Regional simulation of ecosystem CO ₂ and water vapor exchange for agricultural land using NOAA AVHRR and Terra MODIS satellite data. Application to Zealand, Denmark

While accurate information on ecosystem CO ₂ and water vapor exchange is available at eddy covariance flux tower sites, methods to expand predictions of CO ₂ and energy exchange to regional or global scales with high fidelity are lacking. The main objective of this study was to examine the applicability of land surface and atmospheric products from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Advanced Very High Resolution Radiometer (AVHRR) for assessing the spatial variation in CO ₂ and water vapor fluxes for cloudless agricultural land pixels at the Island of Zealand, Denmark. The spatial distribution of green leaf area index, direct-beam and diffuse solar radiation and air humidity was inferred on the basis of late morning MODIS data that was combined with afternoon AVHRR data to resolve the diurnal variation in air and surface temperature. These variables were used in a coupled "two-leaf" ecosystem model operating at an hourly time scale.The enhanced vegetation index (EVI) was strongly correlated with field measurements of green leaf area index (r ²=0.91) and remained sensitive to variations in green biomass up to green leaf area indices of 4-5. Evaluation against standard meteorological data showed that instantaneous estimates of air temperature, actual vapor pressure and incoming solar radiation could be retrieved with overall root mean square errors of 2.5°C, 138.3 Pa and 47.7 W m ^-2, respectively. The combination of late morning and afternoon inferences made it possible to resolve the diurnal course in key model parameters, and predicted rates of ecosystem CO ₂ and water vapor exchange were comparable to eddy covariance measurements at a single flux tower. A large spatial diversity in CO ₂ and water vapor exchange was maintained throughout the study period due to significant regional variations in meteorological input variables and large spatial differences in canopy development. The results of this study stress the necessity of pixel based estimates for an accurate evaluation of regional budgets of CO ₂ and water vapor exchange.