soil erosion losses under freeze/thaw and winter …soil erosion losses under freeze/thaw and winter...

SOIL EROSION LOSSES UNDER FREEZE/THAW AND WINTERGROUND COVER USING ALABORATORY RAINFALL SIMULATOR

Linnell M. Edwards1 and J. R. Burney2

'Research Station, Agriculture Canada, Charlottetown, Prince Edward Island CIA 7M8 •andDepartment ofAgricultural Engineering, Technical University ofNova Scotia, Halifax, Nova Scotia

B3J 2X4

Received 21 May 1986, accepted 15 October 1986

Edwards, Linnell M. and J. R. Burney. 1987. Soil erosion losses under freeze/thaw and winter ground cover using alaboratory rainfall simulator. Can. Agric. Eng. 29: 109-115.

Alaboratory rainfall simulator was used to test three Prince Edward Island agricultural soils (varying in soil texture) forrunoff and splash volume and sediment loss under varying conditions of freeze/thaw, ground cover and erosivity. Woodensoil boxes and ancillary collection frames (termed cassettes) were designed tofit four ata time under the rainfall simulator.With bare soil, freeze/thaw significantly increased sediment loss by about 90%: additionally, sediment in runoff variedsignificantly with soil type and, for a loam soil, was 15 and 31%, respectively, ofthe amounts for a fine sandy loam and asandy loam. Where the soil was seeded to a winter ryecover, sediment loss was reduced by70-80% with no significanteffect of soil type or freeze/thaw. Where, however, theerosive force was increased by adding overland flow to simulatedrainfall, there was a significant increase insediment loss even with ground cover. Sediment splash was sampled forall tests,and only ground cover indicated a significant effect.

INTRODUCTION

In the Atlantic Provinces of Canada,as in many other parts of the world, theUniversal Soil Loss Equation (USLE) isroutinely used to predict soil loss on agricultural land. However, as with any statistically fitted regression model, theapplication accuracy of the USLE is confined to the range and nature of its database. In particular, the data base used forthe USLE is predominantly continentalU.S.A. The erosivity factor (R) is a measure of the summed product of the kineticenergy (E) and the maximum 30-minintensity (/) within each storm period overan average year. The data base therefore isheavily biased towards high intensity summer storm conditions as a causative factor

and takes no account of erosion caused bynon-precipitation events such as snow-melt runoff or the stress of alternate freez

ing and thawing.The applicability of the USLE under

the light rainfall conditions of the PacificNorthwest of the U.S.A. was examined byMcCool et al. (1982) and Onstad andYoung (1982). In general it was found thatthe erosivity value (R) and the croppingvalue (C) required substantial modification. In a related sense, Morgan (1979)presented European data which showedsubstantial soil loss for low intensity, longduration storms on saturated soil.

On Prince Edward Island (P.E.I.) soilerosion is a major concern particularly onpotato lands where the crop is often cultivated up and down the slope, and the soilusually remains bare during the cool-season after potato harvesting. The cool season is moist and the soils could easily be at

or near saturation during periods of snow-melt. The cool season of P.E.I, is charac

terized by frequent showers and occurrences of soil freeze/thaw cycles; and mostof the soil erosion in P.E.I, seems to occur

during this period under these predisposing conditions.

The effects of frost action on soil sta

bility have been studied to some extent(Logsdail and Webber1959; SillanpanandWebber 1961; Hinman and Bisal 1968;Benoit 1973; Richardson 1976), and resultshave varied from no effect to an effect ineither direction, viz. improved aggregation or degradation depending on soil texture, initial soil moisture, freezing rate andtemperature. Undercircumstances of slowfreeze/thaw cycling with high initialmoisture in the soil, as typify P.E.I, naturally and as prevailed in this experiment,Bryan (1971) found significant soil physical degradation and thus susceptibility toerosion.

The first documented study on soil erosion in P.E.I. (Harza of Canada Ltd. 1968)advocated, among other things, that theerosivity factor (R) and erodibility factor(K) in the USLE be independently evaluated in the Atlantic Provinces. Wall et al.(1976) computed rainfall erosivity (R)values for Canada east of the Rocky Mountains and Wall and Dickinson (1979) andWall et al. (1983) developed seasonal rainfall distribution curves. All of these are,however, limited in applicability under thestochastic erosivity and erodibility conditions which prevail on P.E.I.

