Investigating variations of vegetation: climatic, geological substrate, and topographic factors-a case study of Kharestan area, Fars Province, Iran

Kharestan basin is regarded as one of the sources of sediment production and erosion at the upstream side of the Middle East's largest embankment dam (Doroudzan Dam) whose vegetation directly affects the quality of water and soil conservation and the ecological health of the environmental. Accordingly, to investigate variations of vegetation, this study used Enhanced Thematic Mapper (ETM) long-term data for estimating Normalized Difference Vegetation Index (NDVI) with 16 days interval for the time period of 2017-2017 in Kharestan basin. Climatic factors (precipitation, temperature, evapotranspiration potential) and topography (elevation, aspect, slope, and compound topographic Index) were considered effective forces on NDVI variations. Spatial distribution of vegetation and its relationship with climatic and topographic factors in geological formations were investigated. Principal component analysis (PCA) was used to investigate collinearity, multivariate regression was used to explore the relationship between climatic factors and vegetation, and geographical weighted regression (GWR) was used to study topographic factors. The results indicated that the spatial distribution of NDVI was < 0.5 in south, north, and west, and > 0.5 (including agricultural and horticultural lands) in the center of the basin. There was a positive correlation between NDVI and precipitation; moreover, the correlations between NDVI, temperature, and evapotranspiration potential were negative. The NDVI value increased with an increase in precipitation, and decreased with the decrease of temperature to the west and north. In all formations, the highest vegetation growth was observed at the elevations from 1900 to 2900 m. The highest NDVI value was observed in Kashkan Formation at elevations of 2050 to 2100 m. The shaded aspects were more suitable for vegetation growth than the sunny aspects. In all formations, the NDVI value increased up to 15 degrees slope, but decreased with the rise in slope (from 16 degrees slope onwards). From among the factors mentioned, precipitation, temperature, and evapotranspiration potential had the greatest impact on NDVI. This finding is useful for biodiversity protection and constitutes a precious input to environmental and ecological research.