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'76 iita VEtAtAis
Proceedings of the
CALIFORNIA WATERSHED MANAGEMENT CONFERENCENovember 18-20, 1986, West Sacramento, California
Technical Coordinators
Robert Z. CallahamWildland Resources CenterUniversity of California
Johannes J. DeVriesWater Resources CenterUniversity of California
WILDLAND RESOURCES CENTERDivision of Agriculture and Natural ResourcesUniversity or California145 Mulford Hall, Berkeley, California 94720
Report No. 11
February, 1987
Myths and Misconcvptions aboutFou'es Hydrologic Systems andCumulative Effects1
R. Dennis Harr2
Among the various documents that have affected
the management of Federal forest lands, perhapsthe two most important from the standpoint offorest plannieg are the National Forest Management
Aot of 1975 OFMA) (USDA Forest Service 1979) andthe National ftvironmental Policy Act of 1970(NEPA) (Counoil on Environmental Quality 1978)3NFMA deteila bow USDA Forest Servioe is to plan
its foraat management activities, while NEPAregmiree environmental assessments orenvironmental impact statements for planned
activities. The current concern over cumulativeeffeota of logging activities can be traceddirectly to the regulations for implementing NEPA.
Watershed apecialiats on interdisciplinary
planning teams have been directed to formulate
guidelioes, determine thresholds, and constructmethodologies that can be used to addressouTelative effaute of proposed activities on soiland wtor resources. Speoialiats from USDA ForestSovqioe and U3DI Bureau of Land Management in
western Oregon, western Washington, and northern
California have been contacting me, and probablyother reaearob hydrologists, in search of theelusive thresholds. In my efforts to provideeasistanoe, I have become aware of a number of
aythe and misconceptions that are circulatingfreely among watershed spec/CAW, and others
oonoerned with forest management as they look for
siapl., prediotive tools. The object of thispaper is to discuss several of the myths andaisoonoeptiona as I see them.
1 Preseeted at the California WatershedManapment Conference, November 18-20, 1986, WestSacrmento, California.
2Reeearoh Hydrologist, Pacific NorthwestResearch Station, Forest Service, U.S, Department
of Agrioulture, Corvallis, Oregon.
Abdtraot: Review of foredt planning documents andcontact with watershed specialists and otherforest land mAnagera in the Paoifio Northwest hasrevealed several myths or miaconceptiona aboutforest watershed management. These are partly theresult of pressures oreated by the level ofplanning required by legislation and by the questfor simple procedures to prediot cumulativeeffects of mannaement activities on soil and waterreaourooa. Myths and misconceptions discussedinclude these: (1) simplicity can be willed onthe forest hydrologic system,'(2) soil compactionof 12 percent of total watershed area constitutesa threshold for detrimental changes in etreamflow,
desynohronizaticn of flows by logging-induceddiversity of anowmelt oonditiona will always bebeneficial to soil and water resources, and
wet-mantle runoff is not affected by
saiusuttlagi_ I will not cite apes:Arlo planning documents or
conversations to illustrate where the myths and
misconceptions have surfaced. Such instances areirrelevant and would only embarrass well-meaning
individuals, many of whom have been givenimpossible tanks. I can assure you that these
myths and misoonceptiona do exist.
SIMPLICITY OF THE HYDROLOGIC SYSTEM
Perhaps the most basic of the erroneousbeliefs is the idea that Jasualaity_samizaalllalion the forealhyludetglq1/1/1m. This beliefencourages the implementation of aimpliatioguidelines, the adoption of arbitrary thresholds
of concern, and the search for all-encompassingmethodologies to predict consequences of forestactivities on water resources. These actionsoccur sometimes with the blessings of hydrologists
or soil scientists but other times over their
objeotions. The belief in simplicity has beennurtured by the rapid increase in the use of
computer simulation models in forest planning andthe desire to accept the output from such models.Another reason for pursuit of simplicity is thecurrent emphasis on planning called for by NFMA;
allot) planning is often conduoted under strict timeand budgetary constraints.
I must point out that, on the average, thesimplistic methodologies may have resulted in
fairly prudent forest management. But rather than
being viewed as merely a first attempt at solving
a problem, they often seem to inhibit furtherinvestigation and development. Also, they tend to
lead forest managers and some specialists to
believe that hydrologic systems really do function
in the manner desoribed by the simplistic
methodologies.
