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International Institute for Geo-Information Science and Earth Observation (ITC) 2017

Quantification of turbulent heat fluxes and the energy balance closure over a flux tower in the semi-arid Savannah area of Naivasha, Kenya

Mutinda, J.M. (John Mbatha)

Titre : Quantification of turbulent heat fluxes and the energy balance closure over a flux tower in the semi-arid Savannah area of Naivasha, Kenya

Auteur : Mutinda, J.M. (John Mbatha)

Etablissement de soutenance : University of Twente International Institute for Geo-Information Science and Earth Observation (ITC)

Grade : Master of Science in Geo-Information Science and Earth Observation

Résumé
Quantification of turbulent heat fluxes is important as they influence regional and global climates. This becomes very vital in semi- arid ecosystems where evapotranspiration accounts for 90% of rainfall losses. The dependability of quantified fluxes as inputs into climate and hydrological models is hinged on energy balance closure and th erefore it is important to quantify and understand sources of lack of energy balance closure ; which remains to be an unresolved problem with many eddy covariance systems. In this study, half hourly turbulent heat fluxes are derived using measurements fro m a flux tower situated in a semi- arid savannah area of Naivasha, Kenya, between the year 2012 and 2014, and over a brief period in 2016. Given that this site is not a full eddy covariance system owing to lack of a gas analyzer, latent heat fluxes are deri ved by inversion of the Bowen ratio derived from relative humidity and temperature measurements. In combination with ground heat flux estimated from a soil heat flux plate and soil temperature profile measurements ; the energy balance closure is computed. C auses of lack of energy balance closure are evaluated, specifically the effects of quality filtering based on friction velocity, effects of flux footprint, effects of Bowen ratio inter- calibration and accuracy in ground heat flux estimation. Representative ness of tower measured net radiation is also evaluated by comparing it with net radiation retrieved from Landsat 8 OLI & TIRS satellite. The overall energy balance ratio (EBR) for three years analysed (2012- 2014) was 58% , while yearly EBR’s were 67% for 2012, 56% for 2013, 51% for 2014 and 49% for 2 months analysed in 2016. Quality filtering of fluxes when u * is less than 0.3 m s- 1 improved the overall EBR for the three years evaluated by 5%. Filtering fluxes emanating from a road surface towards the east of the tower when wind direction is between 60 ̊- 120 ̊ improved the overall EBR further by 6%. Net radiation measured by the tower radiometer is found to be higher than net radiation retrieved from the satellite both at the tower pixel and when averaged ov er the tower source area. Correcting for the bias in net radiation improved the overall EBR by 5%. Combining filtering data when u * < 0.3 m s - 1 , when wind direction is between 60 ̊- 120 ̊ and correcting for bias in net radiation improved the overall EBR for three years (2012 – 2014) to 76% which is an 18% increase. Yearly EBRs increased to 83%, 77% and 69% for 2012, 2013 and 2014 respectively. It is recommended that other possible factors attributed to lack of closure such as advection and averaging time tha t were not analysed in this study be evaluated. Further, it is recommended that a gas analyzer be added to the tower for more accurate measurements of latent heat fluxes.

Mots clés : Energy Balance Closure, Flux Tower, Turbulent Heat Fluxes, Sensible Heat Flux, Latent Heat Flux, Ground Heat Flux, Energy Balance Ratio, Bowen Ratio, Inter- Calibration

Version intégrale (ITC)

Page publiée le 29 janvier 2018