A Comparative Study of NDVI Values Between North- and South-Facing Slopes in a Semiarid Mountainous Region

This paper used Landsat-5 Thematic Mapper (TM) data from 2007 to 2011 to examine seasonal changes in the normalized difference vegetation index (NDVI) in north-facing and south-facing slopes of a small (approximately 1900 ha.) west-draining hydrographic basin in the Guadalupe Valley in northwest Mexico. The analysis was done for the wet and dry seasons of this mid-latitude (32°N) basin with a rugged geomorphology covered by mostly native vegetation (chaparral) with minor anthropogenic disturbance. We used ten Landsat-5 TM satellite images that were preprocessed to correct atmospheric effects, to further estimate radiance and reflectance before the NDVI computation was performed. A LiDAR derived digital terrain model was used to calculate the terrain slope and aspect to further select and classify north and south facing slopes. With the slopes classified, zonal statistics of the NDVI were computed estimating mean and standard deviation for both slope orientations. We found higher NDVI values in the north-facing than in the south-facing slopes, regardless of the season (wet and dry), and also higher vegetation index values during the wet season compared to the dry season. For the period analyzed, we distinguished the signal of extreme dry year during 2007.