Forest hydrologic systems are more complexthan one would believe after reading some of the
methodologies and prooeduree that have beenproposed to predict cumulative effects of loggingon water resouroe3. For example, many of theseprocedures state that a threshold of harvestactivity or intensity will be determined, without
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specifying how it will Lu determined or whether itreally exists or can bu measured. Similarly,implemsnting a methodology for estimatingcumulative effects of harvest operations on waterresources does not mean that such cumulative
effects either exist or can be measured.
Uatershed management research has discoveredgeneral hydrologic principles and basicSimilarities among various hydrologic systems, butit ha, also discovered differences among these
same systems. The same research results that seem
to form the basis for fairly simple views ofhydrolosic systems also indicate how local site
eheraoteristics can interact with basic processes
to dete ,mine a particular watershed's signature;that is, how it expresses itself in terms ofoutflows of water, sediment, and nutrients or in
terms of landslides, channel morphology, and
Oharaeteristics of the riparian zone. Thesedifferent signa t ures can give insight into how agiven giat of forest management activities mightaffeet soil and water resources in a givenwatershed or along a particularly critical reach
of stream. But in our desire to simplify, to
oreate a methodology that will predictconsequences of harvest activities everywhere or
in the average situation, we usually expendconsiderable energy creating a methodology thatpredicts reasonably accurately virtually nowhere.We may implement procedures without providing fortesting or monitoring the results to see whetherthe procedures are, in faot, working. In theprocess, we may even develop a false sense of
►eourity that our methodology can really protectsoil and water resources.
THE TWELVE PERCENT COMPACTION THRESHOLD
Numerous plans, guidelines, and environmentalimpact statements have related the predictedamount of soil compaction to a defined threshold
of compaction totalling 12 percent of watershedarea. Procedures have been developed to summarizecompaction from all activities for the entirewatershed, including the proposed activity. These
procedures commonly contain a recovery function so
that changes in soil compaction over time can beoonsidared more realistically. If the proposedactivity does not raise the total area ofwatershed compaction above 12 percent, the
proposed aotivity is deemed acceptable. The myth
or tieconoeption here ie that 22/1Akaucalangovering_12 peroent 2L watershe d tl_iIhrestold_fsa_491claatal lhanges in §treamflow.
What is this 12 percent figure and how did itachieve its mythical threshold status? Some ofthe cumulative compaction methodologies citeeaveral of my published papers in referring to thescientific, basis for the 12 percent figure, whileothers refer to undocumented studies. I canoritioally examine the 12 percent figure2ttributed to my papers, but I can't say anythingtbokit the undocumented studies.
In 1979 I prepared a handout for the HarvuaLSuhuduling Workshop held in Portland, Oregon. Iused results of three Oregon studies: the Alseawatershed study in the Coast Range (Harr and
others 1975), the Coyote Creek study in the SouthUmpqua drainage of southwestern Oregon (Harr and
others 1979), and the Fox Creek study in the BullRun watershed near Mount Hoacr(Harr 1980) to
illustrate the magnitude of changes in size of
peak flows that are possible after certain loggingactivities. Earlier, Rothaoher (1973) had shownthat 31Z83 of peek flows were mostly unohangedafter clearoutting without roads or ground-basedyarding. Size of peak flows may be inoreased when
logging causes soil disturbance.
I plotted the increase in size of peak flow
over percent of watershed compacted for eightwatersheds, seven of which I used to develop a
relationship between flow increase and amount of
compaction (fig. 1). Amount of compaction rangedfrom less than 5 percent to nearly 15 percent oftotal watershed area, and corresponding flow
increases ranged from less than 10 percent to
nearly 50 percent. In some watersheds, compaction
was determined from postlogging surveys, but inothers, compaction was taken as the area in roads
(including cut and fill surfaces), landings, and
skid trails. To determine flow increases, Icompared prelogging and postlogging regressions ata Clow of 10
29 litars/(seo)(ha) or 100
ft'/(seo)(mi ) at the unlogged watershed.This flow rate was selected arbitrarily, but with
a return period of 8 to 15 years among the study
60
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5 01:(UJ
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40
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0 6 10 15 20 25
PERCENT OF WATERSHED COMPACTEDFigure 1--Relation between amount of soilcompaction associated with logging and increase in
size of peak flow at flow of 10.9 liters(sec)(hd)at unlogged watershed) for seven small watershedsin western Oregon.
