In situ measurements of moisture and salt movement in freezing soils
D. M . GRAYAND R. J . GRANGER
Division of'H.y);drology. Urrivrrsiry ( ~ ' S ~ ~ s k t r r c . / ~ r nSllsk(lroo11.
,lln,
Sosk. Glrltrd(l S7N OW0
.
Received August 16, 1985
Revision accepted Noverllbcr 20, 1985
The paper presents the results of field studies on thc movement of moisture and salts during frcczing of Prairie soils. I t is
shown that large fluxes of water can migritc to thc freezing t'n)nt and move upward into the frozen soil above. The fluxcs arc
largest in light-textured soils (c.g., silt loam) having a water table at shallow depth. However, substantial amounts of soil
moisturc may also move in silty clay, silty clay loarn, and clay soils under dryland farming provided there is sufficient watcr
present to support capillary flow.
The dynamics of soil riioisture transfer under natural conditions as a result of freezing involves rrlovcment of water in both
vapor and liquid phases. In the shallow surface layer of soil, to a depth of 300-400 mrn, vapor flow predominates; in the
depth below, watcr usually moves primarily as a liquid. It is delnonstrdted that the accumulation of icc with time increases
because of the downward movement of the freezing front and the upward movcmcnt of watcr into the fro~cnsoil above. In a
silt loam with large fluxes, the ice content of the frozen zonc rapidly reaches a level (80-85% pore saturation) whcrc
measurable migration ceases. Conversely, in a silty clay the movement of moisture into the frozen soil is observed to continue
throughout most of the freezing period, and the ice content reaches 93% pore saturation. The grcater movement in thc finer
grained soil is attributed to a higher free~ing-pointdepression, a larger numberof capillary pores. and a higher concent~itiono f
soluble salts in the liquid films.
A close association is observed between changes in the ice contcnt and clcctrical conductivity o f a silt loan1 after freezing. In
a silty clay the agreement is less clear, probably the result of the exchange or ions between the migrating liquid water and the
clay particles. Maximum amounts of exchangeable ions moving into a I rn depth of soil by thc freczing action are estimated to
be 11.9 tlha in a silt loam and 15.7 tlha in a silty clay loam.
Data showing the redistribution of water and salts during thawing are also presented and tliscussed.
-
--
L'aniclc prksente les ksultats d'ttudes de terrain sur le nlouvenlent de I'eau et des sels durant Ic gel dcs sols dcs Prairies. I I
cst dCmontk que d'importantes quantitts d'eau peuvent migrer vers le front de congelation ct par ascencion pkndtrer dans le sol
gel6 sus-jacent. Les sols h texture plus legere (p. ex. loam limoneux), ayant une nappe phrkatique peu profonde. produisent les
flux d'eau les plus abondants. Cependant, dcs quantitks consistrdbles d'eau peuvent kgalement se dkplacer dans les sols
composCs d'argile limoneuse, de loam li~nono-argileuxet d'argile sous-jacents a dcs cultures stchcs pourvu qu'il y ait
sunisan~mentd'eau pour assurer I'kcoulelnent capillaire.
La dynamique du transport de I'eau du sol dans dcs conditions naturelles due au gel implique un ~nouvementde I'cau tant en
phase liquide que vapeur. A un nivcau pcu profond dans la couche du sol, soit a une profondeur de 300-400 mm, c'cst le
transfert cn vapeur qui prddomine, a plus grdnde profondeur gknerdlelnent ,l'eau circulc surtout en phase liquide. II est
dCmontk que I'accumulation de glace en fonction du temps augmentc a cause du nlouvement descendant du front de
congtlation et du mouvelnent ascendant de I'eau dans Ic sol gel6 sus-.lacent. Dans le loam lirnoneux ou le flux est abondant. la
quantite de glace dans la zonc gelkc attcint rdpidement un niveau (80-8576 de saturation des pores) ou ccssc toute migration
rnesurablc. Par contre, dans une argile limoneuse le mouvement de I'eau dans le sol gel6 se poursuit durdnt presque toute la
periodc de gel, et la quantitC de glace atteint 93% de saturation dcs pores. Le flux le plus abondant est dans les sols a grain plus
fin, el il est attribut a une baisse du point de congelation, a un plus grand nombre de pores capillaires et a la plus forte
concentration des sels solubles dans les minces pcllicules d'eau.
