IV ENVIRONMENTAL CONSEQUENCES Water Yield From
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IV ENVIRONMENTALCONSEQUENCES Water yield from forests also can be increased through snow capture. CWatershed Management In The Rocky Mountam Subalpine Zone’ The Status of Knowledge,’ Charles Leaf, 1975, USDA Forest Service Research Paper RM-137; “Watershed Management In The Central and Southern Rocky Mountains,” Charles Leaf, 1975, RM-142, *ManagingVegetatronTo Increase Flow In The Colorado Basm,’ Alden Hrbben, 1979, RM-66; and Snow rn Natural Openings and Adjacent Ponderosa Pine Stands On The Beaver Creek Watersheds,” Ffolliot, et al., 1965; and others. Most of the precrpftatron on the Forest occurs as snowfall dunng the winter and spnng months. Snow typrcally falls to the ground or lodges in the needles of the trees Because the Forest IS and and experiences high winds during the winter and spring, most of the snow, especrally that whrch has lodged c-rtree branches or needles, sublrmates drrectly mto the atmosphere, rather than melting More than 70 percent of the blowrng snow evaporates wrthin two miles of Its ongmatron srte However, d the snow IS captured m drifts, more of it melts and passes into the ground Instead of evaporatrng (See Hrbbert, 1979; Leaf, 1975, Ffolkot, 1965). Snow can be captured by cutting rows or patches into the forest canopy. Strips or patches cut into a forest canopy work in two ways. First, they cause the wrnds to swrrl the snow from tree branches into the openings where rt pries Into dnfts. Drifted snow reduces the surface area to volume ratio so that more snow melts than evaporates from the exposed surface. Patch cuts are thought to be supenor to stnp cuts since the dnfted snow is protected more from wmd. Secondly, patch or strip cuts remove some trees, and, therefore, ekminate the transprratron loss from those trees (Hrbbert, 1979) The effects of forest management on water yield, peak flows, low flows, and trming of flow have been studied for more than 60 years. Various studres have shown that Increased water yield occurs at the time of snowmelt as a result of tree removal (“Effect of Clearcutting on Streamflow Generating Processes from a Subalpine Forest Slope,’ CA Troendle, August 1987 Proceedings of the Vancouver Symposium). Water yreld increases are a result of both the acres treated by various srlvrcultural methods and the drstnbutron of harvest areas. They are also a result of surplus water that IS dependant on climatrc condrtions (especially precrprtation), elevatron, and aspect (Hibbert, 1979, pg. 13). Water yreld mcreases were not modeled for proposed ponderosa pme harvestmg on the Forest. The ckmatrc, elevatron, and aspect zones of thus species on the Forest are not favorable for srgnrfrcantly increasing water yreld. IV-25 Aspen Harvest - An important issue in evaluatmg the effect of the alternative trmber management programs on the Forest’s water resources IS the reduced effect aspen harvests have on water yield compared to conrfer harvests. Three factors must be taken mto account. First, aspen resprouts prolrfrcally and quickly revegetates the cut sate.Therefore, the potential for tncreasrng water yield from timber harvest rn aspen stands over an 80 year rotation IS relatively low. Second, rapid revegetation of the cut site lowers the risk of water quality degradatron (Hrbbert, 1979). When entire watersheds of decadent aspen burn within a short penod of trme, peak flows are increased over the whole watershed. These hrgher peak flows may erode bare steep slopes and scour channels. Compared with sustained yield harvesting and mrtrgated water yield increases, catastrophic, fire-caused increases can be much more damagrng. The Effects Tmber Harvesr Wafer He/d lflcreases - For all of the alternative trmber management programs, timber harvest would increase the amount of water flowing from National Forest lands. The addrtronal water would result from (1) peak snowmelt runoff Increases due to greater amounts of snow melted in logged areas and (2) increased late season flows wrth extra baseflow contributions from water no longer transpirmg from sorls in those same areas, Table IV-9 displays the FORPLAN outputs affectrng water-yield-Induced sediment increases rn Rows (1) through (4) and (9) The FORPlAN calculated water yreld increases are shown m Row (5). These outputs were used to calculate cumulatrve acre-feet and percentage water yield increases for suited timber acres. IV-26 Iv ENVIRONMENTAL CONSEQUENCES TABLE IV-9 DECADE I ACTIVITIES AND OUTPUTS WITH POTENTIAL TO IMPACT WATER Alternatives CONSTRUCTION WATER YIELD INCREASE FOR DECADE ONE AFNR (6) CUMULATIVE 30,800* 25,200* 18,700* 35,i oo* 28,800* 30,i oo* TIMBER HARVEST WATER YIELD INCREASE AFNR (7) WATER YIELD 347,573* 279,672* 199,884* 810,770* 509,568* 574,937* FROM SUITED TIMBER AC AFNR (8) CUMULATIVE 8 9* 9.0* 9.4* 43* 5.7* 5.2* WATER YIELD % INCREASE ON SUITED TIMBER AC (9) STEEP SLOPE 0 0 0 820 0 80 ACRES HARVESTED OR ROADED *-includes 17,700 Acre Feet Increase far plannrng years 1983 through 1988 