Baseflow production from partially peat-covered catchments , Jonsvatnet , Mid-Norway

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Baseflow production from partially peat-covered catchments , Jonsvatnet , Mid-Norway

MASAYAYASUHARA &GAUTE STORR0

Yasuhara, M. & Storre,G. 1992: Baseflow production frompartiallypeat-covered catchments, Jonsvatnet,Mid-Norwa y.Nor . geol.unders.Bull.422,15-25.

Due to theirwide arealextentand high waterreleasecapability,peatlandecosystemsmust have asignificant,albeithitherto poorly quantified,influence on baseflow production inpartiallypeat- covered catchments.This studyreportsthe baseflow productioncapabilityof both peatlandsand surroundingbedrock uplandsin mid-Norway,where the percent arealcoverage of peatlands for twenty eightpristinecatchments ranges from 1.3% to25.9%. Theirbaseuow production capabilities ,expressedasspecific groundwa terflux,were calculatedstatistically as averages over all the catchments studied.When the catchment swere relativelywet,the specific groundwater flux generated bypeatlands was as highas0.0073 miday,whichwas eleven times as much as that from thebedrock uplands (0.00067m/day)with a thin and discontinuous soilcover.Theflux from thepeatla ndsdecreased as the catchment s becamedrier,buteven for the dries tperiod inthis study, its value (0.00097 m/day) exceeded thespecitic groundwater flux from the bedrockuplands(0.00023m/day)byfourtimes. The basetlow productio ncapabilityofthepeatlandsprovedto be rather sensitivetochanges in wetnessof the catchments,compared with thepeat-free bedrock uplands.The two different types of bedrock studied,i.e.greenstone/tuffandslate/phyllite, hadsimilarbaseflow productio ncapabilities.

Masaya Yesunere,Hydrogeology Section,Geological Survey ofJapan, 1-1-3 Higashi, Tsuku ba.

Ibaraki305,Japan.

Gaute Storm,Geological Survey of Norway,LeivEirikssons vei 39, Postbok s 3006 -Lade,N- 7002,Tro ndheim,Norw ay

In troduction

Peatlands are very common in areas with a cold,wet climate,Peatlands are classified either as rainwater-fed mires (bogs) or groundwater-fed mires (fens and swamps).They cover about 4% of the earth's total land area, 30%of the land area of Finland,17%of Swe- den,15%ofCanada and 9%of Norway (Sho- tyk 1988).

Physical properties such as the permeability,bulk density,water content and degree of hurniflcaticn of peatshave been measured in various parts of the world (e.g. Boelter 1969, 1972, Romanov et al. 1975, Hobbs 1986, Mulqueen 1986).For most peats,the greatest contrast in permeability is found between the upper, periodically aerated,acrote/m,and the lower,less-permeable cetotelm.The permeability contrast between these is quoted as beingaround three or four orders of magnitude(Ivanov 1981) and between one and eight orders of magnitude (Ingram 1983),

Many studies have been carried out on the chemical compositionof peat waters, indicating acidic and nutrient-poor water chemistry for bogs,but near-neutral to alkaline,relative-

Iy nutrient-rich conditions for fen and swamp waters (Shotyk 1988).The highconcentrations of organiccarbon in drainagewater from peat- landshave recently drawn much attention from researchers in the context of lake and river acidification (McKnight et al. 1985, Gorham et al. 1986,Urban et al. ~989).

Attempts havealsobeen madeto determine the hydrological processes occurring in peatlands during and after rainfall. Their near- saturated soilmoisture conditions makeitlike- ly that saturat ion overland flow will be the most important contributo r to storm run-off generation(Burt et al. 1990),causing the run- off regime of peatlands to be'flashy'in character. The role of groundwater flow in run-off production has largely beenignored in previous studies.

In general, a catchment consists of water- logged systems,suchas peatlands,andterres- trial systems such as mineral soil uplands and/or bedrock uplands (with a thin soil cover).The results of theabove-mentioned studies make it plausible that peatlandsof even a small percentage area can have a decisive

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16 Masaya Yasuhara&Gaute Storm GU•BULL.422.1992

4 'm I

+ N o

I

Frg.!. The Jonsvatnet req- Ion and the studiedeaten- merits.t.ocanon of theIHD representative basin.Saqel- va.IS also shown. 0 = Sa- gelvahydrologica lS aIon.

