Stor m r unof f ~ãî ò f or ested catchments by subsurf ace r outes

Storm runoff f rom forested catchm ents by subsur face routes

R.Z. Whipkey,Northeastern Forest Experiment Station, Forest Service, U . S. Î .À .Columbus, Ohio, U .S.À .

SUMMARY: Subsurface stormfl ow and not overland fl ow is the maj or source î Ã fl ood fl ows fromwetted, unfrozen forest soils of the A llegheny-Cumberland plateau of the eastern United States.Studies aimed at detaining fl ow from the catchment showed à need for more knowledge ofsubsurface fl ow mechanics and of the nature of the porous medium. Small plot and watershedstudies showed greatest quantities and rates of fl ow from fi ne-textured soils and lesser amountsfrom coarse-textured soils. However, hydraulic conductivity of the soil matrix was too low forthe éî ÷ to be mass interfl ow. Further tests showed turbulent storm fl ow occurred primarily fromlarge biological and structural openings in the profi le — even when the soil profi le was unsaturated.Further study of soil channel hydraulics is proposed. Preliminary plot treatments using contourtrenching and backfi ll ing to break soil channel continuity and soil layers have minimized sub-surface storm éî ÷ from the catchment region above the trench.

êéâî ì é : L 'bcoulement des averses subsuperfi cielles et non Ãåñî è1åò åï 1 4å surface est la source

principale des sommets des crues venant des sols forestiers humides non-ge1bs du plateauA llegheny-Cumberland dans les s tats-Unis de ÃÅçÅ. L 'å1è4å de la retention de Ãåñî è!åò åï 1 dans

le bassin versant à dbmontrb la ï åñåââ11å de connaitre plus profondbment le ãï åñàï |ÿ ï å 4åÃåñî è1åò åï 1 subsuperfi ciel et la nature du milieu poreux. Les btudes eff ectubes sur des parcellesexpbrimentales et 4å petits bassins versants ont dbmontrb que Ãåñî è1åò åï 1 atteint de plus grandesvaleurs et de plus fortes vitesses sur les sols à grains fi ns que sur ceux à gros grains. Toutefois laconductivitb hydraulique de la masse du sol est trop petite pour la formation de Ãåñî è!åæåésubsuperfi ciel assez volumineux. D 'autres essais ont prouvb que Ãåñî è1åò åï 1 turbulent desaverses se forme premibrement dans les larges ouvertures d 'origine biologique ou structurale dusol, meme quand le profi l du sol ï 'est pas saturn' . On considbre qu' i l est ï åñåââàï å de continuer

1åâ é èéåâ de Ãàëî ï hydraul ique des pores et des ouvertures du sol . Les parcelles expbrimentalessont entourbes de fossbs de fanon que Ãåñî è1åò åï 1 subsuperfi ciel des rbgions du bassin versantsi tuates en amont des 1î ââåâ soi t rbduit au minimum.

I N T ROD U CTI ON

The assumption that overland fl ow is the major component of the storm hydrograph-regardless of vegetative cover - is implicit in most analytical discussions of fl ood-fl owprediction and computation (À .S.Ñ.Å., 1949). Áî ò å hydrologists have attempted rationalmethods for separating the storm peak hydrograph into surface runoff and subsurfacerunoff ' (Hursh and Brater, 1941). But in the past two or three decades it has become

increasingly apparent that subsurface stormfl ow is à maj or, if not predominant, compo-nent of total stormfl ow from vegetated drainages (Burykin, 1957; Hursh, 1941, 1944 ;Tsukamato, 1961; ×àï 't Woudt, 1954; Whipkey, 1962, 1965).

Subsurface stormfl ow — also called interfl ow, wet-weather seepage, and quick-returnfl ow — has been defi ned as the water that moves laterally through the soil toward à streamchannel during and shortly after the storm. This fl ow diff ers from the usual ground-water fl ow or nonstorm basefl ow in that it travels via subsurface routes to à surfacechannel without entering the groundwater zone. Rates, quantities, and paths of waterreaching the stream depend on rainfall rate and duration as well as on hydraulic pro-pert ies of the soil .

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R .Z . I I 'hip k ey

ÒÍ Å PR OBL EM A R EA

Subsurface stormfl ow is of concern to forest hydrologists because it ãåðãåüåï ãû ï é1ããàãåñ1water that contributes to fl ood fl ow and Æèâ 1â lost to soilwater and groundwater storage.À good example of this is the regional complex of the United States known as theA llegheny-Cumberland Plateau. Eastern Kentucky, although 80 percent forested, isconsidered by the U .S. Geological Survey to have the second highest fl ood potentialin the United States. This plateau region encompasses à thin strip of western Penn-sylvania and West Virginia, most of eastern Ohio and K entucky, and extends througheast-central Tennessee to Northern A labama.