The only available soil loss data forP.E.I, were reported by Himelman andStewart (1979) based on observations of

CANADIAN AGRICULTURAL ENGINEERING, VOL. 29, NO. 2, SUMMER 1987

erosion plots on a fine sandy loam soilbetween 1973 and 1977 inclusive. Yearlysoil loss on a 12and 7% slope averaged 411ha-1 and 19 t ha-1, respectively, underfallow conditions; 20 t ha-1 and 10 tha-1, respectively, under potatoes cultivated along the slope; and 7 t ha~ *and 41ha-1, respectively, across the slope. Furthermore, erosion during the snow-meltperiod of March-April contributed substantially to total annual erosion load, thusstrengthening general observations inP.E.I, that erosion losses are highest during the cool season. Stewart (1979) contends, moreover, that the low erosivityvalue (R) of 50-60 projected from rainfalldata alone seems inadequate to account forannual soil losses in P.E.I. which commonly exceed 10 t ha_i. It would therefore appear that both increased erosivityforces and factors which markedly reducesoil erodibility resistance substantiallyincrease erosion during the cool seasons.

None of the aforementioned studies onsoil erosion in P.E.I, provided definitiveinformation on the quantitative impact ofcool-season meteorologic processes onsoil loss or runoff. The present study was,therefore, instituted using soil boxes andancillary collection frames (termed cassettes) in a laboratory rainfall simulator toinvestigate the effect of freezing/thawingon P.E.I, soils under the influence of a coolseason erosive force and ground cover.

MATERIALS AND METHODS

General

Samples of three agricultural soils ofP.E.I. (Table I), taken following a barley

109

and NH4 concentrations and electricalconsumption. The barn was fitted withcontinuous above-centre pivot rotating doors inthe sidewalls, and a continuous ridge opening.The non modulated system used themrostats,compressed air and air cylinders to open orclose the air inlets. During the winter, barntemperature fluctuations of 8C were observedwithin a 6 to 8 m in period. C 02concentrations fluctuated between 1500 and

3500 ppm depending on whether the inletswere open or closed. NH4 remained ratherconstant at 6 to 8 ppm. Electricityconsumption was 157 kWh over a one yearperiod. The modulated control system usedthermostats, a gear motor and a time delayto step the inlets open or closed. During thewinter, barn temperature fluctuations of aboutIC were noted at the level of the animals.

C 0 2 concentrations varied between 2800 to3200 ppm, while NH4 was 5 to 7 ppm.Electricity consumption was less than 1 kWhduring one year.

87-114 OPTIMIZING HEAT RECOVERY IN AN

AIR TO AIR HEAT EXCHANGER

OPERATING UNDER FROSTING

CONDITIONS

M.R. Bantie and E.M. Barber, Dept. Agric.Eng., Univ. of Saskatchewan, Saskatoon, Sask.S7N 0W0.

Air to air heat exchangers can be used topreheat ventilating air and hence reduceheating costs for livestock barns. However,frost accumulation is a major problem in thisapplication. Currently available frost controlsystems are based on some combination oftime, pressure loss, core temperature orexhaust air temperature. They do not result inan optimal rate of heat transfer, independentof barn temperature and relative humidity.The development and testing of a frostcontrol strategy is presented, based on ameasured instantaneous rate of heat transfer.

The temperature rise of the supply air ismeasured and the rate of heat transfer is

controlled by positioning a damper to regulatethe flow rate of cold supply air. Experimentswere conducted using a 470 1/s counterflowmetal plate heat exchanger. Tests wereperformed for a cold air temperature of -25C,and for return temperatures and relativehumidities of 13 and 25C, and 40 and 75%,respectively. The prototype controller wasconfirmed to operate properly as predicted.Three control parameters were identified as

being critical to the design of a heat transferoptimizing controller; the rate at which thesupply airflow is changed, the amount of heattransfer degradation permitted beforetriggering defrost and the maximum rate ofheat transfer permitted immediately followinga defrost. Further development is warrantedand needed to establish the critical controlparameters, as well as the installation andoperation procedures.

