Effect of Vegetation Degradation on Nitrate Retention Using NDVI and the SWAT Model in the Khemis Basin, Northwestern Algeria

Nitrate retention in a semi-arid watershed was assessed in parallel with the vegetation cover change from 2003 to 2013. The Soil and Water Assessment Tool (SWAT) model was used for simulating hydrology and nitrate retention in the Khemis Basin (375 km2 in NW Algeria). The normalized difference vegetation index (NDVI), derived from remote sensing satellite data, demonstrated a degradation of vegetation from 2003 to 2013. Nitrate retention varied between -0.98 and 0.03 and decreased with vegetation degradation due to the mobilization and leaching of nitrates available from fertilization. The results of the principal component analysis (PCA) and the correlations among water parameters, vegetation, and nitrates indicated that organic nitrate was mobilized into the water yield from surface runoff. Furthermore, organic nitrates decreased in soil water due to mineralization. Moreover, in the semi-arid basin, evapotranspiration contributes to water loss and plays an important role in reducing water yield, which can cause an increase in nitrate concentration in the water. Nitrate from the lateral flow of the basin was the main source of nitrate retained by vegetation, in contrast to surface nitrate, which is easily leached. Basin geomorphology can have an impact on nitrate retention, especially in flatter landscapes, which tend to increase nitrate retention in the soil. According to the correlations and the simulated nitrate balance, agricultural fertilization was the main source of anthropogenic nitrates in the Khemis Basin.