Operational assessment and monitoring of plant cover in large areas commonly relies on vegetation indices (VIs) determined using the functions of the reflectance in the red and near infrared spectral bands. There is a large degree of variability in the spectral characteristics of different types of vegetation and soil/rock for arid environments. This requires the use of a vegetation index in conjunction with the knowledge of the vegetation and land characteristics in the area being assessed or monitored. A previous paper has been devoted to assess desertification trend in the north part of Kenya. The present work concerns the calibration/validation of the Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) for a tropical arid environment using ground-based spectral data and Landsat Enhanced Thematic Mapper (ETM) images. Data from Landsat (ETM) covering a dry seasons over one of the dry ecosystems in Kenya were processed and the relative NDVI and EVI computed. Spectral data measurements were performed by means of a spectroradiometer (ASD FieldSpec PRO) and other land cover characteristics were observed in the field for the same season and area