Spatiotemporal Variability of the Lake Tana Water Quality Derived from the MODIS-Based Forel–Ule Index: The Roles of Hydrometeorological and Surface Processes

Lake Tana, the largest inland water body in Ethiopia, has witnessed significant changes due to ongoing urbanization and socioeconomic activities recent times. In this study, two-decade recordings of moderate resolution imaging spectroradiometer (MODIS) were used derive Forel–Ule index (FUI). The FUI, which ranges from 1 (dark-blue pristine water) 21 (yellowish-brown polluted water), is important fully understand quality trophic state lake last two decades. analysis FUI over a period 22 years (2000–2021) indicates that Tana eutrophic as confirmed by values ranging 11 17. This agreement with (TSI) estimated MERIS diversity-II chlorophyll (Chl_a) measurements for overlapping 2003-2011 period. categorical skill scores show FUI-based classification relative MERIS-based TSI high performance. positive correlation TSI, (Chl_a), turbidity, total suspended matter (TSM) negative relations Chl_a TSM (at shoreline) colored dissolved organic matter. annual, interannual seasonal spatial distribution marked variation. hydro-meteorological, land-use–land-cover (LULC) related processes are found modulate spatiotemporal variability within range lower upper extremes revealed composite analysis. composites obtained terciles extreme percentiles variables representing hydro-meteorological LULC processes. High (poor quality) associated above-normal extremely (85 percentile) bottom layer temperature, wind speed, precipitation, surface runoff, hydrometeorological drought captured standardized precipitation-evapotranspiration (SPEI). contrast, observed during below-normal low (15 skin temperature evaporation. Conversely good (i.e., FUI) was times above sets drivers respectively. Moreover, varies response NDVI/EVI variabilities. relationship between its consistent expected physical under different drivers. instance, displaces algae blooms shoreline whereas intense precipitation increased runoff lead sediment loads. Increasing increases evaporation, thereby decreasing volume increasing insoluble nutrients, while microbial activity, enhancing phosphorus load. events, inflow allow algal bloom, Chl_a, particles increase, vegetation leads an increase non-point sources nitrogen.