19651974

A United States Contributionto the International Hydrological Decade

BIOLOGICAL EFFECTS IN

THE HYDROLOGICAL CYCLE

Proceedings ofThe Third International Seminar for Hydrology Professors

A National Science Foundation Advanced Science SeminarHeld at Purdue University, West Lafayette, Indiana, U.S.A.

July 18-30, 1971

epartment of Agricultural Engineeringgricultural Experiment Stationurdue University/est Lafayette, Indiana 47907

U.S.A.

BIOLOGICAL EFFECTS IN THE HYDROLOGICALCYCLE — TERRESTRIAL PHASE .A

edited by E. J. Monke .

SPONSORING AGENCIES

National Science FoundationUnited Nations Education, Scientific and Cultural Organization (UNESCO)U.S. National Committee for the International Hydrological DecadeUniversities Council on Water Resources (UCOWR)Department of Agricultural Engineering, Purdue UniversityWater Resources Research Center, Purdue UniversityPurdue Agricultural Experiment Station

COOPERATING AGENCIES

American Geophysical UnionAmerican Society of Civil EngineersAmerican Society of Agricultural EngineersAmerican Society of AgronomySociety of American ForestersDivision of Conferences and Continuation Services, Purdue University

SEMINAR COMMITTEES

Seminar Director:E. J. Monke, Professor of Agricultural Engineering, Purdue University

Initial Planning Committee for Seminar Series:W. C. Ackermann, Chief, Illinois State Water SurveyAllen F. Agnew, Chairman of Executive Committee, Universities

Council on Water ResourcesV.T. Chow, Professor of Hydraulic Engineering, University of

Illinois^ J. A. da Costa, Acting Director, Office of Hydrology, UNESCO• John C. Frye, Chief, Illinois State Geological SurveyL. A. Heindl, Executive Secretary, U.S. National Committee for

International Hydrological DecadeV. A. Koelzer, Chairman of Executive Committee, Hydraulics

Division, American Society of Civil EngineersMax Kohler, President, Section of Hydrology, American Geophysical

UnionDean F. Peterson, Chairman, U.S. National Committee for the

International Hydrological DecadeDale Swartzendruber, Representative, Soil Science Society of

America

UCOWR Ad Hoc Committee on ISHP:E. J. Monke, Chairman, Purdue UniversityJ. M. Bagley, Utah State UniversityV. T. Chow, University of Illinois '"•--V. M. Yevjevich, ColoTado State University

1971 Seminar Planning Committee:E. J. Monke (chairman), B. H. Atwell, J. M. Bell, W. R. Byrnes,R. F. Dale, J. W. Delleur, R. L. Giese, J. L. Hamelink, L. F.Huggins, W. L. Miller, R. A. Rao, R. W. Sexton, Dale Swartzendruber,G. H. Toebes, Dan Wiersma, all Purdue University

SUMMARY OF RAINFALL INTERCEPTION BY CERTAIN CONIFERS OF NORTH AMERICA

by

J. D. Helvey*

Patric (1) and Leyton et at. (2) stated that studies of interceptionrobably outnumber those of any other aspect of the forest water balance.>wever, hydrologists disagree on the importance of interception studiesi relation to the water balance of catchments. Proponents of the energyilance method have reasoned that interception and transpiration lossesre compensating because energy used to evaporate intercepted water is)t available for transpiration loss. Burgy and Pomeroy (3) concludedrom their study that evaporation of intercepted water from grass was}mpensated by a similar reduction in transpiration, and interception wasJt a loss additive to normal transpiration. Leyton and Carlisle (4) anddlillan and Burgy (5) presented similar arguments.

