A healthy watershed, as defined here, is an area of land having the structure and density of forest stands necessary to support a diverse wildlife population. In addition, it has the natural stability of geology and soils to maintain the contribution of eroded sediments reaching streams at a level where natural hydrologic processes balance the ability of the system to both store and transport these sediments without degrading aquatic habitats. The current condition of a watershed is the culmination of all cumulative past events and the ecosystem's response to those events, whether natural or management induced. To insure these ecosystems are healthy and sustainable, both agency and private land owners must be able to exchange cumulative watershed effects (CWE) information about their individual activities accurately, quickly and in similar formats. To account for mitigation activities in mixed ownership watersheds, when assessing for CWE, all disturbance activities must be calculated. The accuracy of assessments of this nature are dependent on the quality of data being utilized. Standardized soil and disturbance history data are needed that will provide resource managers, both public and private, the information they need for economically and environmentally sound decision making.
An indexing or screening model was developed as part of SNEP that produces both a natural erosion potential and a sedimentation hazard index. The goal of this model was to utilize geographic information systems (GIS) and remote sensing as the basis for watershed assessment. With GIS coverages and satellite images, the model identifies sites, as small as 30 x 30m (100 x 100 ft), with high potential for producing sedimentation using the hypothesis that risk of erosion is a function of slope, soil detachability and lack of vegetative cover. The risk of sediment yield to streams is increased as a function of road location in proximity to streams and decreases in the presence of riparian vegetation buffers near streams.
This model works at the scale of Calwater planning watersheds of approximately 1,000 to 5,000 ha (3,000 to 15,000 ac) without the need of extensive field surveys. Watersheds are ranked: one, in order of their natural sensitivity to potential erosion and two, predicting the locations where sediments will originate. The model results are produced at relatively low cost, objectively generated, easily updated, responsive to changes in elevation and precipitation conditions, and minimize data corruption. Prediction of the potential for erosion and location of sedimentation sites is the objective of this spatially explicit and temporal methodology.
The model facilitates a screening process to focus attention of resource managers on the most acute potential problem areas. The tool helps the manager to optimize both environmental and economic investment strategies by locating those areas with greatest potential impact on cumulative watershed effects and selecting a mitigation approach that is most cost effective. Thinking of this methodology as an environmental accounting system allows the user to allocate resources to those projects that have the most immediate impact on reducing cumulative effects.
Related SNEP chapter:
Costick, L. A. Indexing current watershed conditions using remote sensing and GIS. Vol. 2, chapter 54, pp. 1421-1430.
