Water Stewardship Information Sources
| ID | 1977 |
|---|---|
| Citation | Willington, RP, Jamieson, DS, and MD Godfrey. 1973. Evaluation of Watershed Deforestation and Harvesting Practices in the Okanagan Basin. Faculty of Forestry, University of British Columbia, Vancouver, B.C. |
| Organization | UBC |
| URL | http://www.paleolab.ca/OKBasinStudy/3%20%281%29%20I%20Water%20Quantity%20in%20the%20Okanagan%20Basin/25%20WQNT1_AppE.pdf |
| Abstract/Description or Keywords | Forest harvesting in the Okanagan Basin can have a wide variety of effects on that area's water quality, quantity and regime. Many of the effects can be related to the intensity of forest removal or, in other words rotation length. It must also be stressed that forestry has earned a poor reputation in the Basin through the proliferation of certain mythological aspects of its effect on streamflow such as: logging dries up the streams, logging causes floods. Albeit logging may have adverse effects on terrestrial waters, but for the most part they are usually very localized and on a Basin basis are not highly significant. This report concerns itself mainly with the effects of forest harvesting on water quality - a characteristic of major importance in the arid Okanagan Basin. In this respect, the Basin can be grossly stratified into three water yield increase zones: - South of Penticton - North of Penticton below 4000' elevation - North of Penticton above 4000' elevation Within these areas, an appraisal of water yield increases from forest harvesting is possible. South of Penticton. In this zone, any water yield increases accruing from high elevation forest harvesting will not be reflected in increased streamflow. This is due to in situ redistribution of water from surplus sites to deficit sites. Thus, any forest harvesting in this zone, as in all other zones, should be concerned with minimizing water quality deterioration - especially sediment discharge from haul and skid roads and thermal pollution by stream exposure. Any increase in harvesting intensity would be reflected in localized site improvement by increasing available soil water. North of Penticton below 4000' elevation. This zone is typified as the Ponderosa pine-parkland community and is a water deficity hydrologic system. As such, any forest harvesting would not reflect in any water yield increases to streamflow. Localized harvesting is probably best directed towards improving the carrying capacity of incorporated range lands. However, in this region soils are particularly sensitive to the disturbance effects of harvesting activities and extreme caution should be exercised in the location and construction of roads to ensure that adjacent streams do not receive high discharges of sediment. This area, because of its generally close proximity to Okanagan Lake, is particularly sensitive to stream temperature increases (approaching lethal limits for fishery) following their exposure by forest removal. In this respect, buffer strips should become a necessary component of forested-range and forest land management. North of Penticton above 4000' elevation. It is in this zone that the greatest potential for water yield increases as a consequence of forest harvesting exists. It is also the region of most intensive forest harvesting, both presently and in the predictable future. Within this zone, snowpack management considerations are an important aspect of land use hydrology. Water yield increases accruing from forest harvesting in the Okanagan Basin were estimated using modelling techniques and extrapolations of research findings from comparable regions. This was dictated by a shortage of data on the hydro-meteorology of the Basin. Interpretation of the data must be carried out very cautiously to avoid proliferation or creation of any more myths. It has been estimated by the B.C. Forest Service that there exists approximately 1.2 million acres of merchantable forest land in the Okanagan Basin in Canada. Of this only about 300,000 acres are in the zone north of Penticton above 4000 feet. Assuming a constant volume of timber per acre, the existing sustained yield management of these lands on a 120 year rotation (i.e., 1/120th of the total harvested per year) yields an annual cut of 10,000 acres for the Basin. This compares with an actual annual harvest of 10,145 acres in 1971. This harvesting rate has been followed since 1963, before which harvesting rates were erratic and less than 25% of sustained yield levels. For clarity, a 40 year rotation (i.e., 1/40th of the total harvested per year) would result in an annual harvesting of approximately 30,000 acres. Water yield increases accruing from forest harvesting in the approximately 300,000 merchantable forest acres of the Okanagan Basin in the zone north of Penticton above 4000 feet on a 120 year rotation have been estimated to be between 3.31% and 4.20%. These increases would only be realized within this zone (Englemann spruce and Subalpine fir forest type) and would likely be consumed in water deficit sites at lower elevations. This includes correction for sequential regrowth effects on increasing evapotranspiration: a period of approximately 40 years for most sites. Similarly, for a hypothetical 40 year rotation the increases are between 9.93% and 12.60%. Forest fire may effect an increase in water yield through reduction of evapotranspiration. The average annual acreage burned over in the Okanagan Basin is 5,377 with a range between 81 acres (1964) and 25,856 acres (1970). Yield increases, calculated on the basis of water yields from the area north of Penticton and above 4000 feet, have been estimated to be between 1.24% and 1.55% annually. However, as large as these streamflow increases may be, they are only for that area designated as merchantable forest north of Penticton above 4000 feet (approximately 1/4 the total merchantable forest and 15% the total area of the Okanagan Basin). Forest harvesting in other zones of the Basin would have no net effect on streamflow quantity. By adjusting the reported percentage water yield increases to a Basin basis for a 120 year rotation, levels by which comparisons can be made and effects evaluated are made possible. Thus, on the basis of the total Okanagan Basin annual water yield increases accruing from forest harvesting range from 0.50% to 0.64%. Similarly, the figures for the effect of fire are adjusted to be between 0.19% and 0.23%. Respective figures for the hypothetical 40 year rotation are 1.50% and 1.91% for forest harvesting and 0.19% and 0.23% for wildfire. These reported annual increases are not cumulative due to the effects of regrowth on evapotranspiration consumption. Since the values reported for the 40 year rotation are never likely to be achieved since a 40 year rotation is too close to the regrowth time of 40 years, it is only relevant to discuss the values reported for the existing and future 120 year rotation forest harvesting rates, The following limitations must be noted with respect to the reported increase in water yields following forest harvesting and/or wildfire: a. Reported increases are too small to be measured by existing streamflow measurement techniques. b. Total water yield increases in major tributaries of the Okanagan Basin will very likely go undetected due to the low percentage yield increase from sustained yield forest management. Any yield increase would only be reflected in a very small ( 50 km ) basins over which significant portions ( 75%) have been harvested or burned. c. Water yield increases accruing from forest harvesting in this area only become usable if they reach a stream channel. Increased water transmitted downslope through the soil mantle would likely be consumed in water deficit sites below 4000 feet elevation, thereby proving useless in augmenting water supplies for other purposes. It has been shown that most of the annual water yield increase occurs in the spring and fall months thereby necessitating improvement an/or extension of reservoirs to hold water over to peak demand periods in the summer season. In conclusion, it can be safely stated that although streamflow increases will accrue from forest harvesting in the Okanagan Basin, the reliability and predictability of these increases will be inhibitory to planning for water supply. The authors feel that forest harvesting, with respect to water supply, should be concentrated in the area of minimizing water quality deterioration and increasing the general environmental stability of those lands upon which forest land management is occurring. |
| Information Type | report |
| Regional Watershed | Okanagan |
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| Project status | complete |
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