Evaluating multi-year, multi-site data on the energy balance closure of eddy-covariance flux measurements at cropland sites in southwestern Germany

Pascal Kremer,Joachim Ingwersen,Thilo Streck,Arne Poyda,Ravshan Eshonkulov,Tobias K D Weber,Hans-Dieter Wizemann,Alim Pulatov,Petra Högy

doi:10.5194/bg-16-521-2019

Abstract

Abstract. The energy balance of eddy-covariance (EC) measurements is typically not closed, resulting in one of the main challenges in evaluating and interpreting EC flux data. Energy balance closure (EBC) is crucial for validating and improving regional and global climate models. To investigate the nature of the gap in EBC for agroecosystems, we analyzed EC measurements from two climatically contrasting regions (Kraichgau – KR – and Swabian Jura – SJ) in southwestern Germany. Data were taken at six fully equipped EC sites from 2010 to 2017. The gap in EBC was quantified by ordinary linear regression, relating the energy balance ratio (EBR), calculated as the quotient of turbulent fluxes and available energy, to the residual energy term. In order to examine potential reasons for differences in EBC, we compared the EBC under varying environmental conditions and investigated a wide range of possible controls. Overall, the variation in EBC was found to be higher during winter than summer. Moreover, we determined that the site had a statistically significant effect on EBC but no significant effect on either crop or region (KR vs SJ). The time-variable footprints of all EC stations were estimated based on data measured in 2015, complimented by micro-topographic analyses along the prevailing wind direction. The smallest mean annual energy balance gap was 17 % in KR and 13 % in SJ. Highest EBRs were mostly found for winds from the prevailing wind direction. The spread of EBRs distinctly narrowed under unstable atmospheric conditions, strong buoyancy, and high friction velocities. Smaller footprint areas led to better EBC due to increasing homogeneity. Flow distortions caused by the back head of the anemometer negatively affected EBC during corresponding wind conditions.

Highlights

Studying turbulent exchange at the land surface is important for assessing water cycling, plant growth, and carbon fluxes of ecosystems and for enhancing soil–crop, climate, and weather models
We evaluated the Energy balance closure (EBC) of long-term EC measurements at six different cropland sites in two contrasting environmental regions in southwestern Germany
25 % of the available energy was not detected by our EC stations, with the lowest annual imbalances of 17 % in Kraichgau region (KR) and 13 % in Swabian Jura region (SJ)

Summary

Introduction

Studying turbulent exchange at the land surface is important for assessing water cycling, plant growth, and carbon fluxes of ecosystems and for enhancing soil–crop, climate, and weather models. The best technique for determining these fluxes is the eddy-covariance (EC) method. It is considered the most direct and accurate measurement of turbulent fluxes in the soil–plant–atmosphere system (Baldocchi et al, 2001; Burba, 2013). R. Eshonkulov et al.: Energy balance closure at cropland sites sured available energy (incoming net radiation minus ground heat flux) is generally higher than the sum of turbulent exchange fluxes (latent and sensible heat). Either the turbulent fluxes are incompletely captured or the measured available energy is positively biased. This gap in energy balance closure (EBC) is a long-standing problem in EC measurements and is one of the most frequently discussed concerns in micrometeorological research (Foken, 2008a)

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