Other investigators reported that evaporation of intercepted waterrom potted tree seedlings was five to 10 times faster than transpirationrom unwetted seedlings (6, 7). The transpiring seedlings had an unre-tricted water supply, and all seedlings in each experiment were sub-ected to the same environmental conditions. Murphy (8) presented con-Lncing evidence that interception is an important loss in addition toranspiration losses. He measured the energy balance of individual treeeaves in a controlled environmental chamber and found that water lossrom\an externally wet leaf was always greater than from dry transpiringeaves. Thus, a given amount of energy was more e'fficiently used invaporating surface water than water from internal parts of the leaf.

*Evidence from certain water yield studies supports the idea that

nterception differences between species are reflected in streamflowmounts. Delfs (9) reported preliminary results of streamflow compari-ons from spruce- and beech-covered watersheds in the Hartz Mountains ofermany. He stated that the beech-covered area discharges almost 9 inchesore streamflow each year than the spruce area, and "it is establishedeyond doubt that the large interception by spruce reduces the amount ofainfall which reaches the ground and thereby the water discharge."wank and Miner (10) reported streamflow results from two watershed exper-ments in which the natural forest of mixed hardwoods was replaced withastern white pine. When the pine was 10 years old, water yield was 2nches less than the expected yield from~thg original forest on one catch-tent, and it was 4 inches less than expected on the other. The authorsound a positive correlation during winter and summer months between de-Teases in monthly streamflow from the pine-covered areas and differencesn interception loss between the forest types. There was no clear rela-:ionship for hardwoods during months when they were leafing out or shed-ling their leaves.

'Hydrologist, Forest Hydrology Laboratory, Pacific Northwest Forest andtange Experiment Station, Forest Service, U.S. Department of Agriculture,fenatchee, Washington.

103

If interception is an important process in the water balance of Iitchments, it is^an important input to water balance simulation. Indeed, .4Lmost all water balance models include a .term for interception loss. 1ifortunately, the methods of investigation and reporting interception 4jjsults have never been standardized, and published values are extremely ];Lfficult to compare. Another problem is that interception studies arerequently made in stands where species, age, density, etc. are homoge-nous. Since vegetation growing on a watershed containing more than aiw acres is seldom uniform, the potential user of interception resultsis the problem of deciding which values to use. Frequently, a new study5 initiated because previously published results were from stands whichLffered in some respect from the stands of interest.

In 1965, Helvey and Patric (11) summarized results from many inter-sption studies conducted in mature hardwood forests of eastern Unitedtates. Since throughfall and stemflow estimates were similar betweentands, each study was considered to be an independent observation withinpopulation of mature hardwood forests. This allowed equations to beambined and used to predict interception losses from measurements ofross rainfall. The summarized equations were considered applicable toractical problems encountered by water manager^ and foresters but notecessarily adequate for research needs. Recent studies (12, 13, 14, 15,6) have, in general, confirmed the applicability of the summarized re-ults. Helvey and Patric (11) suggested that summaries of interceptionata from other forest types may also reveal similarities -not evident iningle study results."

\The purpose of this paper is to combine available interception re-ult^ from coniferous stands growing in North America in order to deter-ine gross similarities and differences in interception losses betweenpecies. The summarized results are expected to be useful for generalroblems such as modeling the water balance to be expected when watershedsre supporting various species or combinations of species, but they mayot be satisfactory for certain other purposes. Therefore, results fromndividual studies are listed in addition to the synthesized values.

The reader is directed to interception summaries by Klttredge (17),blchanov (18), Helvey and Patric (11), and Zinke (19) for discussions ofhe interception process, and to Helvey and Patric (20) for design crite-ia for interception studies. Terminology by Helvey and Patric (11) wille used in this paper.