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arose, it represents a runoff event capable of
moving significant amounts of bed materials. Boththe Coyote Creek and Alsea watershed studies werehampered by not having relatively large peak flowsin both the prelogging and postlogging periods.
There are two major problems with viewing the12 percent figure as a threshold. First, therelationship shown in figure 1 does not indicate athreshold. Instead, a curvilinear relation
between emount of compaotion and increased flow is
shown. A second and related problem is that the
12 percent figure is arbitrary. Flow changes at
10336? amounts of compaction may also causeadverse impacts. According to the simple
relationship shown in figure 1, 12 percent
compaction corresponds to a 32 percent iocreaee in
size of peak flow at the flow of10.9 liters/(sec)(ha) described above. Are we
ready to believe streams can accommodate a
32 percent increase in size of an 8- to 15-yearevent without adverse effeote on the channel?
Undoubtedly many atreame can, but what about those
streams that would be adversely affected by flows
that were only 10 to 25 percent higher after
logging, flows that correspond to only5 to 10 percent compaction? Without reference tothe stream channels in question, we cannotarbitrarily say nothing will happen until the
mythical 12 percent figure is surpassed.
If a threshold is to be used, it must be basedon the physical oharaoteristios of the stream inquestion. Furthermore, the allowable percentageof compaction should be the end product of anymethodology to assess cumulative effects of
harvest activities on atreamflow. A methodologygore defensible than one based on an arbitraryoompaotion threshold should start with the slope,oritloal particle size, and channel morphology,and based on established hydraulic principles,should determine what percentage of a watershedcan be compacted without increasing the erosivepower of the stream in the reach of interest.This methodology would still require a relationbetween logging and flow increase but would noterbitvarily fix amount of compaction it the earns
level for all streams. Some extremely stablestream systems can accommodate much higher flownwithout any degradation of the stream channel, andto restrict harvest operations in auoh watershedsto thg same degree as in watersheds where somechannel reaches are unstable makes little sense.Another paper at this conference develops such amethodology further (Grant 1987).
DESTNCHHONIZINO FLOWS
Another strange belief is that the
Aeavnotarmnizetien mf !Iowa. oeueed 11.1QgiallkaillILCLAUYIL111121.__SnMelt MiitioneoMilialtallnerio i a l to 0011 1114_11111....011.4EAtl.Numerous research results over the past 30 yearsilluetrete how logging can change both snowaccumulation and the energy balance of meltingsnow where net shortwave radiation is the majorsource of energy for melt (e.g., Halverson and
Smith 1974; Troendle 1985). Recently, researchhas shown diffarencee in snow accumulation andsubsequent rate of melt during rainfall whenconvective transfer of sensible and latent heatsare the major source of energy for melt. (Harr1981; Barris 1984).
Although timber removal by logging maydesynohronize flows and cause lower peak flows inthe parent watershed, it may also synchronizepreviously uneynchronized flows. Furthermore,
whether the resultant desynchronizing or
synchronizing is beneficial or detrimental depends
on how flows from various source areas originally
combined to produce streamflow in the parentwatershed. (Here, flow changes are considered
beneficial if the resultant peak flow is lowered
or its duration is decreased and detrimental ifthe resultant peak flow is raised or its durationis increased.)
In figure 2A, streamflowe 1 and 2 combine to
become streamflow 3 at the mouth of the parentwatershed. An analysis of channel conditions and
bed materialo at the confluence indicates a
critical flow at which bed materials start tomove. Figure 2B shows what could happen if
outflow from subwatershed 1 were to 000ur earlier
as a result of logging-induced changes in that
watershed's hydrologic system. Peak streamflow
below the confluence (hydrograph 3) would bereduced 8 percent, and flow would no longer reachthe critical level. According to the definition
given above, such a desynohronization of flows 1
and 2 would be beneficial to stream channel 3below the confluence. But if logging speeded up
runoff from subwatershed 2 rather thanaubwatsrshed 1, peak streamflow below theconfluence would be increased 14 percent, andduration of flow above the critical flow ratewould be increased 35 percent (fig. 2C). Thesechanges would be detrimental a000rding to thedefinition given earlier.
