Kappa values ranged from 0.69 to 0.88 and from 0.36 to 0.88 for the November 2012 and January 2014 image mosaics, respectively (Table 2). Most SVIs performed considerably better than random assignment; kappa values were (exceptions were NDVI and EVI of the January 2014 product). In a comparison of the two image sets, the detection accuracy was higher in the November 2012 products (average kappa, 0.82) than in the January 2014 products (average kappa, 0.64). Regardless of the image set, SVIs often overestimated vegetation, as indicated by the false-positive rate being higher than the false-negative rate. Although a number of SVIs in the 2012 image clearly performed better than those in the 2014 image, VARI performed well for both image sets according to all three measures (kappa 0.875; false positive 15.0%; and false negative 12.2%, on average). VARI and GNDVI showed comparable performance, but VARI was superior to GNDVI because its accuracy between the two data collection dates was more consistent than that of GNDVI (e.g., for VARI, kappa 0.87 and 0.88; for GNDVI, kappa 0.69 and 0.87). In addition, the threshold values of VARI from the two datasets differed by only 0.02; this indicates that the index is stable for detecting vegetation in drylands as well as being robust in various environmental conditions in desert regions. On the basis of both these qualitative and quantitative assessments, VARI was selected for the fractional land cover calculation.