" ' , METHODS ~~~ "" ~-

This summary begins with interception data which may include measure-lents of throughfall, stemflow, and/or litter interception in addition to *;ross rainfall. Sources of data included published reports in profession-il journals, experiment station papers, college theses, unpublished office 3reports, and tabulated data. Published results are usually in the form of.inear regression equations in which throughfall, stemflow, or litter <.nterception are presented as functions of ~gross _rainfall- or_. a form in irtiich throughfall, stemflow, or litter interception are reported as per- ?:entages of gross rainfall. This review will present results as regres- j;ion equations because percentage values have limited meaning unless storm t>ize distribution is known. ,

u'.. . I

104 5

If three or more regression equations have been derived for closedstands of a given species, or group of species, a weighted average equa-tion is presented in this review. The weighting factor is the number ofthroughfall gages, stemflow trees, or plots used in the study. In com-puting the weighted equation, one assumes that sample accuracy is direct-ly related to number of observations made.

RESULTS

Throughfall and Stemflow

Four throughfall and three stemflow equations were found for redpine (Finns .resinosa Ait.). With the exception of Sopper's (21) equation,the results are similar (Table 1). Sopper's study, designed primarily todetermine hydrdlogic characteristics of the forest floor, included threepermanently located gages to sample throughfall. The gages may have beenbiased in location, and the throughfall equation may not be as accurateas the others. Results by Rogerson and Byrnes (22) receive the mostweight in the average equation because their coefficients were derivedfrom 24 throughfall gages and 18 stemflow trees. Thorud's (23) equationfor pruned conditions is not included in the average even though there isonly a minor difference in the coefficients after one-half of the livecrown was pruned.

table 1. Stand Characteristics. Study Design, and Equations for Computing Throughfall and Stenflovfrom Measurements of .Gross Rainfall (F) in Red Fine

Source -

-

V6igt ttt)\Ihorud (23)°Thorud (23)Sopper (21)Rogerson andByrnes (22)

Age

35313119

20

ThroughfallBasal area gages Storma

sq- ft-

17* ,— .—153

160

S663

24

124940

45

Stemflowtrees Throughfall Stemflow

733—18

0.88P-0.05 0.02P0.85F-0.060.86P-0.05C0.76P-0.02

0.89P-0.04

0.04P-0.010.02FC

—0.02F

Weighted average 0.87F-0.04 0.02F

a Equations supplied by letter dated 4-6-64b Equations computed from author'a datac Stand vas pruned—equation not included in average

The first study in loblolly pine (Pinusvtaeda L.) was by Hoover (25).His throughfall equation-(Table 2) is based on only two gages; but sincehe moved them after each storm, many points were sampled during the 1year of study. The smaller than average "b" coefficient in Hoover'sthroughfall equation is partly compensated by a very high stemflow value.Equations by Swank et at. (16) also indicate that considerable quantitiesof water reach the forest floor in this species by flowing down stems,and their results compare favorably with Hoover's for stands of compar-able age. The weighted average equation does^not-include_Rogerson's (26)equation for' the stand which was heavily thinned. ' '

105

Teble 2. Stand Characteristics, Study Design, and Equations for Computing Throughfall and Stemflovfrom Measurements of Gross Rainfall (P) in Loblolly Pine

StemflowThroughfall _ trees (Tr)

Source

Hoovet (25)Rogerson (26)Rogerson (26)Swank et al. (16)Swank et al. (16)Swank et al. (16)Swank et al. (16).

**"

1025255102030

Basal area

Sq. ft.

1031904065110138152

gages

2121220151515

Stores or plots (PI) Throughfall

85 .8546464646

Stettf low

10(Tr)

——5(P1)

5(P1)5 (PI)5 (PI)

0.73P-0.02 0.22P-0.020.80P-0.020.94P-0.02*0.83F-0.030.73P+0.010.76IH0.010.85P-0.00

— .