Both scenarios are plausible, but what I'vefound puzzling is why, without knowing how flowscombine under undisturbed conditions, some peoplebelieve that logging will desynchronize flows andsuch desynchronizing will be beneficial to streamohannela. These two simplified examples wouldseem to illustrate clearly that logging may eithersynchronize or desynchronize flows and that sucheffects may be beneficial or detrimental tostreaaa. In other oases, ohangea in volume andtiming could offeat one another, and the resultantflow from the parent watershed could beunchanged. In large watersheds in western Oregon,analyses of flow changes that coincided withabrupt changes in rates of logging suggest thatflows can be increased 22 to 56 percent in largeparent watersheds (Christner and Harr 1982). Itshould be clear that whatever changes do 000ur asa result of logging cannot automatically be termedbeneficial to the stream system any more than theycan he automatically considered detrimental.Moreover, because stable stream systems cansometimes a000mmodate increased peak flows without
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(.1) SUE/WATERSHEDS
ao PARENT WATERSHED
'
FLOWc—
CRITICALFLOW
aJYr
4
4
CRITICALFLOW
2 4 6 8 10 0 4 6HOURS
100 2 4 6 a 10
Figure 2--Effects of flow changes due to logging.
A, Hypothetical flow at parent watershed 3,comprised of flows from aubwatersheds 1 and 2. B,Hypothetical flow at parent watershed 3 if flow instOwato p shed 1 occurs sooner following timberharvest: Peak flow is watershed 3 is reduced8 psrcenZ and does not reach critical level.Dashed lines are the original hydrographs shown in
A. C, Hypothetical flow at parent watershed 3 ifflow in subwatershed 2 occurs sooner following
timber harvest. Peak flow in watershed 3 is14 percent higher, and duration of flow above the
critical level is 35 percent longer than beforetimbae harvest in subwatershed 2. Dashed linesare the original hydrographs shown in A.
channel changes, even higher flows considered
detrimental in our simplistic analysis here willnot always ba detrimental in the real world.
WET-MANTLE RUNOFF AFTER CLEARCUTTINO
In the 1970's, several published papersdescribed the changes in size of peak flows thatcould follow harvest activities. It has become
common knowledge among hydrologists that early
autumil peak flows will be higher after logging.
WettG. soils that result from drastically reduced
evapotranspiration allow watersheds to be more
responsive to early tall rains. Because less
rainfall is required to recharge soil wateretortge, more can be translated into atreamflow,and this results in wore storm runoff and higher
peak flows. But tall peaks are characteristicallysmall nompar&d to winter peaks flows thattransport most bed materials and reshapechannels. Thus, whether or not fall peak flowsare higher after logging is considered to be oflittle geomorphic significance.
A study by Rothaoher (1973) showed that winterflows after soil water recharge were relativelyunaffected in a clearout watershed without roads,and a goneral belief originated that wet-mantle runoff_ll_not arfegled by clearcul/Ing. Resultsof the Alsea (Harr and others 1975) and CoyoteCreek (Harr and others 1979) studies tended to
support this belief; higher winter peak
atreamflowa in those studies appeared to berelated instead to amount of watershed
compaction.
The belief that wet-mantle runoff is notaffected by clearoutting is not entirelyerroneous; it may be true in watersheds at lowerelevations where snow rarely occurs. Rothaoher
(1973) seemed to know he had looked at only partof the puzzle when he concluded "...harvesting of
timber will result in appreciably increased peak
runoff only under unusual circumstances..." suchas "...a rain-on-snow event in which large
quantities of precipitation coincide with melt ofa larger accumulation of snow in logged areas."
However, even with that statement, Rothaoher stillwas not seeing the entire picture. Not only didhe consider rain-on-snow unusual (which it isn't),he also didn't recognize what removing forestvegetation might do to rate of snowmult duringrainfall.
