Estimating Additional Water Yield from Changes in Management

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Fool Creek, the small (7114 acre) experimental watershed on the Fraser Experimental Forest, descr.ibed earlier, represents one of the most definitive data sets world-wide; docu.me:nting both the initial effect of timber harvest on increasing water yield, and the more subtle reduction in the initial increase in water yield tha.t orcurs as the vegetation recovers or re-•grows over time. Figure 9 represents a double-mass plot (Anderson 1955) of cumulative flow from Fool Creek (the treated watershed) plotted over the cumulative flow from Easit St. Louis Creek (the cantrol watershed). The data for the period 1943-1955 (Point A to B on figure 9) represents the pre- harvest, or calibration, period and as can be noted, the relationship is linear (follows a very straight lirie), and a regression line has been fitted to the calibration data and extended to point C(equality, or a 1:1 relationship, is not required). In 1954-1956, 40 percent of the watershed area was harvested on Fool Creek and resulted iri an average increase in flow of 40 percent. The increase in flow from Fool Creek began immediately. This abrupt change in the relationship between Fool Creek and East St. Louis Creek, as a result of the increase in flow from Fool Creek, can be evidenced by the change in the traj ectory of the double-mass plot at point B. Flow increased on Fool Creek (Y-axis) relative to that for East St. Louis Creek (X-axis) causing the relationship to deflect upward. Since 1956, the clear cuts on the Fool Creek watershed have gradually recovered naturally. By 1984, approximately one-third of the original vegetation biomass had returned. By 1995, approximately one-half or more of the original increase in flow had been lost as a result of regrowth. Regrowth can be documented by the gradual, almost imperceptible, arc present in the double-mass plot between points B and C(figure 9) as the flow returns to the relationship that existed prior to harvest. Eventually, we would expect the double-mass plot to parallel the regression (solid) line fitted to the pre-harvest data. If the recovery line "over compensates", it would imply the younger, more vigorous stand is using more water than the original stand. The Fool Creek watershed f;xperiment sets the standard in the snow zone for hydrologic comparisons that document the effect of forest disturbance on stream flow response both. in terms of initial response and the recovery. Data from highly controlled e:Kperimental watersheds, such as Fool Creek, are usually more definitivf; in demonstrating response than are landscape scale watersheds, normally rnonitored with less rigor and for largely forecasting purposes. i.... c.. ? 17