Improving the energy balance closure over a winter wheat field by accounting for minor storage terms

Abstract

Turbulent fluxes at the land surface measured by the Eddy Covariance (EC) technique are typically considerably less than the difference between net radiation and ground heat flux. This is known as the energy balance closure (EBC) problem. It is crucial for validating land surface models as it provokes substantial uncertainty to the magnitude and partitioning of energy fluxes. The gap in the energy balance calls for searching for additional energy terms in the soil-plant-atmosphere system. To evaluate the contribution of these minor storage terms to the measured EBC, we conducted an experimental study to evaluate the contribution of these minor storage terms to measured EBC in the Kraichgau region in southwest Germany over two consecutive growing seasons (2015 and 2016). The measured and calculated minor storage terms comprised the enthalpy change in the plant canopy (Sc), the air enthalpy change (Sa), the energy consumption and release by photosynthesis and respiration (Sp), and the atmospheric moisture change (Sq). Furthermore, the soil heat storage (Sg) was determined at different locations within the EC footprint and compared to the single point measurements of Sg at the EC station. Calorimetric and harmonic analysis were performed to compute ground heat flux. Sp had the strongest effect in improving EBC due to the high net CO2 uptake during the productive phase of plant growth. In 2015, all minor storage terms together increased EBC by 5.0% on average, with a maximum value of 7.4% in May, while the improvement in 2016 was 6.8% on average and 8.4% in May. Ground heat flux computed with the harmonic analysis based on plate data narrowed the EBC by 3% more than the calorimetric method. In summary, a better EBC can be achieved by considering minor storage terms and applying a harmonic analysis to ground heat flux data. Regarding future research, we recommend to focus on year-round measurements of energy terms because energy stored during the growing season might be lost from the system during the rest of the year. Nonetheless, the significant contribution of minor energy terms to EBC indicates that turbulent energy fluxes are most likely overestimated when all the missing energy is assumed to be turbulent–the typical approach when fluxes are corrected by the Bowen ratio post-closure method for instance.