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University of Twente (2011)

REMOTE SENSING OF EUPHOTIC DEPTH IN LAKE NAIVASHA

Majozi Nobuhle Patience

Titre : REMOTE SENSING OF EUPHOTIC DEPTH IN LAKE NAIVASHA

Auteur : Majozi Nobuhle Patience

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

Résumé
Euphotic zone depth is a fundamental measurement of water clarity in water bodies. It is determined by the water constituents like suspended particulate matter, dissolved organic matter, phytoplankton, mineral particles and water molecules, which attenuate solar radiation as it transits down a water column. Primary production is at its maximum within the euphotic zone because there is sufficient Photosynthetically Active Radiation (PAR) for photosynthesis to take place. The study was conducted in Lake Naivasha, Kenya. Rich in biodiversity, it supports a thriving fishery, an intensive flower-growing industry and geothermal power generation, thereby contributing significantly to local and national economic development. Little is known about the optical properties of Lake Naivasha, and remote sensing methods have not been applied to study the water quality status of this lake. Thus the goal of the research was to estimate euphotic depth (Z eu ) based on attenuation coefficient (Kd (λ)), using remotely sensed data (MERIS). Ocean optics modelling was based on deriving Kd (λ) from in-situ underwater optical measurements of downwelling irradiance at two depths (0.1 and 0.6m) and remote sensing reflectance (R rs (λ)). Based on the relationship between Kd (λ) and 1/R rs (λ), the spectra were systemically characterised into three distinct classes : 430-600nm, 600-800nm and 800-930nm. Model coefficients were derived for each spectral range using insitu data. The models successfully reproduced measured Kd (λ) (R 2 >0.87 and RMSE of 0.97, 0.85 and 0.26m-1 , respectively). A local model was also developed to retrieve Z eu from Kd (620). MERIS match-up data was used to validate the spectral range-based Kd (λ) model. Results gave an RMSE of 0.86, 0.25 and 2.55m-1 respectively. The empirical methods of deriving Z eu was also applied on Kd (490) and Kd (620) products of MERIS, and the Kd (620) method was more accurate ( ε =2.07% and RMSE=0.044m). Finally, maps of Z eu produced revealed that it varies between 0.9 and 1.3m, and that the deepest light penetration is experienced in the Crescent Island part of Lake Naivasha.

Version intégrale (ITC)

Page publiée le 22 novembre 2013, mise à jour le 26 janvier 2018