The Impact of Agricultural Land Use Changes on Surface-Atmosphere Exchange and Boundary Layer Processes in the Northern North American Great Plains

The area of land held in summer fallow has decreased by tens of millions of hectares since the 1970s in the northern North American Great Plains as producers have recognized that avoiding fallow usually confers economic and soil conservation benefits. These widespread shifts in agricultural management have coincided with an observed 6 W m-2 decrease in radiative forcing that is consistent with the effects of fallow reduction on cloud and precipitation formation processes. Research to date has focused largely on the Canadian Prairie Provinces; the extent and timing of management-related changes to boundary layer climate in the U.S. Northern Great Plains have yet to be ascertained. To begin to address this knowledge gap, and to determine the effects of land use trends on energy partitioning at the surface-atmosphere interface, we measured the fluxes of carbon dioxide, latent heat, and sensible heat in dryland winter and spring wheat, irrigated winter wheat and barley, and summer fallow in Montana, USA. We used simple models of boundary layer development driven with surface flux measurements to estimate the impact of agricultural management changes on boundary layer height. Evapotranspiration in the dryland wheat crops was over 100 mm greater than the 275 ± 39 mm observed in the fallow field during the study period. Surprisingly, cumulative sensible heat flux during the growing season was of similar magnitude in the dryland wheat and irrigated barley despite negative sensible heat flux after irrigation events in summer because of the early harvest of the barley crop. Modeled maximum daily atmospheric boundary layer height was up to 900 m higher as a result of fallow versus dryland spring wheat. Results that the timing of agricultural management practices can be as important to surface-atmosphere exchange processes as the crop itself.