—0.09P-0.010.11P-0.030.12P-0.030.04P-0.02

Weighted average 0.80P-0.01 0.08P-0.02

a Stand was thinned—equation not included in average

Considerable throughfall data were collected in shortleaf pine(Finns echinata Mill.) at Irons Fork, Ark,, and at the Bent Creek Exper-imental Forest near Asheville, N. C., but they vfere never published.Because little information is available on stand structural characteris-tics, the results (Table 3) have little value individually, but they areuseful for this general analysis. The weighted average equation isbased on six throughfall and four stemflow equations.

table 3. Stand Characteristics, Study Design, and Equations for Confuting Throughfall and StenflovProm Measurements of Gross Rainfall (P) in Shortleaf Pine

StemflowThroughfall trees (Tr)

Source *

Boggess (27)Coweeta files (28)Irons Fork (29)Irons Fork J(29)lawson (30)"

Rusk (13)

Weighted average

Age

1330(a)30(a)

34

Basal areaS1- ft-

110— '——122

166

-_

gages

,.620712

20

__________

Storms

157233311156352

— : — ==sr

or plots (PI) Throughfall Stenflov

10 (Tr)3 (PI)——16 pines

12 hardwoodo4(P1)

0.90P-0.06 0.10P0.88P-0.050.84P-0.010.94P-0.04

0.94P-0.090.86P-0.06

0.88P-0.05

0.03P-0.01

——

0.02P0.02P

0.03P

a Mature -b Stand contains a hardwood understory

The most intensive study of interception by ponderosa pine (Finnsponderosa Laws.) was by Rowe and Hendrix (31). In 214 individual storms,they sampled throughfall with 24 gages and stemflow from 24 trees. Theirresults are presented in Table 4 along with results from three otherstudies, Alden and Curtis (32) measured throughfall with two gages oneach of nine natural stands which had basal areas ranging between 48 and278 square feet per acre. Since there was no consistent relationshipbetween throughfall and basal area, the coefficients in Table 4 repre-sent all of the data combined. Throughfall and stemflow equations fromOrr's (33) thinned stand are omitted from the weighted average equation.

106

Table 4. Stand Characteristics, Study Design, and Equation* for Computing Throughfall and Steuflovfrom Measurements of Cross Rainfall (P) in Ponderosa Fine

Source

Alden andCurtli (32)

Orr (33)Orr (33)*Rove andBcndrlx (31)

Weighted average

Age

—65-7565-75

65-75

fiaaal area

Sq. ft.

48-27819080

100

Throughfallgages Storms

1866

24

.26-3413623214

Stemflowtrees Throughfall Stemflow

271010

29

..0.91P-0.020.81P-0.050.89F-0.01

0.89P-0.08

0.89P-0.05

0.03P0.06P0.02F

0.05P-0.03

0.04P-0.01

a Stand thinned—equation not included in average

Helvey (34) presented the only equations for predicting throughfalland stemflow in eastern white pine (Pinus etrobue L.). Although through-fall differs very little between 10-, 35-,«and 0-year-old stands(Table 5), stemflow decreases with age. The upward branching habit,smooth bark, and high stem density account1 for the high stemflow valuesin the 10-year-old stand. The weighted average equations for boththroughfall and stemflow are identical to equations for the 35-year-oldstand.

* 5* Stand Characteristics, Study Design, and Equations for Computing Throughfall and Scemflovfrom Measurements of Gross Rainfall (P) in Eastern White Pine

Source Age Basal area

. Sq. ft.

Belvey (34) 10 76Belvey (34) 35 120Belvey (34) 60 153

Weighted average

Throughfallgages Storms Plots Throughfall Stemflow

20 80 5 0.85P-0.05 0.09P15 80 5 0.85P-0.04 0.06P-0.0115 80 5 0.83P-0.05 0.06P-0.01

0.85F-0.04 0.06P-0.01

Throughfall and stemflow equations for conifers with small needles(spruce-fir-hemlock) are presented in Table 6. Geographically, thestudies are widely spaced, from Connecticut (24), to Oregon (35,36), andAlaska (1). Stemflow amounted to about~7 percent of gross rainfall inVoigt's eastern hemlock (Tsuga canadensis (L.) Carr.) and Knutsen'sDouglas-fir (Pseudotsuga mens-iesii (Mirb.) Franco) stand. On the otherhand, stemflow was insignificant in Patric's (1) study area which in-cluded old-growth western hemlock (Tsuga heteraphylla (Raf.) Sarg.) andSitka spruce (Picea sitchensis (Bong.) Carr.). Rothacher (36) alsomeasured very little stemflow from his old-growth stands of Douglas-firand western hemlock.

