The Secchi disk depth (ZSD) is a widely used parameter for evaluating water clarity. Here we propose an improved algorithm, which is based on a new underwater visibility theory, for retrieving more accurate ZSD from remote sensing reflectance (Rrs) in various waters. Two improvements were carried out in the new algorithm. First, we used a hybrid quasi-analytical algorithm (QAA_hybrid) instead of the sixth version of QAA (QAA_v6) for retrieving more accurate total absorption coefficient ( (λ)) and total backscattering coefficient ( (λ)) even in turbid inland waters. Second, we used a dynamic / ratio (i.e., ratio of diffuse attenuation coefficient of upwelling radiance and diffuse attenuation coefficient of downwelling irradiance) instead of using the fixed ratio (i.e., 1.5). The results obtained from in situ Rrs show that the improved ZSD estimation algorithm gave more accurate ZSD estimations, with the root mean square error (RMSE) reduced from 0.2 to 0.1 in log10 unit, mean absolute percentage error (MAPE) reduced from 39 % to 20 % (N=178 with in situ ZSD values between 0.3 -20.8 m). We then applied the improved ZSD estimation algorithm to the 2003-2012 MERIS images for Lake Kasumigaura to further confirm the performance of the improved ZSD estimation algorithm. The 2 results obtained from 19 matchups demonstrate that the estimated ZSD matched well with the in situ ZSD, with the RMSE of 0.11 m and the MAPE of 15%. The improved ZSD estimation algorithm shows a potential to estimate more accurate ZSD values from remote sensing data in various waters.
Lake Malawi is an important water resource in Africa. However, there is no routine monitoring of water quality in the lake due to financial and institutional constraints in the surrounding countries. A combination of satellite data and a semi-analytical algorithm can provide an alternative for routine monitoring of water quality, especially in developing countries. In this study, we first compared the performance of two semi-analytical algorithms, Doron11 and Lee15, which can estimate Secchi disk depth (SD) from satellite data in Lake Malawi. Our results showed that even though the SD estimations from the two algorithms were very highly correlated, the Lee15 outperformed the Doron11 in Lake Malawi with high estimation accuracy (RMSE = 1.17 m, MAPE = 18.7%, R = 0.66, p < 0.05). We then evaluated water transparency in Lake Malawi using the SD values estimated from nine years of Medium Resolution Imaging Spectrometer (MERIS) data (2003–2011) with the Lee15 algorithm. Results showed that Lake Malawi maintained four water transparency levels throughout the study period (i.e., level 1: SD > 12 m; level 2: SD between 6–12 m; level 3: SD between 3–6 m; level 4: SD between 1.5–3 m). The level 1 and 2 water areas tended to shift or trade places depending on year or season. In contrast, level 3 and 4 water areas were relatively stable and constantly distributed along the southwestern and southern lakeshores. In general, Lake Malawi is dominated by waters with SD values larger than 6 m (>95%). This study represents the first overall and comprehensive analysis of water transparency status and spatiotemporal variation in Lake Malawi.
Assessment of radioactive pollution of the Syrdarya river was carried out. A large number of water samples were collected over a twelve-year period from three zones: upstream of uranium mines; around uranium mines; and downstream of the mines. Samples were analyzed for gross α-, β-activity and radionuclide concentrations. Gross α-activity exceeded the permissible level in almost every water sample. Both gross α-and β-activity in Baigekum village and PV-1 mine significantly exceeded safe levels throughout entire monitoring period. Concentrations of 230 Th and 210 Pb surpassed the national intervention levels in almost all water samples. In a number of samples from Baigekum village excessive concentration of 226 Ra was observed. Furthermore, water samples collected from Tabakbulak in the spring of 2009 had extremely high levels of radionuclides. In general, elevated levels of radionuclides had been observed around the uranium mines and down the stream of Syrdarya since 2008-2009 when industrial-level production started at Zarechnoye, Khorasan and Irkol uranium deposits. The results suggest that radioactive pollution of Syrdarya in Kazakhstan is primarily caused by uranium mining activities. It is likely that the Syrdarya waters are not only unpalatable for human, but it may also not be suitable for household and agricultural use due to radioactive pollution.
Inadequate electricity access limits human development and affects quality of life in Africa. Hence, the improvement of electricity access has become a cornerstone of the African energy policy. This study proposes the use of satellite derived data of nighttime lighting, population count and Index Decomposition Analysis (IDA) to identify the driving factors for changing in electricity consumption in Africa between 2000 and 2013. The results of decomposition analysis indicated that population growth throughout the continent resulted in increased electricity consumption, while other factors had different effects. The results of this study could contribute to the design of better energy policy to achieve sustainable development of the continent.
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