Report on Water Yeild from Forest Management Letter

Home Browse Search

{,. (1) Little if any, of the water theoretical;ly produced by cutting thousands of acres of trees will eait Forest lands. Troendle 2000, pp. 11 ("As watershed size increases, the changes in flow documented to occur at the point of impact, or on-site, become less detectable.") and 44 ("It would seem unlikely that the simulated changes in flow following timer harvest ' simulations would be detectable at any streamgage on the North Platte River. Neither is it likely that the simulate3 increases in fl.ow could actually be detected as they exit NFS land, assuming a gauge were present to mor?itor them."} • The water produced would come almost exclusively during?runoff and more in wet than drv years• Id., p. 3("Changes in flow resulting from forest disturbance in the snow zone have always occurred on the rising side of the hydrograph, or early in the runoff season... . In addition, the largest increase in seasorial flows, following timber harvest, occur during the wettest years."). So, not only is there no increase in base flows, but there would be an increase in spring flooding. Neither of these is a desirable outcome. - The strategy rec?uires cutting at teast 2:0% of a watershed's trees. Troendle 2000, p. 16 ("Generally, it has been assumed that 20-25 percent of the vegetation on a fully forested small watershed[] has to be harvested in order to generate a detectable response at the streamgage."). and p.17 (describing sl.udies with 40% vegetation removal). Thinning that removes less than 25% of the tree:s in a watershed does not increase water yield, and in at least one instance, selective logging of 40% of a snowrunoff dominated watershed produced no increase in annual yield. Rhocies and Purser 1998, citing Marvin 1996. ? Finding the necessary acreage on the existinQ Forests is unlikelv. Troendle 2000, p. 15 ("[E]xtensive land areas suitable for ivater yield augmentation are not readily available on National Forest System lands in the inland west."). Most of the studies described in the report-as is true for most of the experiments described throughout the literature-are smaIl demonstration projects of several hundred acres, not landscape level experiments. Given the Forest Service's roadless initiative, finding appropriate blocks of land for 120 consecutive years would seem even less likeiy. • Because western forests are water short overall, much of the water freed by the timber harvest doesn't reach the stream; rattier remainin a? nd replacement vetz,etation use it. Troendle 2000, p. 13 ("In all cases, .... total water use by the remaining vegetation increased."). Forest cutting may potentialiv reduce baseflows and increases in yield are mostly transient. Rhodes & Purser, citing Hetherington, 1982. He reported that a decrease in annual yield and baseflow in some years following a 40% clearcut. Tnterestingly, one study that the new report does not cite for this outcome is a 1985 presentation by Troendle on Deadwood Creek, cited in Rhodes and Purser 1998. While initial statistically significant increases in annual yieid occurred follawing 30-40°ro vegetation removal, the increased flows conelated better with variations in precipitation, and within four years there was no statistically significant increase in yield across the entire wzLtershed. Increases in baseflow are particularly short- tived. Rhodes & Purser 1998, c:iting Everest & Harr 1982.