107

Table 6. Stand Characterlatica, Study Design, and Equations for Computing Throughfall and Stemflow fromMeasurement* of Gross Halnfall (F) in the Spruce-Fir-Hemlor.k Type

ource

El-son (35)*Rothacher (36)Patric (1)Kniiuen (37) c

Volgt (24)d

Burroughs (38)

Weighted average

P C B

Douglas-fir (b)Douglas-firWestern hemlock (b)Vestern healockSltka apruce (b)Douglaa-flrWestern hemlock 30-35Eastern hemlock 35Engelnann spruce-

alpine fir 200

Sq. ft.

_288-508

77-490163

—166

Throughfall

23244085

—

4224-32

3426——

Sttufluwtreea (Tr)

_

10 (IT)5 (PI)4(lr)9(Tr)_

Throughfall

0.77P-0.020.83P-0.050.77P-0.090.83P-0.080.68P-0.040.72P-0.030.77P-0.05

' '

0.003P0.01P0.07P0.07?-0.01

0.02P

• Equation conputed froib unpublished daifib Naturec Equation computed fron theela datad Equations aupplled by letter dated 4-6'64• Equation aupplled by letter dated 11-14-67

Table 7 summarizes weighted throughfall plus stemflow equations forclosed (not thinned or pruned) stands of conifers, along with averageequations derived by Helvey and Patric (11) for mature nixed hardwoods.Helvey and Patric found that throughfall was greater in hardwoods during\winter than in summer, but studies in conifers have failed to detect a'Vonsistent difference between seasons. Coefficients for pine speciesdiffer somewhat, but there is a compensation between "a" and "b" coeffi-cients, i.e., the smaller than average "b" coefficient for loblolly pineis compensated by a smaller than average "a" coefficient. When theequations are solved for a given amount of gross rainfall, the resultsbetween species are very similar.

Table 7. Summary of Throughfall Plus Stemflow Equations and Computed Throughfall andStemflow (T+S) at 1.0 Inch of Gross Rainfall (P) by Species

Species Average equation T+S (P - 1.0 inch)

Red pineLoblolly pineShortleaf pineEastern white pinePonderoaa pine

Average (pines)Spruce-fir-hemlockMature mixed hardwoods:

Growing seasonDormant season

a Prom Helvey and Patric (11)

T+S-0.89P-0.04T+S-0.88P-0.03T+S-0.91P-O.O5.T+S-0.91P-0.05~~T+S-0.93P-0.06T+S-0.90P-0.04T+S-0.79P-0.05

T+S-0.94P-0.04T+S-0.97P-0.02

Inch

0.850.850.860.860.870.860.74

0.900.95

108

The individual storm equations in Table 7 can be used for computingannual or seasonal throughfall plus stemflow by solving the equationsafter substituting annual (or seasonal) gross rainfall and multiplyingthe constant terra by the number of storms in which gross rainfall equalsor exceeds the constant term. Canopy interception loss is the differencebetween gross rainfall and throughfall plus stemflow for the period ofinterest added to the total rainfall delivered in storms which aresmaller than the constant term in the equations.

Litter Interception

Data for moisture relationships in the forest floor of conifers arepresented in Table 8. According to these limited data, field capacityof litter (water held against drainage) averages about 215 percent byweight and annual losses range from 2 to 17 percent of gross rainfall.The data are too limited and too restricted in geographic location tojustify computing average values.

