Measured Midsummer Spatial Heterogeneity in Ecosystem-Atmosphere CO2/H2O Exchange for Selected Crop Systems of the Southern Great Plains

Patterns of land use and management are likely to dominate the spatial heterogeneity in cycles of energy, carbon, and water in ecosystems of the Southern Great Plains. They may couple to regional climate and hence feedback to ecosystem performance. We report recent progress on measuring and modeling spatial heterogeneity in land surface-atmosphere exchange for different crops in the footprint of a flux system mounted on a 60 m high tower. For three weeks in July 2001 we measured energy, CO2 and water vapor fluxes in four agricultural fields containing senescing sorghum, senescing soy, mowed pasture, and tilled wheat near the DOE Atmospheric Radiation Measurement Program central facility near Lamont, Oklahoma (36.605 N, 97.485 W). Following an initial inter-comparison of three eddy covariance flux systems in the sorghum field, two flux systems were moved so that the other fields were observed for at least four days each. During the observations we also measured soil microclimate, plant biomass, and leaf area index, and collected air, water, and soil samples for physical, chemical, and stable isotope analyses. There were large differences in the diurnal cycles of carbon and water fluxes. Net pre-noon CO2 exchange in the sorghum (a C4 plant), soy (a C3 plant), pasture, and tilled wheat fields were roughly 20-35, 4-6, ~ 0, and ~ -1 (umol m-2 s-1) respectively. Water limitation was readily apparent in the sorghum and soy fields where afternoon net carbon exchange were reduced to about 10 and -2 (umol m-2 s-1) respectively. We will also present a comparison of the field scale measurements with the continuous flux measurement record from the instrument located on the ARM central facility 60 m tower.