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

[摘要] 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 toparameterize and test models. In the present study we cross-checked theevapotranspiration data obtained with the EC method (ET_EC) againstET rates measured with the soil water balance method (ET_WB) atwinter wheat stands in southwest Germany. During the growing seasons 2012 and2013, we continuously measured, in a half-hourly resolution, latent heat(LE) and sensible (H) heat fluxes using the EC technique. Measuredfluxes were adjusted with either the Bowen-ratio (BR), H or LEpost-closure method. ETWB was estimated based on rainfall, seepageand soil water storage measurements. The soil water storage term wasdetermined at sixteen locations within the footprint of an EC station, bymeasuring the soil water content down to a soil depth of 1.5 m. In thesecond year, the volumetric soil water content was additionally continuouslymeasured in 15 min resolution in 10 cm intervals down to 90 cm depth withsixteen 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 withthe result of the WB method (3.3 ± 0.3 mm day⁻¹). ET_ECadjusted 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 theET_WB by 24 and 48 %, respectively. In 2013, ET_WB wasin best agreement with ET_EC adjusted with the H post-closure methodduring the periods with low amount of rain and seepage. During these periodsthe BR and LE post-closure methods overestimated ET by about 46 and70 %, respectively. During a period with high and frequent rainfalls,ET_WB was in-between ET_EC adjusted by H and BRpost-closure methods. We conclude that, at most observation periods on oursite, LE is not a major component of the energy balance gap. Our resultsindicate that the energy balance gap is made up by other energy fluxes andunconsidered or biased energy storage terms.

[学科分类] 地球化学与岩石