Table 8. Moisture relationships In the Forest Floor of Various Aged Stands of Some Coniferous SpeciesStand

Source Species age Maximum Field capacity Annual lossesX of gross

Years I by weight Inches precipitation220 — —225 — . —221 2.0 5— 0.16-1.1 7-27180 — —178* — —230 1.2 2230 1.8 3230 2.2 4215 3.7 6.8215 — —215 2.0 5.0

a The average of four study plots

DISCUSSION AND CONCLUSIONS

For more than 100 years, hydrologists have derived empiricalrelationships to correct rainfall measurements made in forest openingsor above vegetative canopies for canopy interception losses. Unfortu-nately, the methods of investigation and of reporting results have neverbeen standardized, and a rigorous statistical-comparison of results be-tween studies usually is not possible. Therefore, only gross similar-ities and differences in canopy interception loss between coniferousspecies were attempted in this analysis. The summarized results areexpected to have value in practical problems such as modeling the waterbalance because most watersheds are covered with a mixture of speciesand stand conditions.

As noted by many authors, starting_with_Hoppe (4,4), canopy inter-ception loss is greatest in the spruce-fir-hemlock type, intermediatein pine, and least in broad-leaved deciduous forests (Table 7).

109

Morris (39)Morris (39)Sopper (21)Alden (40)Lovdermilk (41)Bale! («2)Helvey (34)Helve? (34)Belvey (34)Rusk (13)MeA (43) .. .Swank et al. (16)

White pintRed pineRed pineFonderosa pinePine-fir-cedarHemlock-fir-cedarEastern white pineEastern white pineEastern white pineShortleaf pineLoblolly pineLoblolly pine

13-2720—30-70——103560——5-30

——352——3121

——————

Because surface area index also is greatest in the spruce-fir-hemlock type (45), intermediate in pine species, and least in deciduousforests, surface area index is an important common denominator for ex-tending interception results to ungaged areas. Previous attempts tocorrelate interception values with basal area have not been very success-ful, except where the stand was heavily thinned, e.g., Rogerson (26).

These summarized results have important implications for the water-shed manager when maximum water yield is an important consideration.Fir and hemlock species not only prevent more water from reaching theforest floor, but they may also transpire more water than pine species.For example, Lopushinsky and Klock (46) found that the transpirationrates of Douglas-fir and grand fir seedlings were less affected by mod-erate soil drying than transpiration rates of ponderosa pine and lodge-pole pine. These responses to soil drying apparently were the result of

, species differences in stomatal control because Lopushinsky (47) foundthat the stomata of Douglas-fir and grand fir were less sensitive toincreasing leaf moisture stress than those of the pines. Similar studieshave not been made to compare the water use behavior of spruce and hard-woods, but it is likely that both interception and transpiration lossesfrom Delf's (9) spruce stand exceeded losses from his hardwood-coveredwatershed. The evidence indicates that fir and hemlock are heavy waterusers and that evaporative losses from deciduous forests are smallerthan from pine or the spruce-fir-hemlock type. In fact, Swank et al. (16)have used their equations to show that the current practice of conversionof Piedmont hardwood forests of the Southeast to loblolly pine could re-duce annual streamflow by about 4 area-inches for each acre converted.

> Interception has been studied a long time. Several such studiesaife probably in progress, and others will no doubt be initiated. Thesestudies should not be discouraged if they have a sound practical objec-tive and sampling design and intensity permits a rigorous statisticalanalysis*of the data. More study is needed of the hydrologic relation-ships of the forest floor because few such studies have been made. Asnew studies are reported, the average equations presented in this reviewcan be refined and the need for additional studies determined.

REFERENCES

1. Patric, J. H. 1966. Rainfall interception by mature coniferousforests of southeast .Alaska. J. Soil and Water Conserv. 21(6):229-231.

2. Leyton, L., E. R. C. Reynolds, and F. B.^Thompson. 1967. Rainfallinterception in forest and moorland. In International Symposium onForest Hydrology. Pergamon Press, N. Y. p. 163-178.

3. Burgy, R. H., and C. R. Pomeroy. • 1958. Interception losses ingrassy vegetation. Trans. Amer. Geophys. Union 39: 1095-1100.

