University of Twente Student Theses


Remote Sensing of the Annual Heat Storage Changes in Lake Tana, Ethiopia

Alemseged, Berhanu (2010) Remote Sensing of the Annual Heat Storage Changes in Lake Tana, Ethiopia.

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Abstract:Lake Tana is the largest lake Ethiopia has and is the source of the Blue Nile. The lake water resources support the livelihoods of over 3 million people living around and have local and global economic and political significance. It is vital to establish a water resources management scheme to ensure sustainable development. For this reason a sound understanding of the lake’s hydrological processes and energy balance is a key factor. It is possible to evaluate the annual heat storage changes and the resulting effect on lake evaporation through retrieving the lake surface temperature from satellite data. First the available methods of water surface temperature retrieval algorithms have been identified. Two split window techniques including the Sun-Pinker and Becker-Li methods were selected for further validation and calibration. The lake surface temperature was retrieved with both methods and results have been checked against ground observations. The algorithm developed by Sun and Pinker for night-time temperature retrieval did not yield acceptable values. Upward temperature jumps in the order of 2 K occurred at sunset, while the reverse happened at sunrise. For this reason the night-time algorithm had to be modified. The lake water surface temperature (WST) has been retrieved with the modified algorithm and the diurnal cycle was analyzed. A final objective of the first part of this study was to identify the most suitable model in terms of input parameters and computational efficiency. A two channel algorithm developed by Becker and Li was found to meet the requirements. However, the coefficients of the model were developed for the AVHRR sensor originally, and needed to be adjusted for the MSG satellite data. This was accomplished by linear regression against the modified Sun-Pinker method. The second phase of this study was to develop a model to simulate the annual heat storage changes of the lake. Accordingly, the WST was determined for a time period covering from December 1, 2007 to May 31st 2009. A total of 25824 thermal images in two TIR bands (10.8μm and 12.0μm) were retrieved at hourly intervals to determine the WST. The model was tuned with satellite based observations to yield the optimal values of lag time and mixing depth which are 40 days and 6m respectively. Lastly, an assessment of the lake evaporation was made using the most common Penman based models. The Morton CRWE and CRLE methods have also been used to compare the outcome of the empirical Morton storage method with that of the satellite derived storage model described here. In this study an insight is gained on the possibilities and prospects of applying remote sensing techniques in combination with land based meteorological observations to accurately estimate the water loss due to evaporation. Key words: Brightness temperature, emissivity, evaporation, heat storage, temperature.
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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