Í åãå the rock strata — sandstone and shale, along with thin layers of clay, coal , andlimestone — are sedimentary. They outcrop at or near the soil surface Soils are residual ,shallow, and low n fertil ity Surface soils are somewhat acid, and their textuxe is mediumto coarse due to clay elluviation. Subsoils are acid, fi ne textured, and moderately toslowly permeable, thus impeding downward movement of water. The subsoil sometimesoccurs only 44 cm below the soil surface, and here localized zones of water saturationcan occur. Where the subsoil layer or rockstrata intersect the soil surface in gullies orslumps, seeps and spr ings may occur . Much of the terrain of the region is rough, roll ingto steeply sloping, and in some places mountainous; and as à result both external andinternal drainage is rapid.

The entire region has à predominately hardwood for estcover-oak (Quercus sp.),yellow-poplar (Li riodendron tulipif era L .), cherry (Prunus, sp.), beech (Fagus, sp.), maple(Acer sp.), ash (Fraxinus sp.), and scattered pine (Pinus sp.). Where the forest has beenprotected from fi re and grazing for at least 10 years, à relatively stable and protectiveforest litter cover protects the soil surface. This in turn helps maintain the excellentwater infi ltration characteristics of soils in this ãåí î â.

PL OT STU D I ES

The primary purpose of our studies has been to fi nd if subsurface fl ow can be detainedand thus reduce fl ood fl ows, promote more soil moisture storage on the site, and encou-rage deep seepage to the groundwater zone. However, so little is known of the hydraulicsof subsurface stormflow in forest soil that we found it necessary to study basic movementprocesses.

Plots were set on sloping forest soil , and à sprinkler system was used to simulaterainstorms; collector troughs were buried at textural horizons in the profi le to collectoutfl ow. By using plots we were able to measure the fl ow from individual plant-soil-water complexes. Antecedent moisture conditions and hydraulic gradients within theplot were measured by multiple-unit tensiometers, and seepage fl ow was recorded at anHS-type fl ume at the downslope face of the plot (Whipkey, 1965).

These experiments were conducted on soils ranging from à fi ne silt loam to à relativelycoarse sandy loam. The surface horizons are well-permeated with roots, root channels,small animal and earthworm burrows, and structural cracks (Gaiser, 1952). Depth to àéî ÷ impeding layer ranges from 44 cm on the silt loam soils to 182 cm on the sandyloam soils. Plot slopes range from 19 to 42 percent, respectively. The most recent plotsare 13.7 meters wide by 81 meters long and represent à unit width and length of à forestedcatchmentÄ (The long plots are operated only under natural rainfall conditions becauseof the impracticabil ity of applying simulated rainfall to long sections of hil lside).

ST U D Y FI N D IN G S

Approximately 130 simulated storms were made over à 4-year period. Rainfall intensitiesranged from 12 to 76 mm per hour. Length of storm varied from 60 to 150 minutes.

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at 183 cm was set on clay occurring at that depth. At ï î time during the fi ve stormsdid any outfl ow occur into any of the collection troughs.

Tensiometers set at various locations and depths on the sandy loam plot showed thatthe potential fl ow gradient was essentially vertical , indicating little if any horizontal orlateral water movement. However, 30 hours after the last storm, slow çååðàäå appearedin the bottom trough. This continued at à low but uniform rate for over 10 days. Wefound this fl ow was coming from à shallow zone of saturation that had built up on theclay layer at the 183 cm depth. There was ï î seepage from soil cracks, old root holes,etc., above this depth.

At the other extreme, the silt loam and loam soil plots responded quickly to rainfall(fi g. 1à). In these fi ne textured soils hydraulic conductivity decreases with depth ; andtensiometer readings during the simulated storms showed slow downward water move-ment. Outfl ow started 20 to 60 minutes after the start of à storm, depending on antece-dent wetness in the plot. The plot face was exposed and examined during several simu-lated storms, and we found that outfl ow was coming from large biological and structuralopenings at all depths above the impeding layer.

And whereas the fl ow from the sandy loam began slowly and seeped uniformly for10 days, fl ow from the silt loam plots peaked shortly after rainfall subsided and stoppedwithin 24 to 32 hours after the storm. As successive runs were made on the silt loamsoils, à âÜà11î ú zone of saturation built up on the clay loam fl ow-impeding layer, andthis contributed à steady but negligible drip for several days after each storm.