STRINDFJORDE ^T

effect on both strea m discharge and stream water chemistry from a partially peat-covered catchment. Some recent investigations sup- port this concept from a geochemical point of view.Ande rsen(1987)confirmedthecon tri- bution ofbogs to ahigher pH instream water by comparing partially peat-covered catchments with peat-free catchments. Moreove r.

carbo n export from peatlands in Minnesota proved to be over 13 times greater than that from surrounding peat-free uplands (Urban et al. 1989).

Few studies have.how ever. investigated the relative impor tance of peatlands and peat-free uplands for run-off production from a partially peat-covered catchment. Quantificat ion of the hydrological significance of peatlands for run- off prod uction. especially during a baseflow period . has important implications for water resources development in boreal countries.

Theobjectiveof thisstudy.whichwasunderta- keninSor-Tro ndelagcounty.mid- orwa y.has

been to clarify the baseflow prod uction processes in just such a partially peat-covered catchm ent by a com parison of the baseflow production capabilities of peatlands and of surro unding peat-free uplands.

Site descriptions

Climate, vegetation and geo logy

Thestudywas carriedoutinthe vicinityofthe lake Jonsvatnet to the east of Trondheim.

Norway(Fig.1).TheJonsvatnet region .which includestheIntern ation alHydrologicalDecade (IHD) representative basin. Sagelva. repre- se nts a largel y na turat ecosystem typica l tor southern and centr al parts of orway. The annual precipitation is highly variable. aver- aging 1150 mm for the period 1972 to 1974 ( orwegian Institute of Technology & orwegian NationalCommitteefor the IHD 1975)

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NGU-BULL. 422,1992

Fig. 2. Peatland typical for the lake Jonsvatnet region.Note the surrounding uplands with an extensive area of ex- posedbedroc k.

Baseflowprod uctionfrompartiallypeat-coveredcatchments 17

and about 1700 mm for the period April1989 to March 1990(Norwegian Institute of Techno- logy, unpublished data).

The region iscompose dof lakes,peatlands and bedrock uplands. Peatlands typically lie in depressions surrounded by uplands with a thinand discontinuous cover of glacial till(Fig.

2). The peatlands'generally extend down to cA m below the surface (Reite 1983) and are overlainby Sphagnum mosses. They are classified as raised bogs in the contex t of the swamp- fen-bog transition. The.vegetation is predo minantly Vaccinium (e.g. blueberry),as- sociated withherb and Sphagnum moss under- cover.There areoccasionalstunted and distor- ted pines (Pinus sylvestris).Surrounding bedrock uplands, dominated by natural Norway spruce (Picea abies), are characterized by a very thin soil cover, up to c. 10-20 cm thick, andextensiveareasofexposed bedrock.Allu- vium is sometimes found in the close vicinity of streams, and its thickness often reaches 20 centimet res or more.

Bedrock in the area consists of metamor- phosed volcanic and sedimentary rocks of Cambrian to Silurian age (Wolff 1979). They are divided into two categories,the first consis- ting of greens to ne and tuff,and the other of slate and phyllite (Fig. 3, after Reite 1983).

Hereafter,the formeris referr edto as bedrock type I,and the latter as bedroc k type 11.Types I and 11 exhibit different fracture character is- tics. Asis obvious from Fig.4,bedroc k type I is often intensively shattered and to some extent weathered, having arelatively high se- condary porosity and permeability. In contrast,the slates and phyllites of bedrock type

11 have a well-developedcleavage.In addition, they are typically cut by larger scale fracture planes (Fig.5),anditisthese whichare likely to contribu te a degree of perme abil ity to the otherwise relatively low-permeability bedrock . Groundwater flow, therefore, is likely to be different for bedrock types Iand 11; a quasi- diffusive, unifor m flow, in some ways similar to thatin uncons olidatedmateri als,maypredo-

ST RIN D FJO RDEN

~ Bedr ock typ e I

•••••• (G ree nstone and tuff) 0 5km

~ Bedrock typ eII

~ (Sla te and phyll i te)

Fig.3.Geological map (Reite 1983,simplified after Wolf!

