Spatial and Temporal Variations in Fluxes of Energy, Water Vapour and Carbon Dioxide During OASIS 1994 and 1995

Abstract

This paper introduces the micrometeorological field campaigns known asOASIS (Observations At Several Interacting Scales) and then summarizesseveral companion studies that have used the OASIS dataset. Instrumentedtowers, aircraft and atmospheric sondes were used for measurements overthree paired sites (crops and pastures), approximately equi-spaced along an88-km transect in south-eastern New South Wales, Australia, during the australsprings of 1994 and 1995. Measurements included standard meteorologicaldata and the fluxes of solar and net radiation, sensible heat, water vapour andthe greenhouse gases CO2, N2O, CH4. Descriptions of the site, andthe spatial and temporal variations of climate fields and fluxes, are presented.There were strong contrasts in fluxes and surface conductances, evaporationratios and water use efficiencies between the 1994 drought year and the normalrainfall year of 1995. Despite greater incoming solar radiation in 1994 associatedwith less cloud cover, net radiation was lower than in 1995 because of greateroutgoing thermal radiation caused by higher surface temperatures. In 1994 dailysensible heat fluxes were about 50% higher and evaporation rates about half thosefor 1995. Rainfall in the three-month growing season prior to the field campaignswas the key determinant of leaf area index, surface conductances and the fluxes ofsensible and latent heat and CO2. Antecedent rainfall distribution also controlled variation in fluxes and surface properties along the transect within each year. There was a net loss of CO2 to the atmosphere at the drier central sites in 1994, and a net uptake at the wetter north-eastern sites. Both sites recorded uptake of CO2 in 1995, but values were lower at the central site than at the north–east site due to the strong rainfall gradient along the transect in the three months prior to each fieldcampaign. Differences in fluxes between crops and pastures at each site were smallerthan between sites.