Dans le loam limoneux gelc on observe une relation ktroite entre les changen~entsde la quantite dc glace et la conductivitC
tlectriquc. Cette relation est nloins Pvidente dans I'argile limoneuse, due probablemcnt B I'cffet dc I'Cchange des ions entre
I'cau liquide en voie de migration et les particules argileuses. Les quantitcs maximales d'ions Cchangeables ptnttrant une
couche de I m de sol par I'effet de gel ont ett estinites a 11.9 tlha dans un loam limoneux et $ 15.7 tlha dans une loam liinonoargileux.
Nous pksentons et discutons kgalement les rksultats obtenus sur la redistribution de I'eau et dcs scls durdnt le dCgel.
[Triduit par la revue]
Can. J . Earlh Scl. 23. 696-704 (1986)
Introduction
The degradation of soils as a result of salinization and the
potential effects of reducing the area and crop yields of economically viable farmland used for the production of cereal
grains on the Canadian Prairies are well-recognized problems.
Vander Pluym (1982) estimated the area of cultivated land and
rdngeland in western Canada affected by saline seepage to be
of the order of I 500 000 ha, which in 1982 resulted in an
estimated economic loss on areas under dryland farming of
about $303 000 000.In Saskatchewan alone it is estimated
that 160 000 20 000 ha is affected (Crosson 1976). and the
rate of increase is 1 - IO%lyear (Rennie and Ellis 1978).
Although salts can be fonned in place on or near the soil
surface by weathering of parent material, the major accumula-
+
Pnnrd ~n Canada i InnpnnlC au Canada
tions occur when salts dissolved in water are transported to the
surface and deposited when the water evaporates. This process
leads to the development of saline seeps, which are considered
the major cause of salinization in the region. In some cases
farm managcment practices, namely summer-fallowing. have
been implicated as a cause of salinity because of the high soil
moisture Ievels associated with that land use.
The common denominators in salinization of a soil surface
are a source of salts and a mechanism to transport them to the
surface. In defined areas of groundwater discharge the distribution of salts in the soil and the mode of transport can be
closely associated with the groundwater flow pattern and the
stratigraphy and composition of the geological formations
697
GRAY AND GRANGER
SANDY LOAM
SILTY
CLAY LOAM
LOAM
S I L T Y CLAY
S I L T Y LOAM
Ca) SOIL T E X T U R E
I
01
2000
I
SAND SIZES
1.'''
'
'
.
'
I.'.L '
100
I
SILT SIZES
.
'
'
1""
10
'
'
'
I
CLAY
1.'"
1
I
1
- 5
GRAIN S I Z E < Im>
Cb) P A R T I C L E S I Z E DILTRIBUTION
FIG. 1. Soil texture profiles and particle size distribution curves for
study sites: (1) Saskatoon 83; (2) Saskatoon 84; (3) Saskatoon 85; and
(4) Outlook.
*
4
6
through which the water moves. Only recently, Bullock (1985)
proposed a classification system for salt-affected lands based
primarily on hydrological and geological factors. Often, however. the upper limit of saturated flow (under positive head) is
the water table. Salt spots that develop above a static water
table are usually attributed to the upward migration of water by
capillary flow in response to gradients set up in the soil profile
after the withdrawal of water by evaporation and evapotranspiration. Peck (1978) suggested that salinization occurs under
these conditions when the accumulation of salts occurring between infiltration events exceeds the loss by leaching. Therefore, an average minimum net upward flux of water is needed
during the summer months to salinize a soil surface. Using the
steady-state equation describing one-dimensional unsaturated
flow and the relationship between capillary conductivity and
soil moisture suction given by Gardner (1958), it can be shown
that the maximum flux of water by capillary movement to a
soil surface varies directly with the water transmission characteristics of the soil and inversely with the square of the depth to
a water table. Peck used the association to establish the "critical" depth to a water table: that depth from which steady-state
movement would be at least 1.OO mmlday on irrigated land and
0.10 mmlday on dry land. The smaller value was taken for
dryland areas because of the lower frequency of occurrence of
infiltration events. The calculations gave ranges in critical
depth of 0.9-6.6 m on irrigated land and 1.6-31 m on dry
land. The lower values in these ranges are for sand and coarsetextured soils; the higher values are for fine-textured, heavy
clays. As pointed out by Peck, the significance of the results is
the importance of the water table depth to salinization. Numerous other studies (Luken 1962; Bettenay et al. 1964; Benz et
al. 1967, 1968, 1976; Lewis and Drew 1973; Sommerfeldt
and MacKay 1982) also emphasized the role of the position of
the water table and upward capillary flow in the process.