1979).PointsP1andP2correspondtothe locations of Figs.

4and 5,respectively.

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18 MasayaYasuhara&GauteStorre ^GU•^BULL^422.¹⁹⁹²

Fig.4.Outcrop ofgreenst one(bedroc k type I) a pom PI InFig.3.

Fog.5.Outcr op of slate (bedroc type 11)at POint P2inFig.3

minate for type I. whereas a concentrated. preferential groundwater flow through major fractures may be predominant for type 11. It shouldbe noted here that not allthebedrock in the region isintensively fractured. Massive bedrock outcrops having few. if any. significant fractur es were observed. for both bedrock types.at several locationsin the Jonsvat- net region.

Th e studied catchments

Twenty eight pristine catchment s. ranging in elevation from 140 m above sea level (a.s.l.) to about 300 m a.s.1. were selected for the study(Fig. 1,Table 1).They range from0.099 km' to 1.566km' inarea(SIin Table1).Catch- ments Nos.1 to 11and Nos.25 to28dischar- ge into the lake Jonsvatnet either directly or indirectly. while others drain into Hommelvika

bay via the River Homla or its tributaries.

Whenchoosingthe catchments,carewasta - en not to include catchmentswith large open water bodies such as lakes.

The selected catchments are drained by strea ms rated from secon d to fifth order on Strahler's (1952)syste m of classificat ion(Tab- le 1). Of the twenty eight catchments.twelve are underlain by bedrock of type Iandfifteen bythose of type 11.Catchment No.15isunder- lain by bot h type s I & 11.each accounting for appro ximate ly half the catchment' s area .

The area of peatlands within each catchment (Sp in Table 1) was measur ed from maps of scales 1:25000 and 1:15000. which enabled calculation of the peatlands' areal perc ent age(Sp1St)for eachcatchm ent. Catch- ment s 0.13and 0.14.adjacentto each other, have high Sp1St values, 0.259 and 0.236

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NGU -BULL. 422.1992 Baseflo wproduction from partially peat-covere dcatchments 19

Table 1.Sum ma ryofphysical characte risticsofthe studied catc hments andmeasured disc harges for theper iodsA.Band C.Thecatc hment numbercorresponds to that in Fig.1.For notations.see text.

No. Streamorder Bedro ck SI(km2) Sp(km2) Sp/$ t A:O (m3/da y) A:Q/5 t (m/day) B:Q (m3/da y) B:Q/$t (m/day) C:O (m3/day) C:O/$ t (m/d a y)

1 3 11 0.209 0.009 0.043 260 1.24 9 -3 59 2.82e -4 20 9.579 -5

2 3 1 0.337 0.032 0.095 360 1.070-3 410 1.228-3

3 2 1 0.18 1 0.004 0.022 250 1.38 9-3 '40 7.739-4

4 2 I 0.0 9 9 0.005 0.0 5 1 79 7.988 ·4 24 2.420 -4 '2 1.219·4

5 4 11 0.55 4 0.0 3 0 0.054 79 0 1.43 e-3 47 0 8.48 9- 4 250 4.51 9-4

6 2 11 0.16 9 0.013 0.077 '7 0 1.019-3 '20 7.109- 4 29 1.729 -4

7 3 11 0.20 2 0.0 0 5 0.025 '60 7.929-4 110 5.459-4 52 2.579-4

8

•

¹¹ ^0.67⁸ ^0.086 ^0.127 ¹⁰⁰⁰ ^{1.4 79-3} ⁷⁵⁰ ^{1.11 9}^-3 ³⁵⁰ ^{5.169 -4}

9 3 11 0.366 0.053 0.14 5 46 0 1.26e·3 340 9.299·4 10 0 2.739-4

10 4 1 0.467 0.09 9 0.2 12 590 1.269-3 230 4.938-4

11 3 11 0.303 0.05 6 0. 185 30 0 9.90e -4 '60 5.289-4

,2 3 11 0.18 9 0.034 0.180 53 0 2.80 e-3 300 1.599-3 83 4.3 98-4

'3 3 11 0.16 6 0.043 0.2 59 260 1.57 9-3 6' ^3.678^·4

,.

