Fluxes of carbon dioxide, water vapour and sensible heat in a boreal agricultural area of Sweden — scaled from canopy to landscape level

Abstract

Within the framework of the Northern Hemisphere Climate Processes Land-surface Experiment (NOPEX), a field experiment was conducted on agricultural crops, focussing on atmospheric fluxes of carbon dioxide, water vapour and heat above agricultural land within the boreal zone. The fluxes were monitored continuously above three surface types, namely; winter wheat, spring barley and bare soil fallow. These measurements were supplemented by an intensive measuring campaign directed towards an analysis of canopy fluxes in relation to plant production. For this purpose, traditional agroclimatic data, including canopy development and the capture of photosynthetically active radiation, were also collected. Diurnal fluxes from the three surface types are presented and intercompared, and the integrated fluxes from the measuring campaign are also shown. Little difference in the fluxes from grain crops at the later stage of development is actually observed even with the leaf area index (LAI) differing by a factor of two. The canopy use factor of photosynthetically active radiation is found to provide a functional expression for the ratio between leaf and canopy conductance, and this parameter is valuable for scaling carbon dioxide fluxes. By studying the relationship between atmospheric and canopy resistances, it is detected that the evapotranspiration from grain crops depends on radiation rather than water-vapour pressure deficit. This improves the potential use of remote sensing in estimating of fluxes at a landscape level. With the help of airborne radar images and visible and near-infrared satellite data the non-forested parts of the landscape are divided into vegetation classes. It is finally demonstrated how the fluxes of carbon dioxide and water vapour from these classes can be calculated by combining photosynthesis and evapotranspiration models with the satellite-based estimates of leaf area index.