In à further investigation we opened trenches along the side boundaries of the plot,leaving à 122 cm wide strip of unwetted soil between the wetted plot and the trench.Seepage was observed in this open trench in every storm, coming from root holes, soilcracks, decayed root channels, and earthworm holes. This seepage began 15 to 25 minu-tes after rain started (average storm intensity 20 mm/hour). Because this î è1éî ÷ traveledat least 122 cm through the unwetted buff er strip from wetted source to open trench,interconnection between macro-channels and cracks in this forested soil must exist.Tensiometers in the unwetted strip showed ï î appreciable change in hydraulic head atany time; this supported the observation that lateral fl ow was not moving as à massinterfl ow through the general soil matrix.

In stil l another study, pits approximately 160 cm deep were dug in silty clay loamsoils; and small plots of 2 to 4 square meters above the pits were sprinkled at à rate of25 mm/hour ' . In one run seepage was observed from à root hole 122 cm below the soil

surface only 16 minutes after water was first applied. As rainfal l continued over à 2-hourperiod, éî ä started from shallower channels. At the end of 2 hours fl ow was observedfrom all openings up to 43 cm from the soi l surface, but at ï î time did the general soilmatrix between channels become wetted.

On one of these small plots, water was ponded in à small depression. Flow not onlyoccurred from the same root channels at the exposed face, but was also observed comingfrom channels in an unwetted pit almost 9 meters obliquely downslope from the pondedwater source. This seepage occurred only 45 minutes after water was fi rst ponded ; andthis again points up the high degree of interconnection between biological and structuralchannels in undisturbed forest soils.

The foregoing results have led us to conclude that subsurface stormfl ow from thesecatchments comes primarily from interconnected cracks and channels in layered fi ne-textured soils. Where the soils are relatively coarse-textured with uniform permeability,water tends to move downward en mass through the general soil matrix to à fine-texturedlayer or bedrock without concentrating and collecting in çÜà11î ÷ cracks and channels.Depending on the depth to à fi ne-textured fl ow-impeding layer, fl ow from catchment

1. From personal unpublished communication with G. Ì . Aubertin, Soil Scientist , U .S. ForestService, Columbus, Ohio.

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areas having coarse-textured soil is ò î ãå apt to be of the slow but steady type thatsupplies âããåàò éî ÷ during non-storm periods. Where these coarser-textured soils arefound on upper slopes and r idges, it is therefore likely that the entire catchment doesnot supply runoff during the storm period. That only part of à catchment contri butesto storm runoff , particularly the middle and lower portions, has been analytically verifi edby Ò.V.À . hydrologists (Betson, 1964).

CU R R EN T A N D FU T U R E R ESEA R CH

Because these experiments have pointed up the need for detailed information as tohydraulic properties of forest soil , we are ï î è studying (with micro-sections) the eff ectsof treeroot development and decay on physical properties of forested soil at variousdepths. We are also starting à study designed to obtain plastic three-dimensional castsof macro-pores and channels for diff erent tree species and soils at varying depths. Fromthis we hope to learn something of the water-transmission capacit ies of these branching,pipe-like, soil channels.

Research is needed concerning the initial movement of infi ltrated water into largemacro-openings in the surface soil . Because general saturation on the surface soil wasneither observed nor verifi ed Úó tensiometers in our studies (fi g. Ib), fl ow into largeopenings appears to violate the " outfl ow boundary law" (Richards, 1950). There are

several possibilities that should be studied, viz., infi ltrated water movement through theextremely permeable À1 and À2 soil horizons is fast . Possibly the initial resistanceencountered in the less-permeable ÀÇ and Bl horizons causes à momentary buildup ofwater at the surface, creating positive pressures and break-through into macro-channelsFurther, there can be localized areas of saturation at places on the forest fl oor whereheavy drip occurs from the tree canopy. Still further, shingle action of l itter can causeconcentration of water, which then breaks through cracks in plastered l itter . Any or allthese factors in combination would tend to create localized points of positive hydraulicpressure in the surface horizons. Whatever the reason, concentrated f low underpositive hydraulic pressure does occur in unsaturated soils.

