Estimating Additional Water Yield From Changes in Management of National Forests in the North Platte Basin

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u I To evaluate this hypothesis, an area equal in size to the North Fork 100 Yp � q ( acres), was partially cut, removing approximately the same percentage of the forest by individually marking trees. Unfortunately and unlike the North Fork sub drainage, Unit 8 is not independently or directly gauged. The measured flow from the Upper Basin and the North Fork must be subtracted from the flow measured at the Deadhorse Main streamgage to partition out of the flow from the interbasin area, which includes the contribution from the North Slope. Partitioning the flow increases the opportunity for error and decreases the reliability of the experiment, but was done out of necessity to assess response from the North Slope (Unit 8) and only for the period 1981 -1983. After 1983, the combined response of treatments from both the North Fork and the Upper Basin make detection of the hydrologic effect of the North Slope (Unit 8) treatment even more tentative. Not all of the observed responses, were as expected. Peak water equivalent increased in the openings on the North Fork by 18 percent as expected (Troendle and King 1987). Although a significant increase in snow pack could be documented within the openings, the increase did not significantly increase the overall mean for the watershed (Forest plus open). On the Upper Basin, openings are more wind exposed, resulting in a certain degree of scour, and increases in snow pack accumulation at the level of the openings cannot be documented. The real surprise however, was that Peak Water Equivalent in the snow pack on the partially cut North Slope increased 16 percent, representing a 4.8 cm or 1.9 inch, increase snow in water equivalent over the entire 100 acre unit (Troendle and King 1987). Total water yield increased on the North Fork (figure 5) as expected but increases have occurred only periodically on the Upper Basin (figure 6). In the case of the North Fork, increases have averaged 2.5 — 3 inches per year for the period of study. In the case of the Upper Basin, the clear cuts were not as effective in generating an increase in flow. On average, covariance analysis indicates flow has significantly increased, but not all individual yearly responses show that trend. In drier years, flow is not increased detectably (figure 6) on the upper basin. Both the North Fork and the Upper Basin demonstrate that the largest increases occur in the wettest years and that most of the increase occurs in May. The surprise was the response from the North Slope (Unit 8). Partial cutting, or thinning, the stand resulted in a 3.5 -inch increase in flow for the period 1981 -1983. Over 50 percent of the observed increase in flow can be 1