An Evaluation of the Effect of Natural and Human Factors on the Linear and Non-linear Changes in Vegetation Using the Landsat Images of the Khor-Sefidarak Watershed, the Province of Alborz

The aim of current study was to study the trend of linear and non-linear changes in vegetation cover under the influences of climatic factors (rainfall, temperature and drought) and human activity (biological watershed management measures) using satellite images over a period of 20 years (2000-2019) on the Khor-Sefidarak Watershed, the Province of Alborz. Vegetation index (NDVI), drought index (SPI) and PolyTrend algorithm were used by extracting the monthly time series. Then, using climatic and implemented watershed management measures data for the basin, the impact of natural (temperature, precipitation and drought) and human factors on vegetation changes was assessed. The results indicated that 35% of the area had significant changes (92.6% increasing and 7.4% decreasing) in vegetation, and 65% of the area had insignificant changes during the last 20 years. The results also indicated that 87.71% of the vegetation changes in the region had experienced a linear type change and the remaining 6.62 and 5.68% haddemonstrated second and third degree nonlinear changes, respectively. Considering the effects of the two climatic variables on vegetation, precipitation did not show a significant trend over the two decades; however the temperature indicated a significant decreasing trend. Assessing the impact of drought on vegetation change, the largest decline in vegetative cover occurred in 2008, which was strongly influenced by severe climatic drought. Although the vegetation changes have not been affected by climatic factors in the long term, short-term changes in extreme events, including drought, strongly justify changes in the region's vegetation. On the other hand, the results indicated that the watershed management measures have caused changes with a linear pattern; however, the changes and expansion of the residential areas (as human factors) have caused a nonlinear pattern in vegetation. Thus the biological watershed management measures have increased and strengthened the vegetation of the region linearly; however, sudden and rapid changes in the rural areas have caused a non-linear decrease in vegetation.