Crop Control of Evaporative Fraction in the Southern Great Plains

The Science

In the U.S. Southern Great Plains, grassland/pasture and winter wheat are the dominant land covers, but have distinct growing periods that may differently influence land-atmosphere coupling during spring and summer. This study demonstrates that the harvest of winter wheat leads to a dramatic change in the fraction of surface energy transferred to the atmosphere as water vapor.  Prior to harvest, evaporative fraction of winter wheat is strongly influenced by leaf area and soil-atmosphere temperature differences. After harvest, variations in soil moisture have a stronger effect on evaporative fraction. This is in contrast with grassland/pasture sites, where variation in green leaf area has a large influence on evaporative fraction throughout spring and summer, and changes in soil-atmosphere temperature difference and soil moisture are of relatively minor importance.

The Impact

The shift in relative importance of surface variables on surface energy transfer to the atmosphere demonstrates the importance of crop type and growth on the atmosphere in the Southern Great Plains, where previously scientists had thought soil moisture and net radiation were the important variables. These effects of land cover and agriculture should be included in regional and global models for accurate representation of surface energy partitioning and the water cycle in regions with a mix of winter crops such as the Southern Great Plains.

Summary

Researchers synthesized and statistically analyzed many years of land surface observations from more than a dozen DOE Atmospheric Radiation Measurement observational facilities in the Southern Great Plains. They showed that during the winter wheat growing season, the quantity of green leaves and the surface-air temperature difference controlled surface energy fluxes, whereas after harvest, soil moisture was most important. For grass, leaf area was always most important.  Currently, few regional or global earth system models include realistic representations of agricultural processes, and far fewer have developed validated routines for simulating winter crops such as winter wheat. These effects of land cover and agriculture should be included in regional and global models for accurate representation of surface energy partitioning and the water cycle in regions with a mix of winter crops such as the Southern Great Plains.

Principal Investigator(s)

Lara Kueppers
UC Berkeley/Lawrence Berkeley National Laboratory
[email protected]

Margare Torn
Lawrence Berkeley National Laboratory 
[email protected]

Funding

This research was supported by the Office of Biological and Environmental Research of the U.S. Department of Energy under contract DE-AC02-05CH11231 as part of the Atmospheric Radiation Measurement (ARM) and Atmospheric System Research (ASR) programs. Data supporting this research can be found at http://www.archive.arm.gov/.

References

J. E. Bagley, L. M., Kueppers, D. P., Billesbach, I. N., Williams, S. C., Biraud, and M. S., Torn, “The influence of land cover on surface energy partitioning and evaporative fraction regimes in the U.S. Southern Great Plains.” J. Geophys. Res. Atmos. 122 (2017), doi:10.1002/2017JD026740.