3. Results and discussion
3.1. Classification accuracy
The overall accuracy of our classifications was higher than 84%
for all the years. The user and producer accuracy (Story, 1986),
obtained for different vegetation cover classes in each of our thematic
maps, varied between 70e100% and 57e100%, respectively
(Table 3).We focused on user accuracy for the purpose of this study
because it signifies how well the thematic map represents land
cover. Some of the confusion among several of the land-cover
classes is likely because of similarities in their spectral
characteristics.Mesquite shrublands were sometimes confused with desert
shrubland because of overlapping species and similarities in
response to monsoon rains. Mesquite shrublands were sometimes
confused with active agricultural fields, especially fields with
irregular orchards (observed during our accuracy assessment) that
might possibly have a similar spectral response as the mesquite
trees (according to the classifier). Most other land-cover classes
were classified correctly, primarily because of their unique
phenological, biophysical and topographic characteristics that
easily differentiated them from other land-cover classes. For
example, the succulent-stemmed shrublands were identified by
unique components related to elevation and species composition
and active agricultural fields were accurate because the variables
captured phenological change; this was expected because irrigated
crops are not dependent on precipitation to conduct biological
activity.