Geoscience Reference
In-Depth Information
Table 9.1
Land-Cover Class Descriptions for the Upper San Pedro Watershed
Forest
Vegetative communities comprised principally of trees potentially over 10 m in height and
frequently characterized by closed or multilayered canopies. Species in this category are
evergreen (with the exception of aspen), largely coniferous (e.g., ponderosa pine, pinyon
pine), and restricted to the upper elevations of mountains that arise off the desert floor.
Oak Woodland
Vegetative communities dominated by evergreen trees (
spp.) with a mean height
usually between 6 and 15 m. Tree canopy is usually open or interrupted and singularly
layered. This cover type often grades into forests at its upper boundary and into semiarid
grassland below.
Quercus
Mesquite
Woodland
Vegetative communities dominated by leguminous trees whose crowns cover 15% or more
of the ground, often resulting in dense thickets. Historically maintained maximum
development on alluvium of old dissected flood plains; now present without proximity to
major watercourses. Winter deciduous and generally found at elevations below 1200 m.
Grassland
Vegetative communities dominated by perennial and annual grasses with occasional
herbaceous species present. Generally grass height is under 1 m and they occur at
elevations between 1100 and 1700 m, sometimes as high as 1900 m. This is a landscape
largely dominated by perennial bunch grasses separated by intervening bare ground or
low-growing sod grasses and annual grasses with a less-interrupted canopy. Semiarid
grasslands are mostly positioned in elevation between evergreen woodland above and
desertscrub below.
Desertscrub
Vegetative communities comprised of short shrubs with sparse foliage and small cacti that
occur between 700 and 1500 m in elevation. Within the San Pedro river basin this community
is often dominated by one of at least three species (i.e., creosotebush, tarbush, and
whitethorn acacia). Significant areas of barren ground devoid of perennial vegetation often
separate individual plants. Many desertscrub species are drought deciduous.
Riparian
Vegetative communities adjacent to perennial and intermittent stream reaches. Trees can
potentially exceed an overstory height of 10 m and are frequently characterized by closed
or multilayered canopies depending on regeneration. Species within the San Pedro basin
are largely dominated by two species: cottonwood and Goodding willow. Riparian species
are largely winter deciduous.
Agriculture
Crops actively cultivated and irrigated. In the San Pedro River basin these are primarily found
along the upper terraces of the riparian corridor and are dominated by hay and alfalfa. They
are minimally represented in overall extent (less than 3%) within the basin and are irrigated
by ground and pivot-sprinkler systems.
Urban (Low and
High Density)
This is a land-use dominated by small ejidos (farming villages or communes), retirement
homes, or residential neighborhoods (Sierra Vista). Heavy industry is represented by a
single open-pit copper mining district near the headwaters of the San Pedro River near
Cananea, Sonora (Mexico).
Water
Sparse free-standing water is available in the watershed. This category would be mostly
represented by perennial reaches of the San Pedro and Babocomari rivers with some
attached pools or repressos (earthen reservoirs), tailings ponds near Cananea, ponds near
recreational sites such as parks and golf courses, and sewage treatment ponds east of the
city of Sierra Vista, Arizona.
Barren
A cover class represented by large rock outcropping or active and abandoned mines
(including tailings) that are largely absent of above-ground vegetation.
set to 20 where available (van Genderen and Lock, 1977). Work by Congalton (1991) and Congalton
and Green (1999) suggests that sample sizes derived from multinomial theory are appropriate for
comparing class accuracies, with a minimum sample size of 50 per class; however, this goal was
not attainable for rare classes in this study.
After evaluation of selected sample points in each reference data set, an error matrix was con-
structed, comparing map class labels to reference data labels for each LC classification. Overall map
accuracy and class-specific user and producer accuracies were calculated for each class. A Khat
(Cohen's Kappa) and Tau (Ma and Redmond, 1995) were computed for the four error matrices,
followed by a significant difference test (Z-statistic) based on Khat values (Congalton and Green, 1999).
9.3.3
Historical Aerial Photography
Reference data for the 1973 and 1986 LC maps were developed using aerial photography stereo
pairs covering the Arizona portion of the study area (1:40,000 scale). A team, including a photo
