Estimating Additional Water Yield from Changes in Management

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make documenting the role of vegetation on stream flow response: at the landscape level a difficult and often misleading task., ; -: .. For example, Burton (1997) used USGS stream flow records to ass,ess the cumulative impact of timber harvest activities (which occurred over a 12- year period) on stream flow fi•om Brownie Creek in Utah. Using thf; North Fork of Dry Creek as the c;onlzol, Burton (1997) compared the flow for the pre-harvest period (1951- 1960) with the flow for the harvest/post-harvest period (1961-1980) for both watersheds and found flow for the 1961-1980 period was significantly ;greater on Brownie Creek. He concluded the increase was a response to timber harvest. Troendle and Stednick (1999), in a reanalysis of the same data found moving the streamgage location on Brownie Creek in 1960 was r.nore likely the cause of the abrupt change in measured flow (figure 10). The abrupt flow change Burton (1997) detected did occur, but it occurred entirely in 1960 as an artifact of relocating the streamgage and not a reflec:tion of the timber harvest operations that occurred over the next 12-year period (1960-1972). If harvesting were the cause of departure, the change would have been subtle and accum.ulative over the 12-year period; opposite of the trend in figure 9. Figure 10 presents a compariscin of the cumulative flow from Brownie Creek plotted over that from Nort:h Fork Dry Creek. In 1960, an abrupt change in the relationship between the; two streams occurred as an artifact of streamgage relocation. Because the line is quite linear from 1960 ta 1980, one cannot conclude that fiirther increases in flow occurred as a result of the continuous harvesting activitic;s that occurred from 1961-1972. If timber harvest caused the change, one; would expect to see a gradual arc evident in the double-mass plot (figure 10). Leaf (1999) used a combination of long-term USGS stream flow data and NRCS snow-course data to document what he also considered to be the long-term effect of vegetat:ive re-growth or in-growth, causing diminished stream flow in the North F'latt:e River Basin. Leaf (1999) utilized records from the North Platte River- at :Saratoga, Wyoming, the North Platte R.iver at Northgate, Colorado, and the Laramie River at Woods Landing, Wyuming. In his Table 3, Leaf (1999) suinmarized analysis of the three gauge records and documents decreases iin flow at all three stream gauging stations. His inference is that increased forest density caused a measurable decrease in stream flow. There is no que;stion that increases in forest density cause a ?. 19