lydia letaru

Abstract We detail our attempts at empirical modeling of MODIS derived Chlorophyll a (Chl a) distribution on Lake Victoria in East Africa and consequently its trophic status. This was motivated by the need for Lake Victoria specific algorithms, as the current satellite based standard algorithms overestimate derived Chl a. In situ Chl a data was hence collected in three field campaigns in November 2014, March 2015 and July 2015. In situ reflectances were collected during the July campaign only. We first developed models from in situ reflectances and in situ Chl a, which when applied to MODIS bands performed dismally (R2 = 0.03). We then proceeded to derive empirical models by directly comparing MODIS bands with in situ Chl a based on data collected in November 2014 and July 2015. The March 2015 dataset couldn’t be used due to cloud cover hence no matchups could be obtained. The best model derived (R2 = 0.88) was based on the ratio 488 nm/645 nm, and was then used to determine the trophic status of Lake Victoria using Carlson’s Chl a Trophic State Index (TSI). The results show that large areas of the lake are mesotrophic with eutrophic displays closer to the shores. The modeled TSI was then validated against in situ TSI derived from the March dataset and posted an 80% matchup. One of the main challenges, however is the prevalence of cloud cover, which hinders synoptic mapping of the lake. That notwithstanding, the study demonstrates the potential of earth observation in providing accurate TSI information for improved management of Lake Victoria.

Trophic status (TS) is a water quality indicator effectively determined from Secchi Disk Depth (SDD). SDD is an in situ method of determining water clarity which is cumbersome, expensive and limited in time and space. This prompted the exploration of satellite imagery which provides a means around these challenges. Whereas algorithms for SDD retrieval from satellite imagery have been developed, their performance has not been tested on Lake Victoria. Therefore, this study aimed at determining the best SDD retrieval algorithm for MODIS satellite imagery from which the lake’s SDD and TS were determined. To this effect, five algorithms were explored and their output was compared to the in situ data collected on 27 July 2015. The multi band model performed best with a R2 of 0.709, RMSD of 0.295 m, RPD of 29.278% and bias of 0.593 m. This algorithm was applied to MODIS Aqua monthly and yearly aggregates from 2013 to 2017 and the trophic status was determined using Carlson’s Trophic Status Index (CTSI). The outputs showed that the water closer to the lake’s shore had a shallower SDD as compared to that in the middle of the lake and the CTSI values indicated that the lake presents as mesotrophic with persistent eutrophic hotspots at the shoreline. It was concluded that Lake Victoria was exhibiting water quality issues and recommended that they are dealt with to prevent its further degradation.