Recent rain-on-snow research has shown thatchanges in both snow accumulation in the transient
snow zone and rate of anowmelt during rainfall cancombine to cause more water to enter soil inlogged areas than in forests. Often, snow
intercepted by the forest canopy melts in the
canopy and reaches the forest floor as meltwater
or as clumps of wet snow. Where trees have been
removed, snow accumulates on the ground where it
melts much more slowly and i9 more likely to beavailable to melt later during rainfall. Becausethe convective transfer of latent and sensibleheats is commonly greater in logged areas wherewindspeeds are higher, more energy is available tomelt snow in many logged areas. During oneaccumulation-melt sequence in a plot study in theOregon Cascades (berris 1984), snow waterequivalent in a clearout was twice that in the
forest. And during rainfall, energy to melt snowduring rainfall in the clearout was 40 percentgreater than that in the forest.
SUMMARY
The myths or misconceptions about foresthydrologic systeus duaoribed abuvu are only four
of thus() I have encounterod while working withforest hydrologists and other forest land managersover the part feu years. As conflicts among
for g et resource uses and among forest interestgroups intensify, more procedures andmethodologies designed to assist the planning
peooaaa or to regulate harvesting activities will
probably be technically scrutinized more closely.tichnteal lound.less of the pro . oeduree andletholologiea will become more critical and will
depend in part on how free they are from
misconceptions and myth. I hope this discussion
h►a helped dispel some misunderstandings about
forest hydrologic systems and the cumulativeeffeots of harvest activities.
REFERENCES
Barris, Steven N. Comparative snow accumulation
and melt during rainfall in forest and clearcutplots in western Oregon. Corvallis, OR: OregonState Univ.; 1984. 152 p. M.S. Thesis.
Chrietner, .7,; Harr, R. D. Peak streamflows fromthe transient snow zone, western Cascades,
Oregon. In: Proceedings, 50th annual westernsnow conference; 1982 April 19-23; Reno, NV;Fort Collins, CO: Colorado State Univ.; 1982;27-38,
Council on Environmental Quality. Regulations forimplementing the procedural provisions of the
National Environmental Policy Act. GovernmentPrinting Office No. 43 FR 55978-56007.Washington, D.C.: Council on EnvironmentalQuality, Executive Office of the President.
1978 . 44 p.
Grant, Gordon. A rational approach to assessingcumulative watershed effects on stream
channels. 1987.
Halvorsen, Howard G.; Smith, James L. Controlling
solar light and heat in a forest by managingshadow sources. Res. Paper PSW-102. Berkeley,CA: U.S. Department of Agriculture, ForestService, Pacific South:iest Forest and Range
Exywiment Station; 1974. 14 p.
Harr, R. Dennis. Streamflow after patch loggingin small drainages within the Bull Run MunicipalWatershed, Oregon. Ras. Paper PNW-268.Portland, OR: U.S. Department of Agriculture,Forest Service, Pacific Northwest Forest andRange Experiment Station; 1980. 16 p.
Harr, R. Dennis. Some characteristics and
consequences of snowmelt during rainfall in
western Oregon. J. Hydrology 53:27?-304; 1981.
Harr, R. Dennis; Harper, Warren C.; Krygier,
James T.; Hsieh, Frederic S. Changes in storm
hydrographs after road building andclear-cutting in the Oregon Coast Range. Water
Resour. Res. 11(3):436• 444; 1975.
Harr, R. Dennis; Fredriksen, Richard L.;Rothacher, Jaok. Changes in etreamflow following
timber harvest in southwestern Oregon. Res.Paper PNW-249. Portland, OR: U.S. Department ofAgriculture, Forest Service, Pacific NorthwestForest and Range Experiment Station; 1979. 22 p.
Rothacher, Jack. Does harvest in west slopeDouglas-fir increase peak flow in small foreststreams? R03. Paper PNW-163. Portland, OR: U.S.
Department of Agriculture, Forest Service,Pacific Northwest Forest and Range ResearchStation; 1973. 13 p.
Troendle, C. A.; King, R. M. The effect of timberharvest on the Fool Creek watershed, 30 yearslater. Water Resour. Res. 21(12):1915-1922;1985.
U.S. Department of Agriculture, Forest Service.Report of the Forest Service--National ForestSystem Land and Resource Management Planning.Federal Register 44(181):53928-53999.Washington, D.C.: U.S. Department of
Agriculture, Forest Service; 1979.
U.S. Department of Agriculture, Forest Service.
National Environmental Policy Aot; RevisedImplementing Procedures. Federal Register
50(121):26078-26104. Washington, D.C.: U.S.Department of Agriculture, Forest Service; 1985.
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