soil water movement as affected by deep freezing1

SOIL SCIENCE SOCIETY OF AMERICA

PROCEEDINGSVOL. 33 MAY-JUNE 1969 No. 3

DIVISION S-l —SOIL PHYSICS

Soil Water Movement as Affected by Deep Freezing1

RICKARD S. SARtz2

ABSTRACTKnowledge of water movement into and through frozen soils

is needed for studying the hydrologic behavior of northernwatersheds. Soil water was logged by the neutron method inboth sandy and silty soils over four winters in southwesternWisconsin. Bonded frost depth, which ranged from 8 to 120 cmat maximum penetration, was measured concurrently by frostprobe or resistance blocks. Changes in soil water took placethroughout much of the frost season, even with deep soil frost.One series of data showed that water may infiltrate and percolatethrough more than 60 cm of hard-frozen ground. However,frozen ground did impede percolation, causing a buildup ofwater in the frozen zone during spring melt. Although neutronreadings increased at upper levels during the frost season, mostof the increases could not be accounted for by decreases atlower levels. Thus, they appeared to result more from infiltra-tion than from upward migration as reported by others.

Additional Key Words for Indexing: soil frost, infiltration,percolation, interflow.

SINCE THE early work of Bouyoucos and McCool (3)suggested that soil freezing causes upward movement

of water to the frost line, migration of soil water with freez-ing has been a popular subject for research. Althoughwater may also move gravimetrically through frozen soil,this process has received much less attention, probablybecause of the general assumption that soil freezing reducesinfiltration to negligible amounts. Bonded frost may berelatively impermeable (1, 9, 13). On the other hand,ground freezing does not always reduce infiltration as muchas is generally assumed (6, 8, 11).

From a study in North Dakota using the neutron meter,

1 Contribution from the Forest Service, USDA, North CentralForest Exp. Sta., St. Paul, Minn. Presented before Div. S-l, SoilScience Society of America, Nov. 13, 1968, in New Orleans, La.Received Nov. 12, 1968. Approved Jan. 1, 1969.2 Principal Hydrologist, Forest Watershed Lab., North CentralForest Exp. Sta., Forest Service, USDA. The Laboratory is main-tained in cooperation with the State of Wisconsin Departmentof Natural Resources.

Willis et al. (15) reported that freezing of the soil appar-ently had little, if any, effect on the accumulation of waterat the depth of freezing. However, in another study, in ahigh water table area, Willis et al. (16) found that a dropin the water table level was accompanied by increases insoil water in the frost zone above the water table. Similarresults were reported by Benz et al. (2). Their data alsoshowed that soil water increases were greater than themaximum amount available owing to the water table drop.This suggested that the additional water may have beendrawn from artesian sources. Ferguson, Brown, and Dickey(5) found that upward movement to the frozen zone in asilty clay loam soil was related to tension. They found noupward movement on plots where the soil water tensionwas greater than 5 atm. Gary (4), who compared thermallyversus hydraulically induced flow, concluded that thermalwater flow was too small to account for the net movementof soil water into the frost zone reported for field soilsunder winter conditions.

This paper gives the results of a series of studies on soilwater movement in field profiles under winter conditionsin southwestern Wisconsin. Measurements were made overfour winters on three different soils types at five sites. Allstudy plots are in areas where the normal gound watertable lies at least 5 m below the ground surface.

EXPERIMENTAL METHODS

Soil water was logged by neutron probe in aluminum accesstubes using a 4.6-mc Ra-Be source the first 3 years and an 80-mcAm-Be source the last year. Measurements were made at 15-cmintervals to depths of 120 or 150 cm. The 15-cm depth readingswith the Ra-Be probe were corrected for neutron escape as de-scribed by Sartz and Curtis (12). Frost depth was determinedby sampling with a soil tube or frost penetrometer, or by resist-ance blocks (10). Sampling interval was normally 1 week thefirst 3 years and 4 days the last year, with more frequent sam-pling during periods of expected change.