5 11 0.8 3 6 0.19 7 0.23 6 130 0 1.569-3 350 4.199-4

'5 5 1111 0.9 62 0.13 8 0.14 3 960 9.98e- 4 550 5.728 -4

'6 5 11 1.56 6 0.134 0.08 6 140 0 8.94e -4 370 2.3 69·4

'7 3 11 0.269 0.0'9 0.071 '20 4.46e-4 65 2.42 e-4

'8 4 11 0.601 0.0 08 0.013 47 0 7.8 2e-4

'9 4 11 0.712 0_040 0.05 6 8'0 1.14e-3 45 0 6.3 2e-4 300 4.2 1e-4

20 3 I 0_132 0.02 1 0.15 9 '40 1.06e- 3

2' 3 I 0.243 0.030 0.12 3 280 1.15 e-3

22 3 1 0.199 0.0 23 0.116 380 1.91e-3 '50 7_54 e-4 98 4.9 2e-4

23 2 1 0.108 0.00 3 0.028 56 5.1ge-4 52 4.8 1e-4 ,7 1.5 7e-4

24 3 1 0.313 0.02 3 0.07 3 350 1.12e-3

25 3 11 0.2 11 0.0 2 3 0.' 09 180 8.53e -4 '20 5.6 ge -4

26 2 1 0.100 0.003 0.03 0 110 1.100-3 42 4.20e-4 26 2.600 -4

27 3 1 0.18 5 0.019 0.103 30 0 1.62e -3 120 6.4ge-4 68 3.68e-4

28 3 1 0.'7 0 0.0 '7 0.100 20 0 1.18e-3 ,60 9.410- 4 7' 4,18a-4

Bedrocktype Fie ldcondi tion!Obesrva tio nperiod Abbreviations

B(intermedia tecond.) !29-31 Aug.&23 Sep.•1989 I: Greenstoneandtuft

11:Slate andphylUte

A(wetcond.)

C(dry cond.)

!23Fob.&1Mar..199 0

f4-6Nov.&14 Nov.• 1989

St:Totalarea Sp:Peatland area

a:Streamdischarge

respectively, indicating that. peattancs cover about a quarter of their.areas. At the other extreme, only 1.3 % of catchment NO.18·s area is covered by peatland.

As an example, the spatial distribution of peatland in catchment NO.11 is shown in Fig.

6. Two large-scale peatlands, both of which generate streamflows,can be found;one on the upstream part of the main valleyand the other on a terrace in a middle of the valley side with a very thin soil cover, if any. The thickness of the peats, as measured using a soil auger,ranged from around 20 cm at the marginto 340 cm(or more)and 270 cmrespec- tively at the centr e of the peatlands. Except at the marginalparts of the peatlands,auger- soundi ng typically revealed the existence of highly decomposed clayey peats of conside- rable thickness below low to moderately decomposed soft peats. The latter, soft peat layer, was c. 220 cm thick for the terrace peatland and c. 330 cm thick for the valley- bottom peatland). Highly decomposed peats are also reported in the lower parts of peatland profiles in other countries (e.g. Ingram

1967,1983,Romanov 1968,Chason & Siegel 1986,Ivanov1981),where theirvertical hydraulic conductivities were found to be very low (10-⁶to 10-⁷m/sec.,or evenlower). Despite the lack of hydraulic conductivity measurements in thisstudy,the above-mentioned facts make groundwater exchange between peats and surrounding bedrocks unlikely on a quantita- tively large scale in the Jonsvatnet region.

Run-off measure ments

Stream discharge in baseflow periods was measured three times between summer 1989 and winter 1990 at each of the catchment outlets .The first measurementswere underta- kenon 29-31August and 23 September,1989 (termed period B);the second on 4-6 and 14 November, 1989 (period Cland the third on 23 February and 1 March, 1990 (period A).

Dischargefrom each catchment was measured once during a given period. The discharge from catchment NO.2 was found to remain approximately constant during each of the three sampling periods,and henceitwasnot

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20 Masaya Yasuhara&Gaute Storm

x

y 350

NGU - BULL.422.1992

Fig.6.Spatial distri bution of peatlands in catchment No.11. Surrou nding uplands arecharacterized by a very thin soil cover and extensive areas of ex- posedbedrock.Indicated figures(in cm) snow thethickness of the pea s.