As à fi rst step in reducing storm runoff from forested catchments, we dug ï àããî ÷trenches approximately 30 ñò wide by 125 cm deep across the full width of our plots.The trenches were backfi lled with the heterogeneous mixture of displaced soil material .Backfilled soil was used rather than plastic or asphalt as the latter create à wall eff ect,which in turn provides à pathway for fl ow. It was felt that trenching would break thefl ow continuum provided by biological and structural channels and or any hydraulicpathways created on natural textural discontinuities and bedding planes. Subsequentspr inkler tests showed that total subsurface é î ãò éî ú from plots bisected by à trenchwas about half of that before trenching.

Tensiometers placed behind the backfi lled trench showed à buildup of saturat ion orà " mound-eff ect " . However, this temporary mound-eff ect was generally gone within

6 hours — the water presumably seeping around the ends of the trench. Further experimentswill confi ne the plot trenches to exclude lateral escape of water around the ends. Even-tually this technique will be studied for an entire small catchment and the eff ect on thestorm hydrograph of natural storms will be studied.

C O N C L U S I O N

Our studies have shown that subsurface é î ï ï éî ú — not overland fl ow — is the predo-minant contributor to high stream stages arising from forested catchments in theAllegheny-Cumberland plateau. Subsurface é î ãò éî ÷ appears to Ãî 11î ú subsurface paths

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provided by biological and structural channels. Where the soil is coarse-textured andrelatively deep, fl ow is generally à function of the matr ix hydraulic conductivity ratherthan lateral fl ow through channels.

Further research is needed on the geometry of the hydraulic passages provided byinterconnected biostructural channels and the eff ects of tree-root growth and decay onthe surrounding soil medium. Further investigation is needed on biological and mecha-nical techniques for detaining quick subsurface stormfl ow on the catchment and reducinghigh stream stages while at the same time promoting soil water recharge and deep seepage.

R E FE R E N CE S

American Society of Civil Engineers. (1949): Manual 28, Hydrology H andbook .Âèòêî ]÷, R.P. (1964): What is watershed runoff ? J. Geophys. Res., 69, ðð. 1541-1552.Âû ãêêï ÷, À . Ì . (1957): Seepage of water from soils in mountainous regions of the humid sub-

tropics. From Pochvouedeni e, 12, ðð. 90 97, English translation.Î ì ÿèê, R.N . (1952): Root channels and roots in forest soils. Soi l. Sci . Soc. Amer . Proc., 16,

ðð. 62-65.HURSH, Ñ.R. (1944): Report of subcommittee on subsurface fl ow. Trans. Amer . Geophys. Union

Trans., Part × , ðð. 745-746.HURsH, Ñ.R. and Âêëòèê, Å.F. (1941): Separating storm hydrographs from small drainage areas

into surface and subsurface fl ow. Amer . Geophys. Union Trans., Part Ø , ðð. 863-870.HURSH, Ñ.R. and H OOVER, Ì .D . (1941): Soil profi le characteristics pertinent to hydrologic

studi es in the southern Appalachians. Soi l Sci . Soc. A mer . Proc., 6, ðð. 414-422.RICHARDS, Ü.À . (1950): Laws of soil moisture. Amer . Geophys. Uni on Trans., 31, ðð. 750-756.Tsv KAMATo, Y . (1961): An experiment on subsurface fl ow. Õàð. Soc. Forestry. J., 43 (2), ðð. 61-68.VAN 'ò Woum , Â.D . (1954): On factors governing subsurface stormfl ow in volcanic ash soils,

# Z . Amer . Geophys. Union Trans., 35, ðð. 136-144.WARD, J. Ñ. (1964): Turbulent fl ow in porous media. Proceedi ngs, À . S. Ñ.Å., Í ó 5, 90, ðð. 1-12.WHIPKEY, R.Z. (1962): Subsurface stormfl ow in forest soil . Paper presented at Amer . Geophys.

Uni on, Washington, D .Ñ., A pril 24-28, 1962.WHIPKEY, R.Z. (1965): Measuring subsurface stormfl ow from simulated rainstorms — À plot

technique. 1.1.S. Forest Service, Central States Forest Exp. Sta. Res. N ote CS-29, 6 ðð., i llus.WHIPKEY, R.Z. (1965): Subsurface ç1î ï ï éî ÷ from forested slopes. I nt. Assoc. Sci . Hydrology,

X th Anne. ÂèÏ ., 2, ðð. 74-85.

asSc vSsroN

Prof. À .À . Áî êî üî ÷ (USSR):Have you noticed the forest infl uence on maximum éî ä depending on fl ood frequency ?

Reply by Mr . R.Z. %í è êèò:No, l can' t reply to this with any certainty. There have been few investigations on thisforested catchments - so 1 couldn' t answer this question as I understand it .

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