Measurements the first year (1961-62) were made in Heschsandy loam, an- alluvial-colluvial soil formed from outwash ofboth weathered sandstone and silty loess. Although mapped assandy loam, soil texture varied by depth. Three access tubeswere installed about 5 m apart, two in an area of bluegrass sod

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334 SOIL SCI. SOC. AMER. PROC., VOL. 33, 1969

with a dense cover of matted grass, the third in bare soil. Tobetter show the effect of soil freezing on water movement, wehoped to induce deep frost penetration around one tube, mod-erate penetration around another, and to prevent freezing, ifpossible, around the third. We did this by changing the coverand removing snow. The adjusted cover on the three plots wasbare soil, matted grass, and matted grass covered with a 15-cmlayer of alfalfa hay. Soil water profiles were logged 15 timesduring the frost season.

In 1963-64 two other sites were chosen for study. Both wereon Fayette silt loam (valley phase), a loessal soil. One site wason a northwest slope of 20%, the other on a southeast slope of25%. The cover on both sites was mowed meadow. Three accesstubes were installed at each site, 12 m apart on the northwestslope and 3 m apart on the southeast slope. Soil water profileswere logged 19 times.

In 1964-65, one tube at the Hesch sandy loam site (the grassplot) and one at each of the two Fayette silt loam sites wereused. The profiles were logged 14 times.

In 1967-68 two other areas were chosen for study, both onlevel land. The soil on one was Plainfield fine sand, a river ter-race soil formed from sandy outwash. The soil on the other wasFayette silt loam, ridgetop phase. The cover on the Plainfieldsite was grass and weeds and on the Fayette site, alfalfa mead-ow, both of medium density. The cover was hand-clipped inthe fall, and the clippings were scattered over the study area sothat the cover was alike on both study sites. Two access tubes3 m apart were installed at each site. These profiles were loggedabout 65 times, using the Am-Be probe.

3 A. W. Krumbach, Jr., I960. The effect of freezing and thaw-ing on soil moisture, bulk density, and shear strength under openand forest conditions. Ph.D. Thesis. Michigan State University,East Lansing.

RESULTS

Since the data for the separate years of study are distinct,the results are discussed separately.

1961-62

Frost was first found in the soil on December 18, andmaximum depth was reached about March 12. (The term"frost" means bonded or concrete frost.) The cover treat-ments influenced frost penetration as planned. Maximumdepths were 96, 71, and 7 cm on the bare, grass, and hay-covered plots respectively. On the bare and grass plots thesoil was frozen continuously from the first week in Januaryuntil about the third week in March. The ground startedthawing then, and the frost was practically gone by April17. The hay-covered plot was nearly frost free most ofthe winter.

Despite the generally subfreezing weather, changes insoil water took place throughout much of the frost season,even on the two plots that had deep frost (Fig. 1-3). Theearly season losses probably resulted from percolation tolower depths rather than from evaporation. Rainfall in thepreceding 3 months totalled 32 cm; so the soil was abovefield capacity [as defined by Veihmeyer and Hendrickson(14)] at the beginning of the frost season. Infiltration fromsnowmelt apparently resulted from surprisingly short peri-ods of above-freezing temperatures, as reported by Krum-bach.3 Increases from a midwinter thaw showed up in the

Initial Water Content(Percent By Volume)


Soil Water Change(Percent By Vol.)

fc! 60

I FrostZone


Fig. 1—Frost penetration and soil water changes by depths onbare plot.

Fig. 2—Frost penetration and soil water changes by depths ongrass plot.

SARTZ: SOIL WATER MOVEMENT AS AFFECTED BY DEEP FREEZING 335Initial Water Content{Percent By Volume)

FrostZone


Fig. 3—Frost penetration and soil water changes by depths onhay-covered plot.

measurements made on January 29 and February 5 (Fig.1-3). The smaller increase on the bare plot may resultpartly from snow removal to induce deeper freezing. Theincreases resulted from a 2-day thaw on February 3 and 4.If we assume that the frost measured on February 5had reached that depth before percolation began, with 51cm of bonded frost on the grass plot, 25 mm of water stillpercolated into the soil. The percolated water tended toremain in place in the frozen zone on this plot (Fig. 2),while on the barely frozen, hay-covered plot the additionalwater was lost through percolation during the following3 weeks (Fig. 3). Note that the percolation loss from the30- to 60-cm zone on the hay-covered plot did not produceany increases at lower levels.