--..

--

' -

'"

""=

:::

-

300

-

-:::

270

x

I\ ):J Pe a t

IIIIIII

Exp osed bedrock

o

²⁰⁰ ⁴⁰⁰ ⁶⁰⁰

Distance (m)

thought necessary to corr ect stream discharges from other catch ments for their slightly different times and dates of measurement.

A snow cover 5 to 20 cm thick lay on the ground during period A. No snow melting was,however,taking place during thatperiod due to the extre mely low prevalent tempera- ture. In the centre of the valley-bottom peatland of catchment No. 11 (Fig. 6), the water table was observ ed to be very close to the peatland surface for period B, but 5-10 cm below the surface for periodC.Unfo rtunate ly.

the snowcover prohibitedwatertable observa- tion during period A. Run-off measurements were commenced only aft er ensuring the ab- sence ofsatur ation overland flowon thepeat- land surf ace.

For smallstrea ms,thestream flowwasinter- ceptedfora specific periodusingalarge plastic bag, which enabled fairly accurate calculation of the discha rge. The velocity-area method was adopted for the larger streams with more discharge.A straight reach of stream with a smooth shoreline, and no weed s or large rocks in the water. was chosen. A sectionof the stream was subdividedvertical- ly into three to six segments of 0.3 - 0.5 m width. The surface velocity was determined

three times at the midpoint of each segment by letting afloa t flow aknown distance(1- 2 m) along the stream. The depth was also measur ed at the midpoint of each segment.

The average surface velocity was converted to the average velocity for the entire profile of each segment by multiplying by a factor of0.85(recom mended by theJapaneseSoci e- ty of Civil Engineers 1985 for a stream less than 0.7 m deep). The discharg e for each segment was calculated by multiplying the aver age velocity by the depth. The process was repeated for each segment of the cross- section, and the total stream discharge was calculated as the sum of the discharges for each segment.

To confirm the accuracy ofthe velocity-a rea method, it was compa red with the «plastic bag»method on 29^thAugustfor the discharge from catchmentNo.2.Closeagreement between the velocity-area method (350 m'/day) and theplastic bagmethod (340m'/day)wasobtai- ned. Th is in dica t e s th a t the vel ocity- a r e a me t- hod canbe satisf actory, ifapplied ina sensib- le way. Measur ed stream discharges (0) for the three periods A,BandC are summarized in Table 1.

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NGU•BULL.422.1992 Baseflow production from partiallypeat-coveredcatchments 21

Apr.May Jun.Jut Aug.Sep.Oct.Nov.Dec.Jan.Feb.Mar.

Fig.7. Daily precipitation and run-off from the catchment Sagelvafor the periodApril1989 to March1990(measured at site0 inFig.1).Thefigure is based onthe unpublished data ofthe Norweg ian Institute ofTechnology.Arrow sindi- cate thethree periodswhen fieldobservationsin thisstudy wereundertaken.

1990 1989

~ ¹⁰⁰

Cl! 80

.~ ~

~ ⁶⁰⁴⁰

CIl'-'- 20 ? ,' .I,

"-

o, 0

120000

~_Cl! ¹⁰⁰⁰⁰⁰ B C A

~ ₈₀₀₀₀

=H~ -

§.

CIl 60000

~Cl! 40000 t::u .!!!

Cl 20000

Figure 7 illustrates variation in daily run-off from April 1989 to March 1990 for the catchment Sagelva (3.4 km' in area), which is the only catchment having continuous recor ds of run-off in the Jonsvatnet region . The three measurement periods in this study are also shown in the figure.Itis clear from Fig.7that all the run-off measurements were carried out during baseflow periods ,between run-off pe- aks.A flow durationcurve(Fig.8),whichshows the number of days that certainvalues of discharge are equalledor exceeded,was derived from the daily run-off data of Fig. 7 for the catchment Sagelva. The average discharges during the periods A, Band C in Fig.7corre- spond to the 175-day , 215-day and 265-day flow values, respectively. The studied catchments at the time of the run-off measurements were,therefore,in relatively wet, inter - mediate and dry soil-mo ist ure cond itions for the periods A, Band C, respectively.