Notwithstanding the fact that the transport of salts to the soil
surface in response to evaporation and evapotranspiration during the summer months is likely an important cause of saliniza-
tion of Prairie lands, it is postulated that salts transferred by
soil moisture migrating to the freezing front during winter may
also contribute to the problem. Recently, Gray et al. (1985)
reported on a field study of soil moisture changes during winter, normally that period extending from mid-November to
mid-February, in the Brown and Dark Brown soil zones of
Saskatchewan. They measured large upward fluxes of water in
response to the freezing action. In an irrigated silt loam having
a water table at shallow depth, increases in the average moisture (ice) content of 11% by volume were measured; in a
heavy, silty clay under dryland farming, an increase of 6%
by volume was observed.
The phenomenon of moisture migration in response to a
temperature gradient during freezing and thawing of unsaturated soils is a complex problem involving the thermodynamics of the soil -ice-water matrix and has been studied by
many researchers (for example, see Ferguson et al. 1964;
Hoekstra 1966; Harlan 1973; Groenevelt and Kay 1974; Jame
1977; Loch 1978; Mageau and Morgenstem 1980). Despite
the large number of studies that havebeen conducted, a clear
consensus is lacking on the relative importance of the different
modes of transport, i.e., liquid or vapor. Most investigators
have assumed capillary flow, whereas Kay and Groenevelt
(1974) and Nakano and Miyazaki (1979) have demonstrated
the significance of vapor to the total amount of moisture transfer. Knowledge of the mode of water transport is essential to
an understanding of salt movement by soil freezing because the
major amounts of soluble salts will be transported by liquid
flow, with minor amounts of self diffusion.
Only a limited number of studies have been undertaken of
the interaction between water and salt movement during freezing, and very few of these were under field conditions. Cary
and Mayland (1972) worked with small, disturbed columns of
unsaturated soil that were incubated for 3, 6. and 9 weeks
under different temperature gradients and found that water and
salt moved from warmer to cooler areas. They assumed movement occurred primarily through unfrozen films by diffusion
along their concentration and thermal gradients and presented a
theory of water and salt transport that uses the physical properties of a soil in the unfrozen state. Recently, in Japan, Mizoguchi and Nakano (1985) showed that after freezing the electrical
conductivity profiles of a wet soil (43% by weight) that had
been treated with a NaCl solution changed in a manner analogous to the soil moisture patterns. In a dry soil (2 1% by
weight) no noticeable redistribution of moisture or salt was
detected. In a field study on a silt loam, Cary et ul. (1979)
reported increases in electrical conductivity caused by the upward movement of water in response to freezing. Likewise,
Benz et al. (1967, 1976), in studies of soil water movement
and soil salinity changes conducted in North Dakota, alluded
to the potential of translocated water by the freezing process to
increase the total salts in a soil profile.
The study reported herein was directed to the following objectives: (1) to investigate the association between the vertical
distribution patterns of moisture and salt in situ at different
times during the natural freeze cycle; (2) to use the correlation
(item 1) for defining the primary mode of moisture transfer;
and (3) to assess the potential of the freezing process to contribute to the causes of soil salinization.
-
-
Field measurement program
The measurement program was conducted during the winters
of 1982-1983, 1983- 1984, and 1984- 1985 in fields offal-
698
CAN. 1. EARTH SCI. VOL. 23, I986
low and stubble near Outlook and Saskatoon, Saskatchewan.
The sites at Outlook were under border dyke irrigation, and a
water table was present at shallow depth, between 2.5 and
4 m. The soil was an uniform, silt loam to a depth of 1.3 m,
underlain by a sandy loam (see Fig. 1). Both soil and water
conditions were conducive to rapid, large fluxes of water by
capillary movement. At Saskatoon three sites at different locations were installed: site 83 (1982- 1983) was predominantly a
silty clay; site 84 (1983 - 1984) was a silty clay loam; and site
85 (1984- 1985) was a medium-textured loam (see Fig. I ) .
The exact depth to the water table at each location was not
measured, although it is known to be greater than 4 m.
