University of Twente Student Theses


Evaporation over fresh and saline water using SEBS

Abdelrady, Ahmed Ragab (2013) Evaporation over fresh and saline water using SEBS.

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Abstract:Evaporation over large water bodies has a crucial role in the global hydrological cycle. Evaporation occurs whenever there is a vapour pressure deficit between a water surface and the atmosphere. In addition the available energy needs to be sufficient for vaporization (to enable a phase change from liquid to gaseous state). Different models have been developed to estimate the evaporation process over water surfaces using earth observation data. Most of these models are concerned with the atmospheric parameters. However, these models do not take into account the influence of salinity on the evaporation rate. Water salinity affects both the density and latent heat of vaporization of the water body and as a consequence reflects on the evaporation rate. The current models do not consider the difference in the energy needed for vaporization. For this purpose, an energy balance model is required. Several energy balance models, such as the surface energy balance system (SEBS), that calculate daily evapotranspiration exist, They estimate the heat fluxes by integration of satellite data and hydrometeorological field data. SEBS has the advantage that it can be applied over a large scale because it incorporates the physical state of the surface and the aerodynamic resistances in the daily evapotranspiration estimation. Nevertheless, this model has not used over water surfaces. The goal of this research is to adapt SEBS to estimate the daily evaporation over fresh and saline water bodies. In particular, 1) parameterizations required for water heat flux and sensible heat flux (through the roughness heights for momentum and heat transfer ) need to be updated, 2) upscaling to daily evaporation needs to be investigated and finally, 3) integration of the salinity factor to estimate the evaporation over saline water needs to be performed. Eddy covariance measurements over the Ijsselmeer Lake (The Netherlands) was used to estimate the roughness heights for momentum and heat transfer at respectively 0.0002 and 0.0001 m. Application of these values over Victoria and Tana Lakes (freshwater), in Africa showed latent heat to be in a good agreement with the measurements, with RMSE of 19.7 and 35.5 [W m-2] and rRMSE of 4.1% and 4.7 %, respectively. Afterwards, the validity of salinity adapted model was tested over different study areas using ECMWF data. It was found that for the original SEBS model and salinity-adapted model over Great Salt Lake, the RMSE are 0.62 and 0.24 [mm 3h-1], respectively and the rRMSE 19% and 24%. The evaporation reduction of the Great Salt Lake and the oceans are 27% and 1 %, respectively. In conclusion, SEBS model is adapted to calculate the daily evaporation over fresh water and salt water by integration the salinity factor in the model.
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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