4. Leyton, L., and A. Carlisle. 1959. Measurement and interpretation•of interception of precipitation by forest__stands. Intv Assoc.Hydrol., Symposium Hannoversch-Muriden, Publication 48, p. 111-119.

110

5. McMillan, W. D., and R. H. Burgy. I960.. Interception loss fromgrass. J. Geophys. Res. 65: 2389-2394.

6. Sykes, M. 1960. Some observations on the effects of foliage wettingon the water relations of Scots pine. Unpublished thesis, ForestryDep., Oxford Univ., England.

7. Thorud, D. B. 1967. The effects of applied interception on transpi-ration rates of potted ponderosa pine. Water Resources Res. 3(2):443-450.

8. Murphy, C. E., Jr. 1970. Energy sources for the evaporation ofprecipitation intercepted by tree canopies. Ph.D. Diss., Duke Univ.,Durham, N. C.

9. Delfs, J. 1967. Interception and stemflow in stands of Norwayspruce and beech' in West Germany. In International Symposium onforest Hydrology. Pergamon Press, N. Y. p. 179-185.

10. Swank, W. T., and N. H. Miner. 1968. Conversion of hardwood-coveredwatersheds to white pine reduces water yield. Water Resources Res.4(5): 947-954. „

11. Helvey, J. D., and J. H. Patric. 1965. Canopy and litter inter-ception of rainfall by hardwoods of eastern United States. WaterResources Res. 1(2): 193-206.

12. Pan, C. S. 1966. A study on the interception of rainfall throughnatural hardwood forest. Taiwan Forest Res. Inst. Bull. No. 131,

\ 20 p.

13. Rusk, C. R. 1969. Interception of precipitation by forest canopies'and litter. M.S. Thesis, Univ. Tenn., Knoxville. 76 p.

14. DeWalle, D. R., and L. K. Paulsell. 1969. Canopy interception,stemflow, and streamflow on a small drainage in the Missouri Ozarks.Univ. Missouri Res. Bull. 951, 26 p.

15. Brown, J. H., Jr., and A. C. Barker, Jr. 1970. An analysis ofthroughfall and stemflow in mixed oak stands. Water Resources Res.6(1): 316-323.

16. Swank, W. T., N. B. Goebel, and J. D. Helvey. 1972. Interceptionloss in loblolly pine stands in the Piedmont of South Carolina. J.Soil and Water Conserv. (In press.r)__

17. Kittredge,. J. 1948. Forest influences.' McGraw-Hill Book Co., N. Y.394 p.

18. Molchanov, A. A. 1960. The hydrologlcal role of forests. Acad.Sci. U.S.S.R. Inst. For. 407 p. (Transl. from Russian by IsraelProgram for Scientific Translations, 1963).

19. Zinke, P. J. 1967. Forest interception-studies in the UnitedStates. In International Symposium on Forest Hydrology. PergamonPress, N. Y. p. 137-161.

Ill

20. Helvey, J. D., and J. H. Patrlc. 1965. Design criteria for inter-ception studies. Symposium for Design of Hydrological Networks, Int.Assoc. Sci. Hydrol., Publ. 67, p. 131-137.

21. Sopper, W. E. 1963. Effects of the forest floor of a red pineplantation on the disposition of summer rainfall. Ph.D. Diss., Penn,State Univ., University Park.

22. Rogerson, T. L., and W. R. Byrnes. 1968t Net rainfall under hard-woods and red pine in central Pennsylvania. Water Resources Res.4(1): 55-57.

23. Thorud, D. B. 1963. Effects of pruning on rainfall interception ina Minnesota red pine stand. Forest Sci. 9: 452-455.