The principal measurements made at each site were soil
moisture, salinity, and temperature at different times during
fall, winter, and spring periods.
Soil moisture
Changes in soil moisture were obtained by monitoring
changes in soil density with a two-probe density meter. With
this method, 50 mm diameter plastic access tubes spaced
304 mm apart (centre to centre) are placed vertically in the
soil. A radioactive source is placed in one of the tubes, and a
detector at the same depth in the other. A count is then made of
the number of attenuated photons striking the detector in a
period of 1 min. The soil density can be calculated from the
reading, knowing the unattenuated intensity of the nuclear
source, the moisture content, and the radiation attenuation
coefficients for soil and water. This method provides nondestructive sampling of the density of a volume of soil approximately 50 mm wide, 250 mm long, and 20 mm (or 40 mm)
thick. By assuming the mass of the soil cube remains constant,
i.e., no major structural changes occur, changes in density can
be attributed to changes in the mass of water contained within
the volume. The equivalent moisture change is calculated from
the density changes, assuming a water density of 1000 kg/m3.
Density measurements were taken at 20 mm increments of
depth to -1 m and at 40 mm increments between 1 and 1.6 m.
Profiles were measured at regular intervals (usually 3 weeks)
during the freezing and thawing periods. Core samples taken at
the time the access tubes were installed were used to establish
the initial readings of soil dry density and moisture content.
In the data presented below, it is considered that errors in
measurement of the moisture change are relatively small compared with the magnitude of the flux. Repeatability tests of the
system(s) in the field gave a standard error of estimate of 2.5
mm of moisture in a 1 m profile. Polyvinyl chloride (PVC)
tubing was selected for the access tubes because it has a thermal conductivity of the same order of magnitude as a natural
soil. Hence it is assumed that under the relatively weak temperature gradients likely to exist between soil and pipe at
depth, the amounts of water that would move either vertically
or laterally to the tubes are small. In addition, there was no
visible evidence of significant frost heave of the soil surface or
ice lens development in the frozen soil cores from any of the
measurement sites. Because of these observations and the fact
that the reading of the surface layer (0-20 mm) was discarded, it was assumed any errors introduced to the calculations by changes in soil density as a result of soil structural
changes were small.
Salinity
Soil cores 60 or 100 mm in diameter were taken at each site
to the depth of soil moisture measurement. These cores were
sectioned to 100 mm lengths, and each sample was tested for
pH, electrical conductivity (saturation extract), and ion content
(Na, Ca, Mg, K, C1, and SO,) by the Soil Testing Laboratory,
Saskatchewan Institute of Pedology. Usually at least three
cores were obtained: in the fall, near the time of maximum
accumulation of ice, and following snowmelt, near the time of
seeding of the annual crop. At least one core was taken directly
between the tubes used for the soil moisture measurements.
Soil temperature
At each site a soil temperature probe equipped with an automatic recorder was installed. The thermistors attached to the
probe provided temperatures at depths of 25, 50, 100, 200,
400, 800, and 1600 mm.
Results and discussion
Water rnovernerlt during freezing
Figures 2 and 3 show soil temperature and moisture changes
on successive dates in the silt loam soil at Outlook and the silty
clay at Saskatoon during the freezing period of the winter
of 1982- 1983. The data illustrate several features of the
dynamics of moisture migration in freezing soils.
(I) In the absence of midwinter intiltration events, moisture
losses from a shallow soil layer adjacent to the surface are
common over winter. The depth of the layer is usually less
than 300 -400 mm, and the moisture loss is attributed to vapor
flow across the soil-air or soil-snow interface caused by
high temperature gradients.
(2) The flux of water migrating to the freezing front is much
larger in the light-textured silt loam than in the silty clay. Gray
et al. (1985) have discussed the major factors affecting the
magnitudes of the fluxes in Prairie soils.
(3) The plane dividing the zone of accumulation of ice and
the unfrozen soil is sharply defined and coincides approximately in depth with the freezing front (0°C isotherm). This
result is in agreement with the findings reported by Jame
( 1977).