Fig. 8. Flow durat ion curve for the catchment Sagelva.

Arrow s indicate the num ber of days (onthe abscissa )that aver age discharge values forthe three perio ds A.B and C(Fig.7)are equalledor exceeded.

I

~

^I

I^I

I

^.~^'C^C1)

I

E

~

Qi

E

^C1)

-

^>-

~ ~

\

^cl: ^CO ^o

\ r-

^I

Analytical proced ures

The magnitude of the baseflow production capability of both peatlands and surr ounding bedrock uplands was analysed by statist ical methods. For simp licity, it was assumed that each catchm ent consists onlyofpeatlandsand peat-freebedrock uplands (Fig.9). Itis further assumed that only two flow components, groundwater flow from the peatlands (flow a) and groundwater flow from the bedrock uplands (flow b), contribute during a baseflow period and that the flows a and b are indepen- dent. These flows discharge via stream beds to produ ce the baseflow from the catchment.

The contribut ion of alluvium and/or moraine to baseflow production was assumed to be negligible in this study,due to the limited areal extent of these lithologies.

The total areaof the catchment,St[dimens i- on L'],can be exp ressed as

120000

~_Cl! ¹⁰⁰⁰⁰⁰

~ 80000

.§.

60000 CIl~

Cl! 40000 J:::U

.!!,!

20000 Cl

oo ^{10 0} ²⁰⁰

Day s

300 365

(1)

where Spand Sb[L'] represent the areas of the peatlands and the bedrock uplands, respectively. Stream disch arge, Q [L'IT], at the outlet of a catchmentisthe sum of the groundwater disch arg e from the peatland s (flow a) and that from the bedroc k uplands (flow b).

Therefore,

(2)

whereqpand qb[(L'/T)/L'or LIT] arethe specific groundw ater f1uxes (groundwater release per unit time and area) from the peatla nds and the bedrock uplands, respectively. Rear- rangement of equations (1) and (2) yields

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22 Masaya Yasuhara&GauteStotre

Peat e Sp, qp )

Q ~

NGU·BULL.422.1992

I I I I /

/

/ / /

.".,/ Bedrock

,,;

"'"

Fig.9.Simplifiedrepresentation of baseflowgenerationprocesse sbytwo qroundw aterflow components.Le.flow a from the peatlands and flow b from the bedr ock uplands.Becauseof the presenceoflow-permeability.highly decompo sed clayeypearsin thelower parts ofpeatlandprofiles.groundwaterexchange between peatsandbedrock(flowc) is assumed to be negligible.For othernotations.see text.

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Q/S,[LIT] is the specificdischa rgeof thecatchment andSp/St is thearealperce ntage of the peatlands in the catchme nt.

If the value of Q/S,and the corresponding Sp/St value is known for many catchments, one can obtain qpand qsvalues graphica lly.

By plotting Q/St values against SplS, values (Q/S, on the ordinate ). a least-square linear regression line can be drawn. The intercept on the Q/Staxis is qb.while the slopeof the regress ion lineis(qp-qb).Therefore,thespeci- fic groundwater flux from the peatlands, qp, can also be calculated.By this procedure,the qpand qb values. representing the basefl ow production capabilities of the peatlands and the bedrock uplandsrespectively.are estima- ted as averages over all the catchments studied.

Results and discussion

The measured disch arge from a catchm ent was converted into its specific discharge (Q/

St) by dividing by the catchment's area. Cal-

culated values are'shown in Table 1. Tilt?

Q/S,value for each catchmentwaslargest for period A (wet conditions) and smallest for period C(dry conditions),withanintermediate value for period B. Q/S, is plotted against SplStfor each of the three periods in Fig.10.

All the catchments are shown in Fig. 10 for each period regardles s of the bedrock type.

