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Title: Energy Balance and Turbulent Flux Partitioning in a Corn-soybean Rotation in the Midwestern U.S.

item Hernandez Ramirez, Guillermo
item Hatfield, Jerry
item Parkin, Timothy
item Prueger, John
item Sauer, Thomas

Submitted to: Journal of Theoretical and Applied Climatology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/24/2009
Publication Date: 1/16/2010
Citation: Hernandez Ramirez, G., Hatfield, J.L., Parkin, T.B., Prueger, J.H., Sauer, T.J. 2010. Energy Balance and Turbulent Flux Partitioning in a Corn-soybean Rotation in the Midwestern U.S. Journal of Theoretical and Applied Climatology. 100:79-92.

Interpretive Summary: It is important to study the amount and distribution of net solar energy in agricultural systems because this may help to understand processes such as air temperature changes (warming or cooling), plant heat stress, soil evaporation, and plant transpiration. A study about energy use, movement, and distribution was done during four years in corn and soybean fields in central Iowa. When the two crop canopies were at their fullest, soybean canopy dedicated more energy to evaporation-transpiration than corn. There was much less energy used to warm the air-canopy layer in soybean than in corn. Perhaps, all these differences are because corn grows tall and upright, while soybean is denser and shorter, so a mature soybean canopy may cover the ground more than corn. Another important feature related to energy movement in this study was a type of wind speed that measures air mixing. Corn was higher than soybean in this type of wind speed measurement. This also can be because of differences in the form and development of the two crop canopies. This research is important to scientists and policy-makers interested in the effect of cropping systems and land use on energy and water balance as well as in climate change.

Technical Abstract: Energy balance at soil surface-canopy interface is critical for better understanding of water balance and changes in regional weather patterns; however, limited long-term, year-round studies have been conducted in agricultural fields. This study was carried out to assess energy balance closure and partition of turbulent heat fluxes in corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] cultivated in two adjacent fields in biannual rotation. From 2004 to 2007, we continuously monitored energy balance components by permanent flux stations equipped with eddy-covariance systems, net radiometers, and soil heat flux plates in both corn and soybean production fields near Ames, IA. Within growing seasons, plant morphological and phenological characteristics appeared to impact magnitude and partition of energy fluxes in our study. As canopies gradually developed, a shift in turbulent fluxes occurred with decreasing sensible heat and increasing latent heat, but with much more pronounced effect in corn than in soybean fields. Conversely, in mid growing season and as both canopies progressively senesced, sensible heat in general increased and latent heat decreased; however, soybean fields exhibited higher latent heat as well as much lower sensible heat than corn. These temporal variations in both amount and partition of quantified turbulent fluxes translated into a pronounced energy imbalance for soybean (0.80 ± 0.07) and an enhanced closure for corn (0.98 ± 0.04) in August and September. Within the same period, discrepancies between crops in closure results and partly in turbulent fluxes were closely associated to friction velocity with mean values of 0.34 and 0.28 m s-1 for corn and soybean fields, respectively. With full-developed canopy conditions, these differences in friction velocity may probably be explained by greater aerodynamics roughness in corn vs. soybean. These results suggest the controlling-role of vegetation on surface energy balance parameters under non-limiting soil water availability conditions. Thus, large-scale land use changes such as expansion of cropland as response to the current increasing demand for grain may strongly alter regional water balance and weather patterns.