Articles | Volume 21, issue 7
Hydrol. Earth Syst. Sci., 21, 3401–3415, 2017
Hydrol. Earth Syst. Sci., 21, 3401–3415, 2017

Research article 10 Jul 2017

Research article | 10 Jul 2017

Analysing surface energy balance closure and partitioning over a semi-arid savanna FLUXNET site in Skukuza, Kruger National Park, South Africa

Nobuhle P. Majozi1,2, Chris M. Mannaerts2, Abel Ramoelo1,5, Renaud Mathieu1,3, Alecia Nickless4, and Wouter Verhoef2 Nobuhle P. Majozi et al.
  • 1Earth Observation Group, Natural Resources and Environment, Council for Scientific and Industrial Research, Pretoria, 0001, South Africa
  • 2Department of Water Resources, Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, Enschede, 75AA, the Netherlands
  • 3Department of Geography, Geoinformatics and Meteorology, University of Pretoria, Pretoria, South Africa
  • 4Nuffield Department of Primary Care Health Sciences, University of Oxford, Oxford, OX2 6GG, UK
  • 5University of Limpopo, Risk and Vulnerability Centre, Sovenga, 0727, South Africa

Abstract. Flux towers provide essential terrestrial climate, water, and radiation budget information needed for environmental monitoring and evaluation of climate change impacts on ecosystems and society in general. They are also intended for calibration and validation of satellite-based Earth observation and monitoring efforts, such as assessment of evapotranspiration from land and vegetation surfaces using surface energy balance approaches.

In this paper, 15 years of Skukuza eddy covariance data, i.e. from 2000 to 2014, were analysed for surface energy balance closure (EBC) and partitioning. The surface energy balance closure was evaluated using the ordinary least squares regression (OLS) of turbulent energy fluxes (sensible (H) and latent heat (LE)) against available energy (net radiation (Rn) less soil heat (G)), and the energy balance ratio (EBR). Partitioning of the surface energy during the wet and dry seasons was also investigated, as well as how it is affected by atmospheric vapour pressure deficit (VPD), and net radiation.

After filtering years with low-quality data (2004–2008), our results show an overall mean EBR of 0.93. Seasonal variations of EBR also showed the wet season with 1.17 and spring (1.02) being closest to unity, with the dry season (0.70) having the highest imbalance. Nocturnal surface energy closure was very low at 0.26, and this was linked to low friction velocity during night-time, with results showing an increase in closure with increase in friction velocity.

The energy partition analysis showed that sensible heat flux is the dominant portion of net radiation, especially between March and October, followed by latent heat flux, and lastly the soil heat flux, and during the wet season where latent heat flux dominated sensible heat flux. An increase in net radiation was characterized by an increase in both LE and H, with LE showing a higher rate of increase than H in the wet season, and the reverse happening during the dry season. An increase in VPD is correlated with a decrease in LE and increase in H during the wet season, and an increase in both fluxes during the dry season.

Short summary
The study analysed the quality and partitioning of a 15-year surface energy dataset from Skukuza flux tower. The yearly mean energy balance ratio (EBR) was 0.93, with the dry season having the lowest ratio. Night ratio was lower than daytime, with analysis showing an increase in EBR with increase in friction velocity, which is also linked to time of day. The energy partitioning showed that sensible heat flux is the dominant portion in the dry season, and latent heat flux during the wet season.