Energy balance closure on a winter wheat stand: comparing the eddy covariance technique with the soil water balance method

Abstract. The energy balance of eddy covariance (EC) flux data is typically not closed. The nature of the gap is usually not known, which hampers using EC data to parameterize and test models. In the present study we cross-checked the evapotranspiration data obtained with the EC method (ET_EC) against ET rates measured with the soil water balance method (ET_WB) at winter wheat stands in southwest Germany. During the growing seasons 2012 and 2013, we continuously measured, in a half-hourly resolution, latent heat (LE) and sensible (H) heat fluxes using the EC technique. Measured fluxes were adjusted with either the Bowen-ratio (BR), H or LE post-closure method. ETWB was estimated based on rainfall, seepage and soil water storage measurements. The soil water storage term was determined at sixteen locations within the footprint of an EC station, by measuring the soil water content down to a soil depth of 1.5 m. In the second year, the volumetric soil water content was additionally continuously measured in 15 min resolution in 10 cm intervals down to 90 cm depth with sixteen capacitance soil moisture sensors. During the 2012 growing season, the H post-closed LE flux data (ET_EC =  3.4 ± 0.6 mm day⁻¹) corresponded closest with the result of the WB method (3.3 ± 0.3 mm day⁻¹). ET_EC adjusted by the BR (4.1 ± 0.6 mm day⁻¹) or LE (4.9 ± 0.9 mm day⁻¹) post-closure method were higher than the ET_WB by 24 and 48 %, respectively. In 2013, ET_WB was in best agreement with ET_EC adjusted with the H post-closure method during the periods with low amount of rain and seepage. During these periods the BR and LE post-closure methods overestimated ET by about 46 and 70 %, respectively. During a period with high and frequent rainfalls, ET_WB was in-between ET_EC adjusted by H and BR post-closure methods. We conclude that, at most observation periods on our site, LE is not a major component of the energy balance gap. Our results indicate that the energy balance gap is made up by other energy fluxes and unconsidered or biased energy storage terms.