20 years MODIS-NDVI monitoring suggests that vegetation has increased significantly around Tehri Dam reservoir, Uttarakhand, India

Large dams have immediate impact on surrounding riparian vegetation due to inundation by reservoir. The effect however may vary beyond impoundment region in an undulating topography. Present work assesses the vegetational changes between years 2000 and 2019, around Tehri dam reservoir area across elevation zones. The work uses 20 years Moderate Resolution Imaging Spectroradiometer-Normalized Difference Vegetation Index (MODIS-NDVI), and covers over 1600 km(2) around one of the world's highest dams built in the western Himalayas. The study area is divided into 13 different zones. Each zone has a 100 m altitudinal difference; from 815 m up to 2015 m altitude. It is observed that in the first (impoundment) zone there is a significant decline in vegetation in last 20 years. Beyond this zone, a significant increase in vegetation up to 1515 m altitude is recorded. The geographical area from reservoir up to this altitude is around 700 km(2). Although vegetation beyond 1515 m exhibits an increasing trend but the increase is not statistically significant (confidence interval < 90%). The overall NDVI exhibits an annual increase of 0.0011 for the last 20 years. It is concluded that the negative impact of reservoir on vegetation is limited up to 815 m altitude while the positive impact is observed between 815 m and 1515 m. No significant impact is observed above 1515 m.