After the thaw of early February, soil water changedlittle until the spring melt period, which began about March8. The March 12 readings showed a 20-mm increase fromthe previous readings on the hay-covered plot. However,increases on the two frozen plots did not begin to show upuntil 2 weeks later. By then the ground had begun to thawfrom both above and below.

Because of the frequent changes in soil water that re-sulted from percolation losses or infiltration gains, anyupward migration that might have taken place may havebeen obscured. However, it would seem that if significantamounts of water had actually moved upward from lowerlayers, Fig. 1-3 would show it. The initial increase at the15-cm depth and the corresponding decrease at the 30-cmdepth on the bare plot indicate possible upward movement(Fig. 1). However, there were decreases at other depths on



10-1

12/9 12/16 12/30DATE

Fig. 4—Soil water changes by depths, plot 3, silt loam, south-east slope—December 9, 1963 to January 27, 1964. Shadingindicates probable translocation.

this plot; and there was a similar decrease at the 30-cmdepth on the hay-covered plot (Fig. 3), where the 15-cmdepth also showed a decrease. Thus, the water loss at the30-cm depth on the bare plot probably resulted from per-colation rather than upward migration. From December18 to January 29, the soil had frozen to a depth of about45 cm on the bare and grass plots, and had not yet frozenon the hay plot. The distribution of water on the two datesdoes not show any change that could not be explained bypercolation alone. Frost in the ground did affect the distri-bution of soil water, however. The frozen soil impededpercolation, causing a buildup of water in the frozen zoneduring the thaw period.

1963-64

The winter of 1963-64 was milder than the winter of1961-62, with low precipitation, intermittent snow cover,and frequent thaws. Soil freezing began about December16 and reached a maximum depth of 30 to 37 cm aboutFebruary 24. Only minor changes in water content tookplace during the soil freezing period. But the changes re-sulted in part from what appeared to be upward movement,particularly from the 30-cm zone to the 15-cm zone (Fig.4). The upward movement took place between December16 and 30, during which time frost penetrated from about2 cm to 15 cm. It was most evident on plots 1 and 3 at thesoutheast site, but the same pattern showed up to somedegree at all three plots on both sites. The maximumchange was 5% or 8 mm. Translocation may also haveoccurred between the 45- and 30-cm zones, but such move-ment was less pronounced and the time of occurrencevaried from one plot to another.

During a thaw in March plot 3 on the southeast slopegained 60 mm of water in a 2-day period. Soil water in-creased at all levels, but the greatest change was at the30-cm depth, where the volumetric content rose from 34to 45%. The ground was bare of snow on March 2, and

336 SOIL SCI. SOC. AMER. PROC., VOL. 33, 1969


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5-1

DECEMBER JANUARY FEBRUARY

Fig. 5—Soil water changes by depths in fine sand profile during soil freezing period, 1967—68.

only 13 mm of rain fell before the plots were remeasuredon March 4. Resistance blocks and thermistors installed30 cm from each soil water tube indicated more thaw onplot 3 than on the other two, but penetrometer measure-ments showed that bonded frost still occurred betweendepths of 8 and 30 cm on the site.

1964-65

Frost started to form by November 30 this year, andreached maximum depths of 90 cm in the sandy loam plotand 75 cm in the silt loam plots by March 1. The mostnoteworthy change in soil water took place on the sandyloam plot between November 30 and December 14. Frostdepth on December 14 was 15 cm. Snowmelt and rain dur-ing the interval produced an estimated infiltration potentialof 15 mm. But water content increased 38 mm, mostly inthe upper 30 cm of the profile. Changes in water contentafter December 14 were mostly small increases, whichvaried considerably by depth and location.

