements of the comparative near-surface climatology for large clearings and adjacent areas of undisturbed natural forest. Finally it overviews preliminary plans for a major international study in Amazonia under the World Climate Research Programme and the International Geosphere Biosphere Programme.

Tropical Deforestation: Albedo and the Surface-Energy Balance

Recent micrometeorological measurements for Amazonian rainforest are reviewed, emphasising those aspects of the radiation and heat balance which are likely to change with deforestation. The possible consequences of such deforestation are considered by examining the sensitivity of the surface energy balance to changes in those parameters which would be most drastically altered.

The Soil-Vegetation-Atmosphere Interface

The Scientific Plan for GEWEX draws together and enhances a range of climate research activities which have the specific scientific objectives: to determine the hydrological cycle and energy fluxes by means of global measurements of observable atmospheric and surface properties; to model the global hydrological cycle and its effects on the atmosphere and oceans; and to develop the ability to predict the variations of global and regional hydrological processes and water resources, and their response to environmental change.