Evaluation of the impact of surface residue cover on single and dual crop coefficient for estimating soybean actual evapotranspiration

Single and dual crop coefficient methods are used in conjunction with grass reference evapotranspiration (ETo) to estimate actual crop evapotranspiration (ETc). However, the impact of soil surface residue cover on the accuracy of ETc estimated with these methods is not well understood. The objective of this study is to evaluate and compare the accuracy of the FAO-56 single crop coefficient (single-Kc) and dual crop coefficient (dual-Kc) methods for estimating soybean [Glycine max (L.) Merr.] ETc in a partially residue covered field. The study was conducted at the University of Nebraska-Lincoln, South Central Agricultural Laboratory (SCAL), Nebraska, during the 2007 and 2008 growing seasons. The field was under reduced-tillage (ridge till) on a silt loam soil and irrigated using a subsurface drip irrigation system. Evapotranspiration flux (ETm) above the crop canopy was measured using a deluxe version of a Bowen ratio energy balance system (BREBS) and ETo was calculated with the Penman–Monteith method. The single-Kc and dualKc-estimated ETc values, both unadjusted for residue cover, were compared to ETm. The unadjusted FAO-56 Kc values performed poorly as the single-Kc underestimated ETm during the initial crop growth stage by 21% in 2007 and 33.6% in 2008 while the dual-Kc overestimated ETm during the same growth stage by 16.8% in 2007 and 16.5% in 2008. Extended simulations were conducted to determine the magnitude by which ETc is reduced for each 10% of soil surface covered with crop residue. Downward adjustments in soil water evaporation (Es) for every 10% of the soil surface covered with crop residue improved the accuracy of ETc estimated by the dual-Kc method. The largest changes in ETc due to adjustments in Es occur during the initial stage of the growing season. The best estimates for seasonal ETc were obtained by reducing Es by 5% for every 10% of surface covered with residue in 2007 (R2 = 0.77, RMSD = 0.87 mm d−1, E = 0.94) and 2008 (R2 = 0.83, RMSD = 0.84 mm d−1, E = 0.95). Greater improvements in the accuracy of estimated seasonal ETc were obtained by reducing Es by 2.5% for each 10% of surface covered with residue during the initial stage and by 5% during the rest of the crop growth stage. These results suggest that the more computationally-involved dual-Kc method with adjustments in Es for each 10% of surface covered with residue improves the prediction of ETc in fields with soil surface residue cover, especially during the initial growth stage. Inaccurate selection of percentage reduction in Es can result in substantial overestimation or underestimation of seasonal ETc by the dual-Kc method.