87-115 TEMPERING AIR USING AN AIR-SOIL

EXCHANGE SYSTEM

M.G. Britton, T.V. Murray, M.T. Burns, Dept.Agric. Eng., University of Manitoba, Winnipeg,Manitoba R3T 2N2, and M.A. Stumborg,Agriculture Canada Research Station, SwiftCurrent, Saskatchewan S9H 3X2

Research into the technical and economic

feasibility of tempering ventilation air forcontrolled environment structures using soil atshallow depths as an energy storage mediumhas been underway since 1985. The researchfacility and experimental results aresummarized. The demonstrated air temperaturetempering potential is outlined, and theapplication to an energy intensive animalproduction building in both summer and winteroperation is discussed, along with apreliminary design approach.

87-116 HIGH PRESSURE VENTILATION

INLETS FOR ANIMAL HOUSING

J.J. Leonard and F. Kloster, Dept. Agric.Eng., University of Alberta, Edmonton, AlbertaT6G 2G6.

In order to obtain suitable trajectories of coldair jets for minimum ventilation during winter,high air velocities are required. Generallythese velocities cannot be achieved usingconventional exhaust fans and inlet designs.One possible alternative is to use a highpressure centrifugal fan to provide minimumventilation during cold weather. Laboratorymeasurements of air flows and distribution

using such a fan and a radially dischargingceiling inlet are described. The results ofthese measurements are used to assess the

feasibility of practical systems, and to suggestpossible designs.

87-117 PRODUCTION OF BIOGAS IN A 50 m3DOWNFLOW FIXED FILM DIGESTOR

216 CANADIAN AGRICULTURAL ENGINEERING, VOL. 29, NO. 2, SUMMER 1987

230

905

PLAN

SCALE:

500

NOTE:

1. ALL VALUES IN mm2. MATERIAL: 22 gge g.i.

GZr„

130t»

mrEND

ELEVATIONELEVATION

Figure 2. Splash frame which fits over a soil box shown in Fig. 1.

N0Tr

PLAN

ELEVATION

scale:

200

1. Al L VALUES IN mm

2. MATrKIAL: ?? gge g.i.

SILL FLEVATION

Figure 3. Runoff collector which fits overthe sill of a soilbox shown in Fig. 1.

drop-former perspex boxes. The entireapparatus was enclosed in an insulatedcooling chamber.

The rainfall drop-formers consisted of aset of 430-mm-square x 30-mm-deep

hollow perspex boxes based on the designof Chow and Harbaugh (1965). Fournozzles in the upper surface of each boxinjected water at rates equivalent to 12.5,25, 50 and 100 mm h ~' over the area of the


box. Drops were formed at the ends of 25-mm lengths of 0.58-mm i.d. polyethylenetubing spaced on a 32-mm matrix in thelower side of each box. Each of the inlet

nozzles was connected to the equivalentnozzle in each of the other boxes in the 2 x

7 matrix of boxes. Water flow to each of thefour sets of nozzles was controlled by asolenoid valve. By opening combinationsof the four controlling solenoid valves,rainfall could be instantly applied over thesoil trough area at rates from 25 to 187mmh-1 in 12.5-mmh-1 increments. Rainfallcould also be stopped instantaneouslywhen the master switch closed all the supply solenoids. The soil trough beneath therainfall boxes could be tilted up to a 5%slope.

Procedure

Soil boxes were filled in the field atthree sites of different soil types. Care wastaken to maintain the original profile orientation of the soil. The boxes were taken to acommon outdoor site, cover crop treatments seeded to Kodiak winter rye at 4 gbox-' (140 kg ha- x\ and left under natural meteorologic conditions until mid-November when they were moved to thelaboratory. All boxes were kept at nearfield capacity through regular watering.Rainfall simulator runs were done in three

time blocks: first, with bare soil treatments; second, with early seeded treatments; and third, with late seededtreatments. For each time block, 12 boxes

HI

112

Figure 4. Assembled soil cassette in rainfall simulator trough (left) and minus splash frame (right).

-R 15 INSULATION

•REFRIGERATIONUNIT

-RIBBEDALUMINUMSHEET(WALLS ANDCEILING)

HINGE-

INLET NOZZLEx 4/BOX

MANIFOLDx4

•2x7 MATRIX OF HOLLOW0.43 m SQ. PERSPEX BOXES

-TROUGH SUPPORT ROD x 4

SOIL TROUGH

Figure 5. Rainfall simulator in insulated and refrigerated enclosure.

