Topographic Controls on Recent Vegetation Vigor and Decline in Nepal’s Mid Hills: A MODIS NDVI Time Series Analysis of Sindhuli District (2017-2023)

Abstract

Prabin Gauli

Topography exerts strong controls on vegetation vigor in mountainous regions, yet district scale assessments integrating recent NDVI trends with explicit topographic and climatic drivers remain limited in the Hindu Kush Himalaya. This study quantified spatiotemporal patterns of vegetation vigor and decline in Sindhuli district, Nepal, using MODIS MOD13Q1 NDVI time series from 2017 to 2023. Annual NDVI composites were generated, and linear regression was applied to derive pixel wise trend slopes. Topographic variables (slope, sin(aspect), cos(aspect), and solar radiation proxy) were extracted from the SRTM 90 m DEM.

Climate covariates (annual rainfall from CHIRPS and mean temperature from ERA5 Land) were added. A random sample of 3,000 points was extracted for statistical modelling using ordinary least squares regression and Random Forest analysis. Spatial autocorrelation was tested using Moran’s I. Maps were produced in R using the terra and ggplot2 packages. Results showed that mean NDVI across the district was 0.62 ± 0.11, with 28.4 % of the area exhibiting a negative NDVI trend indicative of vegetation decline. Solar radiation proxy emerged as the strongest predictor of both mean NDVI and NDVI trend (p < 0.001), followed by slope and annual rainfall. North and west facing slopes with high solar exposure displayed the most pronounced decline, while south facing slopes showed greater stability or slight improvement. Direct comparison between 2017 and 2023 confirmed a district wide NDVI decrease of 0.04 units. These findings demonstrate that topographic position, particularly solar radiation exposure, together with rainfall, is a primary driver of recent vegetation decline in Nepal’s mid hills. Given the widespread presence of Eucalyptus plantations in Sindhuli district, the observed trends have direct implications for fuelwood productivity, rural livelihoods, and sustainable plantation management. The study provides the first district level topographic and climatic analysis of vegetation vigor in the Nepalese mid hills and offers practical recommendations for site specific plantation planning. The open source GEE and R workflow developed here can be readily adapted for monitoring other mid hill districts in the Himalaya.

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