For all three periods,there is a generalposi- tive correlation between the area l percentage of peatland (SplS,)and the specificdischarge from thecatchment (Q/S,).The regressionline for period A has the steepest slope and the largest Q/S,-axis interce pt, fo llowed by those for periodsBandCrespectively.The regression lines in Fig.10 are expressed as

Q/S,= 0.0080 SplSt+0.00067(r'=0.52,n= 16) :for period A

Q/S,= 0.0044 Sp/St+0.00043(r'=0.65,n=28) :for period B

O/S, = 0.0012SplS, + 0.00023(r'=0.30,n= 22) :for period C

The equations allow us to estimate qp and qb values for each period according to the procedure describe d above (Table 2). The

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NGU·BULL. 422.1992 Baseflow production from partially peat-covered catchments 23

Fig.10.Q/SI plottedagainstSp/SIforthethree periods A.

Band C,with calculated regressionlines.

Sp/St

previous section, the water table in the cen- tral parts of the peatlands in catchment No.

11 was observed to have fallen to 5-10 cm below the surface for period C. It appears that the water table,which had been located at the ground surface or in the more permeable uppermost parts of the acrotelm for periods A and S, fell below a 'critical level' in period C. As the result, a rapid decrease in the trans miss ivity seems to have dramatically lowered the specificgroundwater flux from the peatlands.

Althoug h the specificgroundwater fluxfrom the peatlands diminishedsharp ly as thecatch- ments became drier,it was still four timesas large as that from the bedrock uplands even for the driest period in this study (Table 2).It is, therefore, concluded that the peatlands behave as a 'sponge' with respect to water retention andrelease.Thisresultisindisagree- ment with the findings of Chapma n (1965), Tomlinson (1979) and Surt et al. (1990), who were in doubt about the gradual release of groundwa ter to streams during drier periods and stressedthatpeatlands were poorsupp li- ers of baseflow.

On the other hand, the bedrock uplands proved to bea relatively stable contributor

w

baseflow production compared with thepeatlands. Therefo re, the drier the catchments are,the more important the contribution of the bedrock uplands to baseflow production be- comes.This may result from a significant secon dary groundwater storage capacity in the bedroc k (Le.the numerous fracturesdissecting the massive primary rocks).

The bedro ck uplands consist of two different lithologies,Le. greenstone and tuff (type

Table 2. Summary of the specific grou ndwat er flux from ^0.003

I

bedrockuplands and peatlands for theperiodsA,BandC. ^A

I ^A^(wet)

Period BedrockUplands Peatlands Ratio (0Sr.a,OOBOSpISt..O_0006i )

Flux (m/day) Flux (m/day) -;::

0.002 A~-

'"

A(wet) 0.00067 0.0073 1:11 ~

B(intermediate) 0.00043 0.0040 1:9 ~

•

C(dry) 0.00023 0.00097 1:4

Ui<, 0.00 1 Q

correlation coefficient between Q/S. vs.Sp/St is not statist ically significant for the period C. Therefore, the obtained qp and qbvalues for period C in Table 2 must be regarded as somewhat uncertain compared with those for periods A and S.

For all of the three periods, the peatlands contributed muchmore to baseflowproduction per unit area than the bedrock uplands, with the largest specific groundwater flux, qp, during period A (wet conditions). The specific groundwater flux from the peatlands at this time , 0.0073 m/day, was'about eleven times as large as qb(0.00067 rh/day),the flux from the bedrock uplands. Under dry conditions (period

Cl ,

the specific groundwater fluxes fro m both the peatlands and the bedrock uplands were substantially reduced,to'0.00097 m/day and 0.00023-m/day respectively. Even for the dry period C,the specificgroundwa ter flux from the peatlands was still larger than that from the bedrock uplands, but only by a factor of four times.From period A to period C,the specificgroundwater flux from the bedrock uplands decreased by approximately two-thirds , whereas that from the peatlands declined by about seven-eighths. This implies that the peatlands are relatively unstable with respect to baseflowproductioncapability compared to the peat-free bedrock uplands.

This rapid decreas e inspecificgroundwater flux from the peatlands for drier conditions may be due to the low ering of the water table within the peats and the resultant sharp decrease in groundwater flow tow ards streams. According to Romanov et al. (1975), hydraulicconductivity tends to decrease rapidly in the acrotelm,from 10"-10" m/sec at 1 to 2 cm depthto 10" m/sec at the lower bounda- ry of the acrotelm. In addition, Tomlinson (1979) reported a 'critical level' for the water table at5-6cm below the bog surface,below whichlateral groundwater flow almostceased and run-off sharpl y decrease d.

