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International Institute for Geo-Information Science and Earth Observation (ITC) 2014

Deriving water quality indicators of Lake Tana, Ethiopia, from Landsat-8.

Danbara, T.T. (Teshale Tadesse)

Titre : Deriving water quality indicators of Lake Tana, Ethiopia, from Landsat-8.

Auteur : Danbara, T.T. (Teshale Tadesse)

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 2014

Résumé
The purpose of this study is to derive the water quality indicators of the Lake Tana from the recently launched satellite Landsat- 8 using in- situ measurement and hydro- optical inversion model Hydrosat. In- situ water quality variables and radiometric measurements were carried out in September 2013 which is at the end of rainy season. The downwelling irradiance and upwelling radiance measured in the field and concentration of suspended particulate matter (S PM), coloured dissolved organic matter (CDOM), Chlorophyll- a absorption and turbidity from lake water samples were measured in the laboratory. The field measured Ramses spectral data was analysed and the Landsat- 8 relative spectral response (RSR) has been simulated by interpolated Ramses data. The spectral ranges of Ramses data to work accompany with the first five operational land imager (OLI) bands of Lansat- 8 and the central wavelengths of Landsat- 8 to retrieve the water quality variables were determined. The Hydrosat inversion model is selected for this study. The model derives all relevant water quality variables. For this study the model has been modified to retrieve the water quality indicators of Lake Tana from Landsat- 8 . Also an atmospheric correction scheme for Landsat- 8 has been developed . Results of this study on applicability of Landsat- 8 for retrieval of water quality indicators are promising. Compared to its predecessor newly added bands of Landsat- 8 such as band- 1 and band- 9 have been found useful for the retrieval of the water quality indicators. The field data has been used to calibrate and to validate the derived IOPs using calibration and validation of geophysical observation model (GeoCalVal) . The specific inherent optical property (SIOP) of SPM of Lake Tana has been estimated. The derived backscattering coefficient of suspended particulate matter ( SPM ) and the absorption coefficient of detritus and gelbstoff with the measured concentration of SPM have shown linear relationships with R- squared value of greater 0.7 . The time- space distribution of the water quality indicators of Lake Tana has been investigated for four seasons . The absorption of detritus and gelbstoff is high along the lake shore and the backscattering coefficient of SPM is low across the lake in dry season. In peak rainy season the IOPs are distributed within the same pattern across the lake and their higher values are observed along the lake shore and in the rivers outlet area. At the end of rainy season the absorption coefficient of detritus and gelbstoff is high across the lake and the backscattering coefficient of SPM is high in the west part of the lake. Furthermore, the IOPs are distributed across the lake with the higher value in the nort hern part shortly after the end of rainy season. The study has shown that the most dominant water quality variable which predominantly affects the IOPs of Lake Tana is SPM.  The main source of SPM in Lake Tana is the sediment load from the tributaries and from the erosion of the agricultural land around the lake during rainy season.

Version intégrale (ITC)

Page publiée le 2 avril 2018