24. Voigt, G. K. 1960. Distribution of rainfall under forest stands.Forest Sci. 6: 2-10.

25. Hoover, M. D. 1953. Interception of rainfall in a young loblollypine plantation. USDA Forest Serv. Southeastern Forest Exp. Sta.Pap. 21, 13 p.

m26. Rogerson, T. L. 1967. Throughfall in pole-sized loblolly pine as

affected by stand density. In International Symposium on ForestHydrology. Pergamon Press, N. Y. p. 187-190.

27. Boggess, W. R. 1956. Amount of throughfall and stemflow in a short-leaf pine plantation as related to rainfall in the open. 111. Acad.

\\Sci. Trans. 48: 55-61.

28. Coweeta Hydrologic Laboratory, Franklin, N. C. Unpublished datawhich was collected near Asheville, N. C., 1934-35 (on file).

29. Irons Fork Experimental Forest, Mena, Ark. Unpublished data, 1942-43, 1939-42 (on file).

30. Lawson, E. R. 1967. Throughfall and stemflow in a pine-hardwoodstand in the Quachita Mountains of Arkansas. Water Resources Res.3(3): 731-735.

31. Rowe, P. B., and T. M. Hendrix. 1951. Interception of rain and snowby second-growth ponderosa pine. Trans. Amer. Geophys. Union 32(6):903-908.

32. Alden, E. F., and W. R. Curtis. 1957. Rainfall interception by ninedensities of pole-size ponderosa pine. Unpublished manuscript onfile at Rocky Mt. Forest and Range Exp. Sta., Flagstaff, Ariz., 10 p.

33. Orr, H. K. 1972. Throughfall and stemflow relationships in second-growth ponderosa pine in the Black Hills. Rocky Mt. Forest andRange Exp. Sta. USDA Forest Serv. Res. Note RM-210, 7 p.

34. Helvey, J. D. 1967. Interception by eastern white pinev WaterResources Res. 3(3): 723-729. ' ~~~~

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35. Simson, A. G. 1928. The interception of summer rains by forestcover. USDA Forest Serv. Pac. Northwest Forest and Range Exp. Sta.Res. Notes No. 5, 1 p.

36. Rothacher, J. 1963. Net precipitation under a Douglas-fir forest.Forest Sci. 9(4): 423-429.

37. Knutsen, S. K. 1965. Hydrologic processes in thirty to thirty-fiveyear old stands of Douglas-fir and alder in western Washington. M.S.Thesis, Univ. Wash., Seattle, 167 p.

38. Burroughs, E. R., Jr. Unpublished data supplied by letter datedNovember 14, 1967.

39. Morris, M.- J. 1936. Amount and water holding capacity of litter inrelation to age of coniferous stands. M.S. Thesis, Yale Univ.

40. Alden, E. F. 1968. Moisture loss and weight of the forest floorunder pole-size ponderosa pine stands. J. For. 66: 70-71.

41. Lowdermilk, W. C. 1930. Influence of forest litter on runoff,percolation, and erosion. J. For. 28: 474-49.1.

42. Balci, A. N. 1964. Physical, chemical, 'and hydrological propertiesof certain western Washington forest floor types. Ph.D. Diss.,Univ. Wash., Seattle.

43. Metz, L. J. 1958. Moisture held in pine litter. J. For. 56: 36.\

44.' Hoppe, E. 1896. Precipitation measurements under tree crowns.Mittheilungen Aus Des Forstlichen Versuchswesen Osterreichs, 21(Translated by A. H. Krappe, Div. Silvics, Forest Serv., Transl. No.291, 1935), 76 p.

45. Burger, H. 1925. Holz-laub-und nadelunter-suchungen. SchweizerischeZeitschrift Fur Forstwesen, p. 266-310.

46. Lopushinsky, W., and G. 0. Klock. 1972. Transpiration of coniferseedlings in relation to soil water potential. Abstracts, Meetingof Amer. Soc. Plant Physiol., Western Section, Eugene, Oregon.

47. Lopushinsky, W. 1969. Stomatal closure in conifer seedlings inresponse to leaf moisture stress. Bot. Gaz. 130(4): 258-263.

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