-SOLENOIDVALVE x 4

-WATERSUPPLY

•VACUUMLINE

•SCISSORJACK


were placed at field capacity in cold storage at - 15°C for 10 d, and the remaining12 were subjected only toa laboratory temperature of about 20°C.

For practical reasons, the frozen treatments and the non-frozen treatments wererun at opposite ends of each time block atambient temperatures of 1°C and 20°C,respectively. For each subset of four boxesof any particular soil type within the temperature set, two boxes were selected atrandom to receive rainfall plus overland(upslope) flow, and the remaining two toreceive rainfall application only. Twelvecassettes were then run randomly, four at atime. All tests were run under a rainfallsetting of 50 mm h~l for a period of 30min.

In total, six rainfall simulator runs wereconducted (four cassettes per run) for thebare soil block. Prior to each run, the rainfall rate was calibrated byusing a tarpaulinunder the rainfall boxes for a period of5 min. Fourboxes were then placed in thesoil bin pre-set to a 5% slope. A splashframe was set over each box and a runoffconcentrator inserted over each sill. Anoverland flow applicatorpipe was insertedin each of the appropriate boxes. Unusedpipes were left on the bottom of the rainfallsimulator trough where their dischargepreserved the calibration of the four pipesas a unit. Siliconsealer was used along thejoint between the splash frame and therunoffcollectorto preventleakage. Splashwas collected from the side troughs of thesplash frame and runoff was collectedfrom the end of the runoff collector foreach cassette.

Following each run, the volumes ofrunoff and splash from each of the fourcassettes were measured and separatelyretained for sediment determinationtogether with the wash from the runoffcollector, splash trough and the collectionand measuring vessels.

Sediment dry mass for both runoff andsplashfrom each soil box was obtained byfiltering through a 0.47-jjim filter, thenoven-drying the retained soil at 105°C.Data on runoff volume and sediment lossfrom bothrunoffandsplashweresubjectedto analysis of variance and mean separation.

RESULTS

As reflected in Table II, ground coversignificantly decreased runoff sedimentloss to 17% of that for bare soil for earlyseeded rye and to 28% for late seeded rye,and also significantly decreased sedimentsplash to 36% for early-seeded rye and57% for late seeded rye. The freezing pre-treatment significantly increased runoff

TABLE II. MEANS OF MAIN EFFECTS

Mean value

Runoff SplashVolume Sediment Volume Sediment

Cell (mL) (g) (mL) (g)CropBareEarly seeded

4191a

3917a60.2a10.1/7

434a390a

1.50a0 54/7

Late seeded 4943a 17.0/7 431a O.86/7

SoilLoam 2226/? 9.4c 401a 0 82aFine sandy loam 5888a 41.6a 4\9a 1 03aSandy loam 4931a 30.3/7 434a 1.05a

TemperatureNormal 3287a 21.2/7 391a 0.84aFrozen 4874a 31.0a 444a 1.09a

Erosion agentRainfall 1402/7 12.2/7 4\\a 1 1 laRainfall + overland flow 1299a 46.0a 430a 0.83a

a-c Means followed by the same letter in any column for any treatment are not significantly different at

TABLE III. LEVELS OF SIGNIFICANCE OF MAIN EFFECTS AND INTERACTIONS

RunoffSource of Splashvariation Volume Sediment Volume Sediment

Main effectsGround cover status (GrC) NS ** NS **

Soil type (S) ** ** NS NSTemperature status (T) NS * NS NSErosion agent (EA) ** *M= NS NS

Two-way interactionsGrC x S NS ** NS **

GrC x T NS NS NS NSGrC x EA NS ** NS NSS x T #* NS NS **

S x EA ** ** NS NST x EA NS NS NS NS

Three-way interactionsGrC x S x T NS NS NS NSGrC x S x EA NS ** NS NSGrC x T x EA NS NS NS NSS x T x EA ** NS NS NS

*,**/> = 0.05and/> = 0.01, respectively; NS == not significant at P = 0.05.

