Impact of Forest Service Activities on Stream Flow

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Troendle/Nankervis/Porth Page 3 5/22/2003 tributaries. Laurie Porth, Biometrician with the RMRS, along with C. A. Troendle, had the lead in accomplishing this task. a) Double-mass analysis techniques were used to identify and validate the consistency of long-term stream flow data from as many 9 stream gauging stations in the North and South Platte River basins. Long term NRCS automatic and manual snow course sites were evaluated for consistency of record and paired with the stream gauges. Snow course data, as well as appropriate and representative long-term rainfall data, were then compared with long-term stream flow data to evaluate the stability of long-term hydrologic response at vazious points in the watersheds. As a result, a series of USGS stream gauging locations with long-term databases were identified that would be useful in detecting future change in flow. b) For each of the reference stream gauging sites, an analytical procedure was developed that: 1) characterizes the relationship between pre-1997 stream flow and snow pack accumulation or precipitation at 3 or more nearby sites, 2) allows user input of post-1997 data for stream flow and snow pack or precipitation, and 3) compares graphically and tests statistically whether the post-1997 data departs from the pre-1997 data. Results Management Impacts on Water Yield In the earlier analysis of the historical impacts of forest management activities on water yield from the North Platte River, the data in the stand polygon maps provided by the respective National Forests was aggregated based on similazities in aspect, elevation, species composition, etc. as a means of reducing the data base to a manageable size (Troendle and Nankervis 2000). In the analysis reported on here, Andrew Peavy (GIS Analyst, USFS Inventory and Monitoring Institute, Fort Collins, CO) assisted in developing the data into a GIS platform, Peavy combined the stand polygon maps provided by each forest into a composite for all forests. He then matched and clipped that data using the watershed boundary for the North Platte River. Relative to the eazlier effort, the total azea of the National Forest land in the North Platte River Basin was reduced by approximately 100 acres because of areas of overlap along the boundaries of adjacent Forests. The individual stand polygons were then intersected with the Oregon State University Climatic maps for Colorado and Wyoming to obtain an estimate of the average monthly precipitation input to each of the individual polygons. In the earlier effort, average monthly precipitation for polygon aggregates was estimated using elevation relationships developed from the same climatic maps (see Troendle and Nankervis 2000). Using the GIS to estimate precipitation for each individual polygon is far more robust than the eazlier procedure since it allows maintaining the identity of each polygon but the approach resulted in a 6-percent reduction in the estimate of average precipitation input to the North Platte watershed. As part of this effort, Mel Mehl (Renewable Resources, USFS Region 2, Denver, CO) provided additional information on timber harvest activity that occurred on each of the Forests, by year, from 1997 to 2001. The activity data, also in polygon form, was intersected with the original stand polygon data to describe initial and post harvest conditions in stands tha.t ha.d been impacted between 1997 and 2001. The result was the development of a GIS database containing information on the