Surface Energy Balance, Evapotranspiration, and Surface Coefficients During Non-Growing Season in a Maize-Soybean Cropping System

Abstract

<abstract> Surface energy balance components, including actual evapotranspiration (ET), were measured in a reduced-till maize-soybean field in south central Nebraska during three consecutive non-growing seasons (2006/2007, 2007/2008, and 2008/2009). The relative fractions of the energy balance components were compared across the non-growing seasons, and surface coefficients (K_c) were determined as a ratio of measured ET to estimated alfalfa (ET_r) and grass (ET_o) reference ET (ET_ref). The non-growing season following a maize crop had 25% to 35% more field surface covered with crop residue as compared to the non-growing seasons following soybean crops. Net radiation (R_n) was the dominant surface energy balance component, and its partitioning as latent heat (LE), sensible heat (H), and soil heat (G) fluxes depended on field surface and atmospheric conditions. No significant differences in magnitude, trend, and distribution of the surface energy balance components were observed between the seasons with maize or soybean surface residue cover. The cumulative ET was 196, 221, and 226 mm during the three consecutive non-growing seasons. Compared to ET_ref, the cumulative total measured ET was 61%, 63%, and 59% of cumulative total ET_o and 43%, 46%, and 41% of cumulative total ET_r during the three consecutive seasons. The type of residue on the field surface had no significant effect on the magnitude of ET. Thus, ET was primarily driven by atmospheric conditions rather than surface characteristics. The coefficient of determination (R²) for the daily ET vs. ET_r data during the three consecutive non-growing seasons was only 0.23, 0.42, and 0.42, and R² for ET vs. ET_o was 0.29, 0.46, and 0.45, respectively. Daily and monthly average K_c values varied substantially from day to day and from month to month, and exhibited interannual variability as well. Thus, no single K_c value can be used as a good representation of the surface coefficient for accurate prediction of ET for part or all of the non-growing season. A good relationship was observed between monthly total measured ET vs. monthly total ET_ref. The R² values for monthly total ET vs. monthly total ET_ref data ranged from 0.71 to 0.89 for both ET_r and ET_o. Using pooled data for monthly total ET vs. monthly total ET_ref, R² was 0.78 for ET_r and 0.80 for ET_o. The slopes (S) of the best-fit line with intercept for the monthly total ET vs. monthly total ET_ref data were consistent for all three non-growing seasons, with S = 0.45 ±0.05 for ET_r and S = 0.62 ±0.08 for ET_o. The parity in R² and S across the three non-growing seasons suggests that the same regression equation can be used to approximate non-growing season ET for field surfaces with both maize and soybean crop residue covers. Considering the extreme difficulties in measuring ET during winter in cold and windy climates with frozen and/or snow-covered conditions, the approach using a linear relationship between monthly total ET vs. monthly total ET_ref appears to be a good alternative to using a surface coefficient to approximate non-growing season monthly total ET. The conclusions of this research are based on the typical dormant season conditions observed at the research location and may not be generally transferable to other locations with different climatic and surface conditions.