(4) The accumulation of ice is increased by the downward
movement of the freezing front and the upward migration of
water into the frozen soil. It is interesting to note in Fig. 26-6
the distance water moved upwards into the frozen soil is relatively small. For example, on November 29 the freezing front
was at a depth of 520 mm (Fig. 2a). Between November 29
and December 15 ice accumulated in the soil layer between
300 and 630 mm (Fig. 26 ), and the part in the 300 -520 mm
increment (shaded area) can be attributed entirely to movement
into frozen soil. For subsequent periods during the advance of
the freezing front, i.e., December 15-30 (Fig. 2c) and December 30 - January 19 (Fig. 26) the distances were 80 and
20 mm, respectively. The decrease in the rate of expansion of
the zone of accumulation in both upwards and downwards
directions with time can be directly related to the decrease in
the temperature gradient at depth. The data indicate that the ice
content of the silt loam rapidly reached a level at which "measurable" migration ceased. The average moisture content
when this occurred as estimated from the soil moisture profile
on January 19 in the depth increment 400-800 rnrn,a layer of
relatively uniform texture that was not significantly affected by
moisture exchanges at the soil surface, was 41 % by volume or
82% pore saturation (Fig. 4a). Stoppage of flow may be the
result of ice crystals causing rearrangement of the soil particles
and separation of the liquid films (Hoekstra 1966) or blockage
of the flow paths by ice-filled pores. Note that there was no
visible evidence of ice lens development in the frozen soil
-
GRAY A N D GRANGER
SOIL TEMPERATURE
SOIL MOISTURE CHANGE
< a > N o v . 17/82
t o Nov. 29/82
<b> Nov. 2 9 / 8 2
t o Dec. 15/82
<OC>
<%-Vol
Cc> Dec. 15/82
t o Dec. 3 0 / 8 2
Cd> Dec. 3 0 / 8 2
t o Jan. 19/83
FIG.2. Profiles of soil temperature and soil moisture (ice) changes on successive dates during freezing of a silt loam at Outlook, 1982- 1983.
The soil temperature profiles are those measured at the end of the interval, for example, in (a) on November 29, 1982; the shaded area is the
accumulation of ice above the position of the 0°C isothem~at the beginning of the measurement interval (identified by the number and arrow).
caused by upward migration of moisture into thc frozen soil.
SOIL TEMPERATURE
-20
0
2 0 -20
S O I L MOISTURE C H A N G E
Co> N o v . 16/82
t o Dec. 2/82
< b > Dec. 2/82
t o Dec. 17/82
<OC>
20
0
<%-Val.
Cc> Dec. 17/82
t o Jan. 13/83
>
Cd> Jon. 13/83
t o Feb. 8 / 8 3
FIG. 3. Profiles of soil temperature and soil moisture (ice) changes on successive dates during freezing of a silty clay at Saskatoon,
1982- 1983. The soil temperature profiles are those measured at the end of the interval, for example, in (a), on December 2, 1982; the shaded
area is the accumulation of ice above the position of the 0°C isotherm at the beginning of the measurement interval (identified by the number and
arrow), caused by upward migration of moisture into the frozen soil.
cores. The above discussion implies liquid flow as the primary
mode of transport and is considered valid based on the findings
of Cary and Mayland (1972) and data provided later in this
paper. The growth pattern of ice in the frozen zone of the silty
clay differed appreciably from that found in the silt loam.
Figure 3 shows that ice continued to be redistributed throughout the frozen soil during most of the freezing period. The
average moisture content when measurable upward movement
could not be detected (taken as the moisture content of the
300 - 500 mm soil layer on February 8) was 44 % by volume or
-91 % pore saturation (Fig. 4b). Although the primary mode
of transport cannot be explicitly defined, the possibility that it
was a capillary flow should not be discounted. Many studies
have shown the freezing-point depression of soil water is inversely related to the radii of the soil pores and that significant
amounts of unfrozen water can exist in fine-grained soils at
700
CAN. J . EARTH SCI. VOL. 23. 1986
FROST
FRONT
MOISTURE CONTENT
<a> O U T L O O K
<X-Vol.
<b>
>
SASKATOON
FIG.4. Profiles of saturation levels, soil moisture (ice) content, and
position of frost front: ( a ) Outlook, January 19, 1983 and (b) Saskatoon, February 8, 1983.
temperatures below 0 ° C (Nersesova and Tsytovich 1963:
Yong 1965; Jame 1972). The amount of unfrozen water also
depends on soil temperature and the type and concentration of
ions adsorbed by the soil particles and (or) dissolved in the
water. These findings support the use of the hydrodynamic
model (Harlan 1973) rather than the capillary model (Penner
1959) for explaining the ice accumulation patterns caused by
soil freezing.
