Variation of surface water extent in the great Sebkha of Oran (NW of Algeria), using Landsat data 1987–2019: Interaction of natural factors and anthropogenic impacts

Abstract

The rapid and precise extraction of water information through satellite remote sensing has become a crucial technology in the study of water resources due to its ability to provide substantial and repeatable coverage instantly. This technology is particularly useful in monitoring wetlands, among other applications. It in this perceptive, this research aims to study the surface water extent (SWE) variation of the Great Sebkha of Oran (GSO). A wetland area classified as Ramsar, bordering Oran city. Extended over nearly 40,000 ha, the water extent is influenced by climatic, geological, and hydrological factors, which are typical of endorheic regions in semi-arid and arid climates. However, over the last decade, these natural effects have been overtaxed by anthropogenic impacts caused by the strong urban expansion of the city of Oran. This is particularly evident with the commissioning of the large El Kerma sewage wastewater treatment plant (WWTPK) in 2009. Located at NE edge of the GSO. The function of this plant is to treat a large proportion of the wastewater volume generated by the city of Oran. Nevertheless, for various reasons, a lag in the implantation of the various execution phases of a development project extended to the entire GSO basin has led to nearly a decade; the WWTPK has discharged partially treated wastewater directly into GSO. This had a major impact on the mode of surface water extent change of the sebkha and led to major pollution of the latter. To better understand the (SWE) dynamics, post and pre-discharges from the WWTPK in the sebkha were monitored monthly over 33 years (1987–2019). A consistent database of Landsat sensor images (TM, ETM+, and OLI) was collected and processed in the Google Earth Engine (GEE) platform to extract the SWE in the GSO, which was achieved using spectral indices: Normalized Difference Vegetation Index (NDVI), Normalized Difference Water Index (NDWI1 and NDWI2), Modified Normalized Difference Water Index (MNDWI) and Dryness/Wetness index (DWI). Subsequently, the SWE, correlated with precipitation over the entire GSO watershed, was analyzed and discussed at different time steps: monthly, annual, and seasonal. Before 2009, the precipitation regime study concerning the SWE shows the complexity of the functional dynamics of the endorheic system of the GSO watershed. After 2009, the commissioning of WWTPK had disrupted the SWE variation of the GSO. The maximum average of the SWE during the 2009–2019 sub-period was 169 km2, while it was 122 km2 during the undisturbed sub-period (1987–2008). The use of remote sensing data analysis contributes effectively to this work, representing proven results to protect this sensitive natural environment. They remain necessary to help decision-makers to ensure and preserve the sustainable ecological viability of the GSO and the natural environment of wetlands of the entire Oran sublittoral furrow of Gdyel-Arzew: Daiet Morcelly, Telamine Lake, Sebkha golf Arzew.