Estimating Additional Water Yield from Changes in Management

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watersheds were monitored from 1911 to 1919 and then one of the , watersheds was clear cut. Following harvest, stream flow was increased an average of 1 area inch for the following 7 years. The authors concluded the increased flow was largely a reflection of reduced winter interception loss and that although summer evapotraspiration (ET) by the over story was reduced it was largely offset by increased under story ET. As noted by Leaf (1999), the most classic watershed experiment, in terms of both the length of record and the duration of treatment response, has been the Fool Creek Watershed on the Fraser Experimental Forest, CO (Hoover and Leaf 1967; Troendle and King 1985). Following a 12-year calibration with the control watershed, East St. Louis Creek, approximately 40 percent of the 714 acre Fool Creek drainage was harvested in altemating clear cut and leave strips during 1954-1956. The average hydrograph before and after treatment is depicted in figure l. On average, total seasonal flow increased by 40 percent, average peak flow increased by 20 percent, and most of the detectable change in flow occurred in the month of May (Troendle and King 1985; Troendle et al. 1998). The largest peaks were not significantly increased and the largest increases in flow occur in the wettest or largest flow years (figure 2) (Troendle et al. 1998; Troendle and King 1985). In the case of Fool Creek, "bankfull discharge" increased from an average duration of 3.5 days prior to harvest to more than 7.0 days following harvest (Troendle and Olsen 1994). The most frequently occurring, or lowest, flows were not affected by rimber harvest (Troendle and Olsen 1994). The response at Fool Creek, a small experimental watershed was similar to that of Wagon Wheel Gap and depicts the nature of the change that occurs when the forest in the sub alpine environment is disturbed by harvest, fire, or insect mortality. Fool Creek was harvested over 40 years ago and although the initial response to treatment has diminished as the Forest has recovered, full recovery is not expected to occur for yet another 25 to 30 years (figure 3). Peak discharges expressed as either maximum instantaneous or maximum mean daily flow; from Fool Creek have always been observed to be snowmelt driveri. This is consistent with Jarrett's (1993) observations that floods occurring from watersheds lying above 7,500 feet in elevation in Colorado axe snowmelt driven. Response to summers rainfall events generally represents only 3 to 4 percent of the event precipitation both before and after either timber harvest (Troendle and Bevenger 1987) or fire (Troendle and Bevenger 1995). 4