Larry A. Costick

This dissertation evolved from the Sierra Nevada Ecosystem Project (SNEP) report number 3
 

Keywords:  Remote Sensing, GIS, Natural Resources, Modeling, Watershed Assessment, Soil Erosion,

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Abstract

The purpose of this study was to develop a model that uses resource inventory information to screen and index watersheds for their potential for soil erosion and possible contribution to sedimentation. The model, based on conditions in watersheds of the central Sierra Nevada Mountains of California, produced rankings termed: Natural Erosion Potential Index (NEPI) and Sedimentation Hazard Index (SHI). Both provide indicators of the "current cumulative condition."

In the study, a methodology was designed and tested using Geographic Information Systems (GIS) and remote sensing to map areas with high potential for soil erosion and to locate specific sites where stream sedimentation is likely to occur. Forty watersheds of the Cosumnes and American River Basins were studied intensively. The study included both Camp and Cat Creeks, which have been the focus of intensive ecological and CWE reviews and publication by the Forest Service. The model developed from the study of these 40 watersheds was applied to the 156 watersheds of the Eldorado National Forest (ENF) to rank them for potential erosion and sedimentation hazards. The hypothesis was that risk of erosion is primarily a function of slope, soil detachability, and bare unprotected ground. The risk of eroded soil becoming sediment increases as a function of road location in proximity to streams and decreases in the presence of riparian vegetation buffers near streams. The NEPI uses slope, cover, and soil detachability as the most significant parameters indexing soil erosion, given common climatic conditions. Threshold values established for these parameters provided the link to locations where the probability of sediment reaching the watercourse is high.

Following the classifications, the relative ranking of the watersheds was computed on the basis of: (1), the order of their natural sensitivity or their inherent erosion potential and (2), predicting the probable origins of sediment. It is assumed these indices will be used as guides for future mitigation activity. The results of NEPI and SHI are significantly less costly to produce than standard field surveys, are objectively generated, easily updated, and are responsive to variation in elevation and precipitation distribution over the watersheds. In predicting the potential for erosion of a given unit area of land, the model indexes "current condition," or "watershed health," in Cal-Water planning watersheds (CWPWS) relative to the condition of neighboring watersheds. These models have been designed to be used as a primary screening tool and environmental accounting system that provide objectively generated assessments to decision makers.

The ranking process allows resource managers to focus attention on the most acute problem areas, where the potential for sedimentation is the greatest. This tool allows the manager to optimize both environmental and economic investment strategies by locating areas that have the greatest impact on cumulative watershed effects (CWE) and to select the mitigation most cost effective at the local site. This analysis methodology is an accounting system that provides managers with tools to be used when allocating both human and financial resources to mitigation projects. These tools, among other things, provide locations where mitigation will likely have the most immediate or significant impact on the reduction of sedimentation and ultimately, on cumulative watershed effects.

NEPI and SHI rankings agree with empirical reports of the US Forest Service and private industry. The Forest Service equivalent roaded acres (ERA) values acquired through the use of the ERODA model are not available for all of the Eldorado National Forest watersheds. Where values exist they are not supported by scientific observation and testing, making it impossible to use them as a standard by which to measure NEPI and SHI. For the 77 watersheds that have comparable data, the r values comparing the USFS's natural sensitivity index with NEPI are 0.54 (Costick 1996). ERA and SHI are conceptually different in that ERA reduces all disturbance, regardless of location, to an acreage equivalence of unsurfaced roads, while SHI focuses on multiple parameter values that contribute significantly to erosion only in stream buffer zones where roads are present. The r value correlation for these two assessment methods, however, was 0.34, indicating some areas where both methods agree on erosion hazard.

One of the driving parameters of the NEPI methodology is the lack of vegetative cover or the amount of bare easily eroded soil. This parameter is dynamic and changes during each growing season. Only timely analysis of recent satellite imagery or aerial photos can provide this data for annual or bi-annual watershed ranking. Impacts of the disturbance of terrestrial ecosystems are cumulative; therefore, summing acres or hectares actually disturbed season after season provides valuable data for cumulative watershed effects assessment.

This spatially explicit map product points to critical area in the South Fork American River canyon where U.S. Highway runs between Placerville and Lake Tahoe.  Where the parameters of slope, cover, and soil K-factor exceed all three threshold conditions the cells are colored red.  Where slope and cover thresholds are exceeded the cells are yellow and orange where K-factor and cover are exceed.  Click in the image to see the enlargement.
 
Keywords: cumulative watershed effects; soil erosion; watersheds; resource inventory; watershed management; forest management; GIS; remote sensing of environmental parameters.