1967-68

The data for 1967-68 show the changes that took placein relatively dry profiles of sandy and silty soils during awinter of deep freezing. Both soils (Plainfield fine sand andFayette silt loam) had started to freeze by November 30.Maximum penetration was 120 cm in the sand and 105 cmin the loam. Water content at the beginning of the frostseason was unusually low for this time of year. Normallythe soils in this area are recharged by fall rains, but therewere no fall rains in 1967. Average profile water contentson November 27 were about 10 and 26% by volume forthe two soils.

Soil water changes as measured by the Am-Be probeshowed gains in the upper part of the profile that could berelated to rainfall or snowmelt (Fig. 5 and 6). Valuesbelow 75 cm (to the 150-cm depth measured) showed

no change. Upper level measurements were not correctedfor neutron escape; so the plotted values at 15 and 30 cmin the sand and at 15 cm in the silt are low. Vertical resolu-tion of the probe was found to be 36 cm at 36% and 60 cmat 15% water content. This explains the comparativelysmaller increases at the 15-cm depth in the sand profile.The greater vertical resolution in (sandy) soil also meansthat readings at one depth are influenced by water contentat the next depth. Thus the values at 30 cm would be lowerbecause of neutron escape, but higher because of thegreater water content at the 15-cm level.

DISCUSSIONUnder the conditions of this study, infiltration and per-

colation rather than upward migration appeared to bethe primary mechanisms for water movement in frozensoil. Although some upward movement was evident, thelarger increases at upper levels could usually be related torainfall or snowmelt. All soils studied held water at ten-sions lower than 5 atm, the value given by Ferguson et al.(5) as the point above which they detected no upwardmovement. A tension of 5 atm is equivalent to a volumetricwater content of about 10 to 14% in Fayette silt loam.

Increases in water content above the amount possiblefrom infiltration are puzzling. Where did the water comefrom? Translocation from a water table as suggested byBenz et al. (2) is one possibility, but seems unlikely in alow water table area. Upward movement from unsaturatedsoil below the zone measured is another possibility; but theshort time involved (only 2 days on the southeast slopeplot) would seem to rule out this explanation also. Thatleaves interflow as the only other possible source. Whileinterflow seems a reasonable explanation for the increasesmeasured on the sloping plot, it may be a less likely expla-nation for the increases measured on the relatively flat,sandy loam plot. However, interflow could take place onflat land too, where soil profile differences might produce

SARTZ: SOIL WATER MOVEMENT AS AFFECTED BY DEEP FREEZING 337

Initial Water Content(Percent By Volume


0J

75

DECEMBER JANUARY FEBRUARY

Fig. 6—Soil water changes by depths in silt loam profile during soil freezing period, 1967—68.

differential hydraulic gradients, particularly under frozenconditions. Excavation of the access tube on the sandyloam plot disclosed a void around one side of the tubebetween 45 and 75 cm. This resulted in lower readings atthese depths, but whether it would affect the distributionof soil water is unknown. The increases in December 1964that might be attributed to interflow were found largelyat the 15- and 30-cm depths.

That water apparently moved freely through frozen soilprofiles may seem surprising. However, Krumbach4 foundthat bonded frost usually contained open pores that wouldallow percolation. Our studies showed a decreasing re-sponse to infiltration as the frost season progressed. Thisprobably resulted from decreasing permeability as the frosthardened. Extraction of frozen cores and soil block resis-tance readings showed a definite hardening of the frost withtime, even with no change in water content. Presumably,hardening is caused by growth of the frost crystals withmovement of additional water from the liquid to the solidphase as described by Hoekstra (7). Conversely, the proc-ess of frost softening during thaw is accelerated by perco-lating water. The changes that can take place in bondedfrost suggest that a system is needed for classifying relativesoil density or permeability under frozen conditions.

1 Ibid.

soil water movement as affected by deep freezing1

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