Assessing Temporal Variability of Turbidity in Pampean Shallow Lakes (Argentina) Using Three Decades of Landsat Imagery

Abstract

The Pampean region (Argentina) encompasses thousands of permanent shallow lakes, which can be either in turbid or clear water regimes. The intensification of human activities in this region has led to a general deterioration of their water quality. Remote sensing allows the monitoring of water quality in lakes over large areas and extended periods. The objectives of this study were to analyse temporal variations in turbidity in six shallow lakes using Landsat satellite data (5-TM, 7-ETM+ and 8-OLI) for the period 1987–2020 and to evaluate the impact of total crop area and El Niño Southern Oscillation (ENSO) events on these variations. A semi-analytical turbidity algorithm using reflectance in the red and near-infrared bands was tested against in situ turbidity measurements. The Floating Algal Index (FAI) was used to discard pixels with above-water vegetation. The best performance of the algorithm was achieved using only the near-infrared band for the four turbid lakes, and the red band for the two clear lakes. High vegetation coverage in clear lakes prevented the estimation of the complete turbidity series. A significant positive turbidity trend was observed for turbid lakes during the 2000s, coinciding with a sharp increase in cultivated area. Regarding ENSO, a statistically significant relationship with turbidity was observed for the entire period in one lake, but turbidity peaks coincided with La Niña events in all lakes. This study emphasizes the utility of remote sensing in complementing field studies and thus monitoring regional changes in aquatic systems over long periods.