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International Institute for Geo-Information Science and Earth Observation (ITC) 2009

Open water evaporation estimation using ground measurements and satellite remote sensing : a case study of lake Tana, Ethiopia

Abreham Alebachew

Titre : Open water evaporation estimation using ground measurements and satellite remote sensing : a case study of lake Tana, Ethiopia

Auteur : Abreham Alebachew

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 2009

Résumé
Lake Tana is the largest natural reservoir of fresh water resources in Ethiopia. It is the source of Blue Nile River and is located in the north-western part of the country. It has a surface area of 3000 km 2 with an elevation of 1786m above sea level. Water resources management in Lake Tana and its surroundings is an issue of high significance because of great socio-cultural, ecological and economical values. Evaporation is one of the main components of the water budget of lakes. Accurate estimates of lake evaporation are necessary for water and energy budget studies, lake level forecasts, water quality surveys, water management and planning of hydraulic constructions. Evaporation from open water remains a difficult process to measure or estimate. The major source of difficulty is the fact that the required meteorological parameters are rarely measured over the water surfaces, and the thermal lag between the water and land surfaces renders the land-based measurements less effective in the parameterization of open water evaporation. The use of remotely-sensed data provides a means of obtaining useful information about the evaporating water surface. This study examines open water evaporation losses f rom the lake by different climatic methods and remotely sensed spectral data. The long-term daily meteorological (1995-2007) data from the nearby station of the lake is used as well as the annual cycle of albedo on a monthly time steps produced fro m Terra MODIS satellite image products. In situ measurements of instantaneous energy balance components was collected from the surface of the lake using 3D sonic anemometer and four channel radiometer and also the temperature profile of the water during the field campaign to calibrate and validate remote sensing data analysis. A daily evaporation map is produced from combination of in situ measurements and satellite remote sensing data (Terra MODIS) using the evaporative fr action approach. The average daily evaporation from the lake calculated for a cloud free day of 27 th of September 2008 is obtained as 4.94 mm/day with standard deviation of 0.20 mm/day. The surface temperature exhibits a strong diurnal variability that cannot be captured from polar orbiting satellites that sample each location approximately twice a day. Geostationary satellites provide diurnal coverage, and allow derivation of t he surface temperature cycle. In this study four channels and split-window algorithms are used to re trieve the surface temperature of the lake from MSG/SEVIRI images. The four channel method gives slightly better results than the split window algorithm.

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Page publiée le 23 mars 2018