Impact of Forest Service Activities on Stream Flow

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Troendle/Nankervis/Porth Page 17 5/22/2003 Impacts of 2002 fire and insect and disease mortaGty on stream flow The year 2002 was particularly dry and one of the worst fire seasons in recent history. In addition to the catastrophic losses due to fire, mortality associated with insect and disease attack was also perceived to be elevated. As a separate exercise, the Forests provided additional data on the occurrence of fire and insect and disease (pest) mortality in the North Platte Basin for year 2002. Fire data consisted of the defmition of the fire boundary, and identification of the unburned, lightly burned, moderately burned, and severely burned azeas within the fire boundary. Along with providing a GIS layer depicting the area of burn, the Forests also provided a description of the percentage of pre fire basal area remaining after the fire, as a function of specie composition and fire intensity. In addition to the fire data, GIS coverage of aerial surveys for "red tops" or pest kills, in 2002, was also provided. The aerial extent of the pest incidents was much broader than the eactent of fue but the tree mortality was faz less. The pest data consisted of the spatial definition of the polygon surveyed and numbers of red tops, by suspected pest. Although the area of the polygon was defined, along with the number of dead trees, the total number of trees was not listed so a percent reduction in basal azea could not be calculated. In most cases, only a few trees in the polygons were killed and simple calculations indicated that less than 10 percent of the basal area was impacted, at both the polygon and watershed level. A 10 percent or less removal of basal area will not result in a simulated change in stream flow so the impact of pest mortality was dropped from further consideration, hydrologically. Data for three fires, presumed to have occurred on the North Platte basin was provided. As was done with the "activity" data, the first step was to intersect the fire layer with the watershed boundary layer. One of the fires did not occur in the North Platte basin and was dropped. In total, 3311 acres of forestland were burned in 2002 in the North Platte basin. This consisted of 1283 acres of spruce-fir, 1414 acres of Ponderosa Pine, 431 acres of Lodgepole Pine, 129 acres of Limber Pine, and 54 acres of Aspen. Most of the area burned was of low to moderate severity meaning that 50 to 80 percent of the initial basal azea survived the fire. The burned azea layer was intersected with the original stand description data layer and the specie composition, initial stand basal azea, reduction in basal azea resulting from fire, and polygon precipitation and aspect deternuned for each separate polygon. T'he SAS model was then run to estimate the impact of fire on water yield. We simulated that the fues that occurred in the North Platte basin in 2002 may have caused the 1997 estimate of stream flow of 10.0 inches to increase by 0.01 area inches or 922 acre feet of water per yea.r. This compazes with the estimate of annual increases due to timber harvest activity of about 0.003 azea inches per year. The increase due to timber harvest is accumulative and by 2017 would accrue to 0.05 area inches per year as noted above while the fires of 2002 would be contributing a constant 0.01 area inch. Summary of the Effect of Vegetation Change on Water Yield The most significant factor influencing stream flow from National Forest land in the North Platte River is the amount of precipitation entering the basin on an annual basis. The most significant factor influencing the long-term trends in streamflow response in the basin appears to be the influence of natural changes in vegetation. Forest management activities aze currently having