TABLE IV. MEANS OF MAIN EFFECTS — BARE SOIL

Mean value

Runoff Splash

CellVolume

(mL)Sediment

(g)Volume

(mL)Sediment

(g)SoilLoam

Fine sandy loamSandy loam

1673/7

6179a

4721a

17/7

110a54/7

413a

424a

465a

0.82/7

\.12ab1.97a

TemperatureNormal

Frozen2161b5615a

42/7

79a466a

401a1.38a

1.63a

Erosion agentRainfall

Rainfall 4- overland flow1797/7

6585a

26/7

94a407a461a

1.59a1.41a

a,b Means followed by the same letter in any column for any treatment are not significantly different at

sediment loss by 75%. Sediment loss andrunoff volume varied significantly withsoil type. The loamsoil showed only 20%of the runoff sediment loss and 38% of therunoff volume as compared to the finesandy loam, and 31% of the runoff sediment loss and 45% of the runoff volume as


compared to the sandy loam. When overland flow was added to rainfall (to form asingle erosion agent), runoff sediment lossincreased 277% and runoff volume 420%.As Table III shows, therefore, all of themain effects, viz. ground cover status, soiltype, temperature status, and erosion agent

113

TABLE V. MEANS OF MAIN EFFECTS — EARLY SEEDED

Mean value

Runoff Splash

Cell

Volume

(mL)Sediment

(g)

Volume

(mL)Sediment

(g)

SoilLoamFine sandy loamSandy loam

TemperatureNormalFrozen

Erosion agentRainfallRainfall + overland flow

1143/76163a4147a

3504a4330a

1082/7

6753a

3a

18a

10a

10a

10a

4/7

17a

382a

404a

383a

368a411a

409a370a

0.74a

0.46a/?0.40/7

0.44a0.63a

0.69b0.39a

a,b Means followed by the same letter in any column for any treatment are not significantly different atP = 0.05.

were significant on runoff sediment loss.Soil type and erosion agent significantlyaffected runoff volume. Ground cover status significantly affected sediment splash.Significant two-way interactions on sediment loss occurred between ground coverstatus x soil type, ground cover status xerosion agent, and soil type x erosionagent; while significant three-way interaction on sediment loss occurred between

groundcoverstatus x soil type x erosionagent. Significanttwo-way interactionsonrunoff volume occurred between soil typex temperature status, and soil type xerosion agent; while significant three-wayinteraction on runoff volume occurredbetween soil type x temperature status xerosion agent. Significant two-way interaction on sediment splash occurredbetween ground cover status x soil type,and soil type x temperature status.

Separateanalysis of varianceon data forgroundcover status showedthat, with baresoil, the freezing pretreatment increasedthe sensitivity of the tests. Temperaturestatus had a significant effect on sedimentloss and runoff volume from bare soil, buthad no significant effect on either theearly- or late-seeded (ground cover) treatments. As the treatment means for bare

soil in Table IV show, the freezing pretreatment significantly increased sediment lossas well as runoff volume by 88% and103%,respectively. However, as indicatedin Table V freezing pretreatment had nosignificanteffect on either sediment loss orrunoff under ground cover conditions.

DISCUSSION

This technique was operationally innovative, in that it took the basic Chow andHarbaugh (1965) rainfall simulator designas previously used by Burney and Huggins(1973) and adapted it for more efficientusage. There was an advantage in transportation as well as in handling the samples inthe field and laboratory, in the relative ease

of maintaining soil profile orientation, andin testing four experimental units at once,thus quadrupling the turn-over rate of theoriginal single-trough design.

As indicated previously, Bryan (1971)found significant soil aggregate degradation and thus susceptibility to erosionunder conditions of slow freeze/thaw cycling with high initial soil moisture content,such as typify P.E.I, and as prevailed inthis experiment. Such a finding supportsthe significant increase in runoff volumeand sediment loss on bare soil observed inthis study due to freeze/thaw. The resultsof the application of this technique to theagricultural soils of P.E.I, point unmis-takeably to the importance of ground coverin soil and water conservation. For,whereas with ground cover, temperaturestatus had no significant effect on sedimentloss when the soil was bare, freeze/thawcaused significant increases in sedimentloss and runoff volume as previouslyshown.

Based on the results of this study, therewere distinct differences between soiltypes in theirbasicabilityto resisterosion;but, again with a cover crop, the severity oferosion was reduced to such an extent thatdifferences in sediment loss were insignificant.

The greater resistance to erosion shownby the loamsoil in this studywasbasedona textural advantage. It showed no advantage in organic matter content, but had ahigher clay content than either of the othertwo soils, which is a highly desirablecharacteristic for good aggregate stabilityand, in turn, for successful soil and waterconservation management. It is impractical, however, to manage any soil forincreased clay content, although considerable improvement in stability can beattained through increased soil organicmatter and one of the surest ways to manage for this is to seed a winter cover cropand incorporate it as green manuringafterthe winter.

