Aspen Bibliography

Using the Wrnshyd Procedure to Estimate Long-Term Cumulative Effects of Aspen Clearcutting on Water Yield in Alberta

R. H. Swanson

Document Type

Conference

Source

Proceedings, Western Snow Conference, 62nd Annual Meeting

Journal/Book Title/Conference

Western Snow Conference

First Page

135

Last Page

138

Publication Date

4-1994

Abstract

Snow accumulation, evaporative loss of snow, summer evapotranspiration and ultimately water yield are affected by clearcut harvesting. As harvesting progresses through time, regrowth occurs that limits the effect that wind can have on the transport and redeposition of snow from clearcut areas. Basal area also increases, altering the effect on evapotranspiration. Regrowth can be estimated and input into the hydrology portion of the USFS Water Resources Evaluation of Non-point Silvicultural Sources (WRENSS) procedure to obtain an estimate of the cumulative effects of harvest through time.The hydrology section of the WRENSS procedure indicates that if the height of regrowth is greater than the depth of the snowpack, wind driven snow transport and loss in transport are nil. This generally occurs by the second year after clearcutting in the boreal forest of Canada where aspen regrowth occurs rapidly from root sprouts after clearcutting. Basal area increases to about 30 sq m/ha in the first 30 years after harvest and by year 20 is sufficiently high so that the estimated increases in evapotranspiration are essentially nil. The net effect in WRENSS of the combined effects of clearcutting on snow accumulation, snow transport, loss in transport and evapotranspiration is zero for any single clearcut in aspen forest of the region by 25 years after harvest.The cumulative effects of periodic harvests within a 546 km² watershed over various time intervals were estimated to determine possible harvesting regimes in order to maintain the annual water yield increase below 15% at the first significant user, a criterion established by Alberta Environment to minimize flooding impacts on downstream residents and water users. Three harvesting scenarios were simulated: 1) continuous harvest of 400,000 cu m/yr until all of the merchantable timber is removed (approximately 15 years), 2) continuous harvest at 200,000 cu m/yr for 30 years and 3) continuous harvest at 100,000 cu m/yr over a 60 year rotation. The first scenario is the only one that resulted in a maximum increase (21%) in water yield greater than 15% at the boundary of the watershed. The other two scenarios resulted in maximum water yield increases of 11% and 6% respectively.

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