Co-located optical and thermal surface change in a large desert reservoir: a grid-cell bivariate climatology of twenty-one years of MODIS observations over Lake Nasser

Introduction Large desert reservoirs are dynamic socio-hydrological systems, yet long-term satellite assessments of their surface behaviour have been largely univariate, leaving the spatial co-occurrence of change poorly characterised. This study presents a grid-cell bivariate climatology, applied to 21 years (2005–2025) of monthly MODIS/Terra observations over Lake Nasser, Egypt. Methods Five variables were analysed across 186 lake-and-margin cells: the normalised difference vegetation index (NDVI), an uncalibrated near-infrared optical index (the NIR index), white-sky albedo, and daytime and night-time land-surface temperature (LST). Both indices are uncalibrated optical signals, not measures of vegetation or of chlorophyll-a concentration. Each monthly series was summarised by its trend, abrupt change, intra-annual variability and skewness, and the pairs cross-classified, with spatial autocorrelation accounted for in assessing significance. Results Albedo declined over 94.1% of cells. Daytime LST fell over most of the reservoir whereas night-time LST rose in every cell, and the day-minus-night range narrowed significantly throughout. Both indices declined mainly in intermittently exposed marginal cells, the NIR index showing no consistent direction over permanent water. Intra-annual variability was tightly coupled between the two LST fields (Spearman’s ρ = 0.85) and between the NIR index and albedo (ρ = 0.76). Abrupt shifts in the optical variables were detected in most cells and dated to 2014 and 2017, before the upstream Grand Ethiopian Renaissance Dam began filling, and their directions matched the trends. Against a Landsat-derived inundated-area record, independent of the MODIS/Terra series, the optical signals and the diurnal range varied inversely with reservoir extent; an external MODIS-Aqua chlorophyll-a climatology, from a different platform and retrieval algorithm, showed no consistent relationship with the NIR index, which does not support reading a single near-infrared band as a pigment signal. Discussion The principal patterns were stable across the sensitivity analyses tested, and the framework offers a spatially explicit baseline for co-located optical and thermal change in large reservoirs. The bivariate classification is transferable to other large reservoirs.