Spatial and temporal variations in fluxes of energy, water vapour and carbon dioxide during OASIS 1994 and 1995

This paper introduces the micrometeorological field campaigns known as OASIS (Observations At Several Interacting Scales) and then summarizes several companion studies that have used the OASIS dataset. Instrumented towers, aircraft and atmospheric sondes were used for measurements over three paired sites (crops and pastures), approximately equi-spaced along an 88-km transect in south-eastern New South Wales, Australia, during the austral springs of 1994 and 1995. Measurements included standard meteorological data and the fluxes of solar and net radiation, sensible heat, water vapour and the greenhouse gases CO2, N2O, CH4. Descriptions of the site, and the spatial and temporal variations of climate fields and fluxes, are presented. There were strong contrasts in fluxes and surface conductances, evaporation ratios and water use efficiencies between the 1994 drought year and the normal rainfall year of 1995. Despite greater incoming solar radiation in 1994 associated with less cloud cover, net radiation was lower than in 1995 because of greater outgoing thermal radiation caused by higher surface temperatures. In 1994 daily sensible heat fluxes were about 50% higher and evaporation rates about half those for 1995. Rainfall in the three-month growing season prior to the field campaigns was the key determinant of leaf area index, surface conductances and the fluxes of sensible 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 field campaign. Differences in fluxes between crops and pastures at each site were smaller than between sites.