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

Authors

Lameck O. Odhiambo, University of Nebraska-LincolnFollow
Suat Irmak, University of Nebraska-LincolnFollow

Document Type

Article

Date of this Version

1-2012

Citation

Agricultural Water Management 104 (2012), pp. 221– 234; doi: 10.1016/j.agwat.2011.12.021

Comments

Copyright © 2011 Elsevier B.V. Used by permission.

Abstract

Single and dual crop coefficient methods are used in conjunction with grass reference evapotranspiration (ET_o) to estimate actual crop evapotranspiration (ET_c). However, the impact of soil surface residue cover on the accuracy of ET_c 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-K_c) and dual crop coefficient (dual-K_c) methods for estimating soybean [Glycine max (L.) Merr.] ET_c 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 (ET_m) above the crop canopy was measured using a deluxe version of a Bowen ratio energy balance system (BREBS) and ET_o was calculated with the Penman–Monteith method. The single-K_c and dual-K_c-estimated ET_c values, both unadjusted for residue cover, were compared to ET_m. The unadjusted FAO-56 K_c values performed poorly as the single-K_c underestimated ET_m during the initial crop growth stage by 21% in 2007 and 33.6% in 2008 while the dual-K_c overestimated ET_m 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 ET_c is reduced for each 10% of soil surface covered with crop residue. Downward adjustments in soil water evaporation (E_s) for every 10% of the soil surface covered with crop residue improved the accuracy of ET_c estimated by the dual-K_c method. The largest changes in ET_c due to adjustments in E_s occur during the initial stage of the growing season. The best estimates for seasonal ET_c were obtained by reducing E_s 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 ET_c were obtained by reducing E_s 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-K_c method with adjustments in E_s for each 10% of surface covered with residue improves the prediction of ET_c in fields with soil surface residue cover, especially during the initial growth stage. Inaccurate selection of percentage reduction in E_s can result in substantial overestimation or underestimation of seasonal ET_c by the dual-K_c method.