Although the results of thisstudy makeit clear that, regardlessof the groundcoverstatus, there will be significantly increasedsediment loss with an increase in erosiveforce, there would still be considerablewisdom in establishing a winter groundcover to minimize such loss. The effect oferosion agent on sediment loss has shownsignificant interaction with ground coverstatus; moreover, early- and late-seededcover, respectively, showed 18 and 29% ofthe bare soil treatment sediment loss underthe influence of rainfall plus overland flow.This is a significant saving of soil.

The principle findings of this studyleave no doubt of the quantitative, beneficial impact of winter rye cover on soilerosion, and its ability to significantlyattenuate the effects of such naturallyuncontrollable factors as erosion agent,freeze/thaw and soil type on sloping land.Cover cropping as a management measureto minimize soil erosion has long beenacknowledged globally, and the benefitsofits application in the cool season are wellknown in P.E.I., at least, qualitatively.Nevertheless, popular adoption has beenconstrained, mainly by the late harvest ofpotatoes, although there is some adoptionafter tobacco, corn and soybeans.

If, therefore, potato farmers on P.E.I,are to adopt winter cover cropping andmake it an integral part of annual-cropmanagement, they must be prepared totrade off the higher profits of late potatoesagainst the benefits of cool-season soil erosion control by this means. This will,undoubtedly, involve some sacrifice byfarmers in the short term, but can be justified through stewardship in the pursuit oflong-term benefits to the farm as well as tothe wider resource as a whole.

ACKNOWLEDGMENTSThe authors gratefully acknowledge the

assistance of Allan MacRae and Paul Frame in

conducting the field work and the rainfall simulator runs, and Jack Vissers and Jim Godwin inthe design and construction of the soil cassettes.

REFERENCES

BENOIT, G. R. 1973. Effect of freeze-thawcycles on aggregate stability and hydraulicconductivity of three soil aggregate sizes.Soil Sci. Soc. Am. Proc. 37(1): 3-5.

BRYAN, R. B. 1971. The influence of frostaction on soil aggregate stability. Trans.Inst. Br. Geogr. 50: 71-88.

BURNEY, J. R. and L. F. HUGGINS. 1973.Hydraulics of shallow flows over stableeroded sand surfaces defined by area spectra. Water Resources Research Center Tech.Rep. No. 36, Purdue University, WestLafayette, Ind. 137 pp.

CHOW, V. T. and T. E. HARBAUGH. 1965.Raindrop production for laboratory water-

114CANADIAN AGRICULTURAL ENGINEERING, VOL. 29, NO. 2, SUMMER 1987

shed experimentation. J. Geophys. UnionRes. 70: 6111-6119.

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MORGAN, R. P. C. 1979. Soil erosion. Academic Press, Don Mills, Ont. 113 pp.

ONSTAD, C. A. and R. A. YOUNG. 1982.Erosion characteristics of three north

western soils. Trans. ASAE (Am. Soc.Agric. Eng.) 25: 367-371.

RICHARDSON, S. J. 1976. Effect of artificialweathering cycles on the structural stabilityof a dispersed silt soil. J. Soil Sci. 27:287-294.

SILLANPAN, M. and L. R. WEBBER. 1961.The effect of freezing-thawing and wetting-


drying cycles on soil aggregation. Can. J.Soil. Sci. 41: 182-187.

STEWART, N. E. 1979. Some problems associated with implementing soil conservationprograms. Paper, Soil Erosion Workshop,NSAC, Truro, N.S. 17-19 April. 14 pp.

WALL, G. J. and DICKINSON, W. T. 1979.Soil erosion — predictionin Canada. Paper,Soil Erosion Workshop, NSAC, Truro, N.S.17-19 April. 25 pp.

WALL, G. J., L. J. P. VAN VLIET, and W. T.DICKINSON. 1976. The universal soil loss

equation — a method for predicting soil lossin Canada. Abstract, Annual Meeting, Can.Soc. Soil Sci., Halifax, N.S. 4-7 July.

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for Canada east of the Rocky Mountains.Can. J. Soil Sci. 63: 271-280.

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soil erosion losses under freeze/thaw and winter …soil erosion losses under freeze/thaw and winter...

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