and FORPLAN Alternative maxrmum water yield Increase. Water yrelds shown are Increases over the baselrne total water yreld from the Forest of 2.87 millron acre feet/year. Baseline water yreld IS the runoff that would be expected If all watersheds were rn a natural pristine condrtron. IV - 27 Iv ENVIRONMENTAL CONSEQUENCES Decade one cumulatrve (*) water yield increases are shown in acre feet In Row (6) and by percentage in Row (8). Alternative IE has the most potential to create additional water (17,400 acre feet per year) while Alternatives 1D (1,000 ac ft/year) and 1C (7,500 ac ft/year) would produce the lowest Increases. The exrsting water yield for the sultable trmber acres rn the alternatives are displayed in Row (7). Three of the Alternatrves (lA, lC, and ID) could increase the cumulative water yreld for the suitable ember acres by approximately mne percent The size of thts increase is based on the low number of suitable timber acres rn these alternatrves and on the existrng 17,700 acre feet increase for harvestmg m the years 1983 through 1988. For all the alternative timber management programs, the increased water yields generally would be spread out over the entire runoff cycle. Decreases in fall water yields are not expected. These predrctrons are based on current research (“The Effect of Trmber Harvest on the Fool Creek Watershed, 30 Years Later,‘Troendle and Kmg, Dee, 1985 Water Resources Research Volume 21, pgs. 1915 - 1922). All of the alternatrves schedule aspen for harvest. The preferred Alternatrve, 1G, would reduce conifer hatvestmg and Increase aspen halvesttng The results of conifer harvesting were modeled using HYSED for the origrnal Forest Plan. The results of this modeling are drscussed in the FEIS, pages IV-66 through IV-78. Slgnrflcant Impacts are not expected to resukfrom the Increased aspen harvest if the cuts are dispersed throughout Forest watersheds. Alternatives IE, IG, and IH would not Increase the cumulative water yield Increase for the suitable timber acres more than SIX percent. A SIX percent increase IS wrthin the acceptable limits of 10 to 20 percent conversion of a drainage area to an equcvalent clearcut area that IS recommended for sensitive C classification watersheds (HYSED, October 1981, page 45). Srgmfrcant water yreld Increase impacts are not expected for Alternative I G, but the emphasis on aspen harvest in Alternatrves IE and IH could cause a concentratron of harvest In certain watersheds, Thus concentration of harvesting would Increase the risk of channel damage and degradatton in sensrtrve watersheds. These conclusions are based on aspen harvest research results and water yield research on the Fraser Experimental Forest (The Fraser Expenmental Forest, Colorado: Research Program and Published Research 1937-1985, Alexander, Troendle, Kaufmann, Sheppard, Crouch, and Watkms, Rocky Mountain Forest and Range Expenment Station Report RM-118, pgs. 17, 18, 24, and 25). One Issue that was identified in the scoprng process involved the effects increased water yields on irrigation ditches that exist on the Forest Timber harvest in Fool Creek, Fraser Experimental Forest, Increased peak discharge by an average of 23 percent when 50 percent of the timbered area of the watershed was harvested. However, Fool Creek is a 714 acre watershed and IS not representatrve of the size of the watersheds that are used as tmgatlon water sources CThe Effect of Partral and Clearcutting on Streamflow at Deadhorse Creek, Colorado,” C A. Troendle and R.M. King, 1987 ;lournai of Hydrology 90, pgs. 145 - 157) Forest watersheds that mrght supply irrigatron dkches are typrcally larger, thrrd to fifth order watersheds An averagmg effect takes place in these larger watersheds Sorls, elevation, aspect, and subdrarnage shapes integrate flows to drsperse the trming of peak flow increases rn the larger drainages IV - 28 Iv ENVIRONMENTAL CONSEQUENCES Water yreld mcreases generated by harvestmg subdrarnages are usually insrgnrfrcant when measured at the mouth of the larger drainages. Srnce most of the ditch drversrons are In the larger dramages, increased flows are not expected to have a harmful effect on rmgation dttches. Peak discharges are not likely to effect properly maintamed dttches with diversion structures that have been designed to withstand normal vanation in peak drscharges Mrtigahon, If necessary, can be achreved through tamely drtch maintenance and drversron structure design and management WATER QUALITY How Tlmber On the Natronal Forests sedrment isthe pnmary pollutant created by loggrng and Management Affects road construction activrties. Sediment may be introduced into stream channels Water Quality from soil drsturbrng actrvrtressuch as timber harvest, road burldmg, and site and slash treatments. Another concern IS late summer water temperatures, which have been measured at greater than 70-degrees Fahrenheit at some lower elevatrons on the Forest As water temperatures Increase beyond 70-degrees Fahrenheit, the cold water fisheries resource would be detrimentally affected.