IntheJonsvatnet region, aswas citedin the

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24 MasayaYasuhara&GauteStorre GU -BULL422.199 2

Sp/St

fic groundwate r flux ashigh as 0.0073 m/day, eleven times the flux from surrounding bedrock uplands (0.00067 m/day) with a thin and discontinuous soilcover.The specificground- waterflux from thepeatlands decrease dsharp - ly as the catchm ents became drier, bu even for the driest period in this study, its value appea redto exceed the groundwater flux from thebedrock uplands by four times.The baseflow production capability of the bedrock uplands proved to be rather insensitive to chan- ges in wetnessofthecatchments.The results from this studywould seem to haveimportant implications for water resources planning in partially peat-covered catchm ents. especially for drinklnqwatersupplyand forhydro electrici- ty development during baseflow periods.

Because the peatlands remain so close to saturation,only small amo unts of rainfall are necessary to raise the water table to the surface (e.g. Burt et al. 1990). It is, therefore, plausible that the water table rises rapidly to the surface during rainfall and may generate large amou nts of saturation overland flow, resulting in er drastic increase in the stream !- low production capability of the peatlands.In such a case, the peatlands may become almost exclusive contributors to streamflow generation. On the conrary. this study sug- gests that the value of specific groundwater flux from the bedrock uplands may end to- wards that from the peatlandswhen the drying of the catchments reaches a more advanced stagethanwas observedinthis study.Ouanti- fyingthecapabilitiesofboththe peatlandsand the peat-free bedro ck uplands for these two 'extreme' casesis anobjectivefor futurerese- arch in the Jonsvatnet region.

Acknowledgments

Masaya Yasuhara WIshes to than the Royal orweqian Counc il for SCient ific and Industrial Research ( T F)for gIving rum the opportunity to underta e trus study In orway.The authors gratefullyacknowledge BjornA.Follestad (Geological Survey of orway).Bernt Malme (for mer Sec- tion Leader for Hydrogeology. GU) and Pro fessor U.

Hafsten (Univ.of Trondheim) for usefulcomments on our study and Katsue Yasuhara for her help and supportIn fieldmeasurements.Specialthank s are also due toProf es- sorA.Killinqtve it (NTH) for easy access to the unpublished hydr olog ica land meteorolog icaldatacollectedat thecatch- mentSagelva.

0.3

o. 0.2

o.ooo-- - - - - . - -- - - - , , - - - ---i 0.0

Fig.11.Separate plotsof O'SIfor periodBag ainstSpfSI.

forthe bedrock types I(e)and 11(0).

Concluding rema rks

Peatlands are found to be large sources of baseflow in catchments inthe Jonsvatnetregi- onof mid-Norway.Whenthecatchment swere relativelywet,thepeatlands generat ed a speci- O/SI = 0.0041 SplSI + 0.00046 (r'=0.48, n=

12):for bedrock type I

O/SI = 0.0046 SplS + 0.00041 (r'=0.74. n=

15):for bedrock type 11

The qo values (0.00046 m/day and 0.0004 1 m/day respectively) for bedrock types I and 11 are almost equal. This indicates that both types of bedrock havenearlythe samecapabi- lity forbaseflow production ,despitetheirdiffe- rent fracturing characteristics.

In Japan,Takaha shi(1978) reported specific baseflow production capabilities ranging from 0.0007 to 0.0009 m/day for catchm ents composed of the Paleozoic (partially Mesozo- ic) slate, chert and sandst one.The baseflow production capabilitiesof the bedro ck s observed in the Jonsvatnet region are, there fore.

of the same order of magni tude as those for Japanese catchme ntsof nearly the same age.

although the primary characteristics of the bedrock are likely to be rather different.

I) and slate and phyllite (type 11). Figure 11 attempts to distinguish betwee n the specific gro undwate r flux es from the uplandsfor be~ rock types Iand 11. The regression lines obtained for the period B are

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NGU-BULL.422,1992 BasefJowproductionfrom partiallypeat-covered catchments 25

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