University of Twente Student Theses


Evapotranspiration in water limited environments: up-scaling from the crown canopy to the eddy flux footprint.

Rwasoka, Donals Tendayi (2010) Evapotranspiration in water limited environments: up-scaling from the crown canopy to the eddy flux footprint.

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Abstract:Evapotranspiration in Water Limited Environments (WLE) plays a central role in explaining ecohydrological dynamics and the mass-energy interactions between the land surface and the atmosphere. However, evapotranspiration is a lumped flux of evaporation and transpiration. In WLE, role of tree transpiration is a subject of current scientific and water resources management interest. This work thus, implements for the first time, an integrated eddy covariance, sap flow, eddy flux footprint and remote-sensing based up-scaling approach to investigate tree transpiration dynamics in a WLE. The research focuses on the following thematic aspects (1) the role of transpiration fluxes in dry areas (2) the surface energy balance and (3) the application of eddy flux footprints in tree transpiration up-scaling and mapping using remote sensing. Field measurements were done in the Sardon catchment close to Salamanca, Spain (DOY 249 -269, 2009), where the tree species Quercus Ilex and Quercus Pyrenaica are the dominant vegetation and thus are the focus of this study. Sap-flow was measured and up-scaled using species-specific biometric upscaling functions combined with Quick-Bird imagery. Post-processing eddy covariance data was done to determine evapotranspiration from latent heat flux. Furthermore, 2-D eddy flux footprints were determined using 30-min energy flux data, and discretized over the land surface. Finally, the transpiration contribution to evapotranspiration was determined by up-scaling sap-flow from the tree canopies inside the eddy footprint. The mean measured sap flow for Quercus ilex was ~2.8 litres per hour compared to ~0.4 litres per hour Quercus pyrenaica. Applying eddy covariance it was found that the average evapotranspiration was ~0.21 mm/day. The contribution of tree transpiration was assessed on the basis of two eddy flux footprints, which showed tree transpiration contributions of ~ 40 and ~13%. The differences in the percentages reveal the influence of species heterogeneity. The higher percentage was found from a footprint with a higher density of Quercus ilex. The trees enclosed in one of the studied footprints covered ~11% of the footprint surface area, and contributed ~40% of the evapotranspiration measured by eddy covariance, thereby highlighting the contribution of tree transpiration to the total evapotranspiration . Soil heat flux was found to play a major role in energy flux partitioning as it reaches up to 40-50% of net radiation. A high energy balance closure of 86% was achieved without considering other energy storage terms such as biomass storage. An analysis of the lack of closure revealed the influence of friction velocity, thermal stratification and the proper determination of soil heat flux as key factors. The soil heat flux measurement that did not consider heat storage underestimated the soil heat flux by ~21%. It is concluded that the integrated approach proposed and implemented in this research is a pragmatic and universally applicable approach for combining eddy covariance, sap flow and remote-sensing based tree transpiration up-scaling.
Item Type:Essay (Master)
Faculty:ITC: Faculty of Geo-information Science and Earth Observation
Programme:Geoinformation Science and Earth Observation MSc (75014)
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