J.H.C. Gash

Carbon Dioxide Uptake by an Undisturbed Tropical Rain Forest in Southwest Amazonia, 1992 to 1993

Measurements of carbon dioxide flux over undisturbed tropical rain forest in Brazil for 55 days in the wet and dry seasons of 1992 to 1993 show that this ecosystem is a net absorber of carbon dioxide. Photosynthetic gains of carbon dioxide exceeded respiratory losses irrespective of the season. These gains cannot be attributed to measurement error, nor to loss of carbon dioxide by drainage of cold air at night. A process-based model, fitted to the data, enabled estimation of the carbon absorbed by the ecosystem over the year as 8.5 ± 2.0 moles per square meter per year.

Estimating sparse forest rainfall interception with an analytical model

Gashs analytical model of rainfall interception is reformulated, with improved boundary conditions, to give a better description of the evaporation from sparse forest. The model is tested against data from Les Landes Forest collected during HAPEX-MOBILHY. The new formulation requires an estimate of the evaporation per unit area of canopy, rather than per unit ground area. When the evaporation per unit area of canopy is equated with the estimates of evaporation derived from the Penman-Monteith equation there is an improved description of the observations.

Biogeochemical cycling of carbon, water, energy, trace gases, and aerosols in Amazonia: The LBA-EUSTACH experiments

The biogeochemical cycling of carbon, water, energy, aerosols, and trace gases in the Amazon Basin was investigated in the project European Studies on Trace Gases and Atmospheric Chemistry as a Contribution to the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA-EUSTACH). We present an overview of the design of the project, the measurement sites and methods, and the meteorological conditions during the experiment. The main results from LBA-EUSTACH are: Eddy correlation studies in three regions of the Amazon Basin consistently show a large net carbon sink in the undisturbed rain forest. Nitrogen emitted by forest soils is subject to chemical cycling within the canopy space, which results in re-uptake of a large fraction of soil-derived NOx by the vegetation. The forest vegetation is both a sink and a source of volatile organic compounds, with net deposition being particularly important for partially oxidized organics. Concentrations of aerosol and cloud condensation nuclei (CCN) are highly seasonal, with a pronounced maximum in the dry (burning) season. High CCN concentrations from biomass burning have a pronounced impact on cloud microphysics, rainfall production mechanisms, and probably on large-scale climate dynamics.

Modelling interception loss for two sparse eucalypt and pine forests in central Portugal using reformulated Rutter and Gash analytical models

Abstract Gross rainfall, throughfall and stemflow were measured in Eucalyptus globulus Labill. and Pinus pinaster Ait. stands in central Portugal over two and a half years (from January 1992 to July 1994). The results show that the interception loss is higher in the pine stand (17% of gross rainfall) than in the eucalypt stand (11% of gross rainfall). Interception loss was also simulated by the Rutter model and Gashs original analytical model, but both models overestimated the interception loss from these Mediterranean sparse forests by 29–44%. To improve the description of the rainfall interception from sparse forests, the Rutter model was reformulated. This new version of the Rutter model and a previously reformulated version of Gashs analytical model greatly improved the accuracy of the simulation for both forests, resulting in estimates of the interception loss within 3% of the measured values.

Vegetation, water, humans and the climate: A new perspective on an interactive system.

Land Surface Matter in Climate and Weather: The Climate near the Ground The Regional Climate The Global Climate The Sahelian Climate The Amazonian Climate The Boreal Climate The Asian Monsoon Climate.- How Measurable is the Earth System: The Energy Balance Closure Problem Radiation Measurements in Integrated Terrestrial Experiments Surface Turbulent Fluxes Accuracy and Utility of Aircraft Flux Measurements Boundary Layer Budgeting Vegetation Structure, Dynamics and Physiology Remote Sensing and Land Surface Experiments The Water Balance Concept Use of Field Experiments in Improving the Land Surface Description in Atmospheric Models Further Insight from Large-scale Observational Studies of Land/Atmosphere Interactions.- The Value of Land Surface Data Consolidation: Motivation for Data Consolidation Existing Degrees of Consolidation Achieving Full Consolidation Terrestrial Data Assimilation.- The Integrity of River and Drainage Basin Systems: Responses of Hydrological Processes to Environmental Change at Small Catchment Scales River Basin Responses to Global Change and Anthropogenic Impacts Responses of Continental Aquatic Systems at the Global Scale Case Study 1 - The Amazon Basin Case Study 2: The Elbe River Basin in Central Europe Case Study 3: The Mixed Underdeveloped/Developed Mgeni Catchment, South Africa Scaling Relative Responses of Terrestrial Aquatic Systems to Global Changes.- How to Evaluate Vulnerability in Changing Environmental Conditions: Predictability and Uncertainty Contrast between Predictive and Vulnerability Approaches The Scenario Approach The Vulnerability Approach Case Studies Conclusions.

Comparative estimates of interception loss from three coniferous forests in Great Britain

Abstract Measurements of rainfall interception from three coniferous forests in Great Britain are compared with estimates of the evaporation made with two conceptual models, the Rutter model and an analytical model of interception loss, using data from automatic weather stations mounted above the forests. Estimates of the mean evaporation rate from a saturated canopy are derived by optimisation, to determine an empirical value of this parameter for future use at sites where no meteorological data are available.

An application of the Rutter model to the estimation of the interception loss from Thetford Forest

Abstract The Rutter model of rainfall interception in forests has been used with data from an automatic weather station, to estimate interception loss from Thetford Forest, for twenty-one 4-week periods during 1975 and 1976. The estimates were compared with measurements of the interception loss, with good agreement between observed and predicted evaporation. The sensitivity of the model to the two major characteristics of the forest structure was also investigated.

The measurement and modelling of rainfall interception by Amazonian rain forest

Abstract Measurements of the evaporation of rainfall intercepted by Amazonian rain forest were compared with estimates made by two models, the Rutter model and Gashs analytical model, applied to meteorological measurements made with automatic weather stations mounted above the forest. Neither of the model estimates of interception loss was significantly different from measured interception loss, which amounted to 8.9% of the measured rainfall.

An integrated micrometeorological system for evaporation measurement

Abstract An integrated eddy correlation system is described. This system comprises a multi-headed sensor, a real-time processing unit and software for analysing and storing the data on a microcomputer. The instrument, known as the Hydra, was first used for research in 1981. Since then, experience gained in using the instrument on four separate research projects has been applied to the redesign and production of an improved instrument designated the Mark 2. This instrument is described together with two applications, the first over a millet crop in the Sahel region of West Africa and the second over a pine forest in south-west France during the HAPEX experiment.

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.