Land Cover and Land Use Mapping in the San Lorenzo River Watershed

Background

High spatial resolution multispectral satellite imagery can be a valuable resource for assessing, monitoring, and analyzing watershed conditions, as well as resource uses, by generating land cover and land use maps. Typically, land cover and land use maps are derived from satellite imagery by classifying unique spectral signatures in the imagery according to their associated types of ground cover and land use which are defined by a scientist or resource manager. The spectral signatures result from the distinct manner in which different land cover and land use features reflect and absorb sunlight. The reflectance and absorption properties captured by remotely sensed images are utilized by image processing software packages to differentiate between land cover or land use categories, resulting in a classified map for a defined geographic area. Accurate assignment of spectral signatures requires significant work and knowledge and the final classified map is usually verified by interpretation of aerial photography or ground-truthing to confirm class assignments.

Recently, a land cover map and a combined land use and land cover map were created for the San Lorenzo River watershed in Santa Cruz County from multispectral IKONOS imagery acquired during November 2001. The maps were created by the National Oceanic and Atmospheric Administration (NOAA) Coastal Services Center using a modified classification scheme based on NOAA's Coastal Change Analysis Program (C-CAP) and input from local ecologists and county resource managers. In addition, Santa Cruz County parcel data and other ancillary data sources were used to derive land use categories from the land cover data set. (See the SRP GIS Data Inventory / Download section for access to this data and metadata).

This combined land cover and land use map of the San Lorenzo River watershed was created from IKONOS satellite imagery. The high spatial resolution of the imagery, along with input from local scientists and field data, allowed for mapping of the detailed classes shown here.

Results and Applications

The land cover and land use maps that were created from the IKONOS imagery of the San Lorenzo River watershed provide a record of current watershed conditions and resource uses. They quantify the general land cover (see table) and land use (see table) patterns across the watershed, and can be compared or combined with other data to evaluate changes over time or to identify where research, restoration, or conservation may be desirable.

The maps indicate that the San Lorenzo River watershed is largely covered by evergreens, and that Timber Production Zones (TPZ) is the largest land use class in the watershed, followed by residential development and roads. When these maps are viewed concurrently with a digital elevation model in geographic information system (GIS) software, it is apparent that the evergreens occur predominantly in steep upland areas, while other land uses, such as residential and commercial development, are confined to the lower elevations of the watershed where it is less steep.

The land cover and land use maps also show that residential and commercial development in the watershed is concentrated along the San Lorenzo River and its tributaries. Residential and commercial development can impact salmonid recovery by increasing pollutants, impervious surfaces, and water diversions (see the Santa Cruz Socioeconomics and the Factors Limiting Salmonid Production sections of this Web site for more information). Although impervious surfaces in the watershed account for less than five percent of the total watershed cover, even this small amount may negatively affect salmonid habitat. Studies have shown that conversion of as little as 15 percent of a watershed to impervious surfaces may result in the dramatic decline of aquatic health (Horton 2000).

While the land cover and land use maps reveal that the up-slope areas of the watershed have an evergreen canopy, the IKONOS imagery used to create the maps could not be used to classify what lies beneath that canopy. For example, there may be roads beneath the evergreens that contribute sediment to nearby streams, or there may be invasive exotic plant species that are altering riparian plant communities. It is also difficult to determine the seral stage of the evergreen forest. For these and other reasons, spatial analysis using remotely sensed imagery must often be supplemented by field investigations and other data and evaluations in order to be useful for resource management. Land cover and land use maps derived from high resolution imagery do allow resource managers and planners to ask relevant questions to determine whether more detailed information is required, and to identify locations where on-the-ground information should be gathered.

The San Lorenzo River watershed shares many of the features of neighboring watersheds in Central California, including a history of diverse land uses such as logging, mineral extraction, tourism and recreation, as well as significant physical and biotic diversity and intact salmonid populations. Landscape information gained from analyzing high resolution imagery for this watershed may therefore provide insight into watershed conditions and management for the larger region.

Reference

Horton, T. 2000. Death for streams lies in the pavement. Baltimore Sun.