(5) A measurable gradient in soil moisture at depth below
the freezing front developed in the silt loam early in the freezing period. Figure 2b shows that between November 29 and
December 15 water was removed from storage from the soil
layers immediately below the freezing front, and the moisture
gradient established was sustained through January 19, 1985
(see Fig. 2 c and 4. This sudden change in soil moisture
distribution in the unfrozen soil was due to highly, unsteadystate flow conditions caused by a rapid increase in the temperature gradient between December 3 and December 10. After
December 15, the temperature gradient at depths of 800 mm
and below changed very gradually with time and quasi steadystate flow conditions could be assumed.
The above discussions of soil moisture movement assume
sufficient water in the soil to allow the growth of ice crystals
and capillary movement. In a dry soil, moisture changes at
depth during freezing usually appear as alternating, discrete
layers of moisture gains and losses (Gray et al. 1985).
Salt movement during freezing
Figures 5a, 6a, and 7a show pmfiles of soil temperature,
electrical conductivity (saturation extract), and soil moisture
prior to freezing in the fall and changes in these profiles over
winter (Figs. 56, 6b, 7b) and from fall to spring (Figs. 5c, 6c,
7c). The data for Outlook (Figs. 5, 6), for consecutive winters
(1983- 1984 and 1984- 1985), are from two sites spaced
about 3 m apart.
In Fig. 5b it can be observed that by February 9 the depth of
penetration of frost into the silt loam at Outlook was approximately 950 mm, and a significant amount of water or ice had
accumulated above this depth. The figure also shows a corresponding increase in electrical conductivity (hence salt content) throughout a large part of the frozen zone extending
upward to a depth of about 200 mm. The fact that the zones of
salt and ice accumulations tend to coincide supports the proposition that moisture movement occurred primarily in the liquid
phase. The shallow soil layer at the surface (0-200 mm),
however, shows an increase in moisture but essentially no
change in conductivity. This suggests moisture migration in
the layer as a vapor.
Over winter, the average conductivity of the zones of major
ice and salt accumulation increased from 3.5 mSIcm in the fall
to 5.7 mSIcm in February, and the profile changed from being
slightly saline to moderately saline, as a result of the upward
migration of salts (principally sodium and magnesium sulfates). The net increase in soluble ion content (Na, Ca, Mg, K,
C1, and SO4) between November 9 , 1983 and February 9,
1984 to a devth of 1 m was equivalent to 11.9 tlha.
The interaction between salt and moisture transfer during the
winter of 1984 - 1985 at Outlook is less clear than in the previous year because the profiles (Fig. 6) were complicated by
snowmelt infiltration events between November 20 and December 20. The major accumulations of both exchangeable
ions and ice (due to upward movement) occurred between 400
and 1100 mm. A simple mass balance of exchangeable ions to
a depth of 1550 mm using samples taken on November 4, 1984
and March 4, 1985 agreed within 4 . 5 % . One can conclude
fmm these results that the accumulation of ions in this soil
layer (400- 1100 mm) was due primarily to the downward
leaching of salts from the 0-400 mm layer and the upward
transport by liquid flow from the 1100- 1550 mm layer. The
net increase in exchangeable ions to a 1 m depth attributable to
freezing was 3.5 tlha.
During the winter of 1983 - 1984, in the silty clay loam at
Saskatoon, there was an increase in moisture content in the
150-600 mm increment between fall and mid-March; however, there was no significant change in conductivity above a
depth of 400 mm (Fig. 7b). It would appear the increase in
moisture above 400 mm was due to vapor movement; the
increases in conductivity below 400 mm, which are primarily
due to accumulations of calcium and magnesium sulfates
(-93% by weight), suggest liquid flow. It can be noted,
however, that although there was a substantial increase in conductivity at depths of >600 mm, there was little increase in
moisture. This may be the result of adsorption of soluble salts
from the migrating water by the clay particles. The total increase in dissolved salts to a depth of 1 m caused by upward
movement due to soil freezing was estimated to be 15.7 tlha.
The above results support several propositions regarding soil
moisture and salt migration in the overwinter period, namely;
(1) soil moisture changes in a profile occur as a result of
moisture transfer in both liquid and vapor phases; (2) liquid
water moving to a freezing front is an effective medium for the
upward migration of soluble salts; and (3) substantial amounts
of salts can be transferred to the crop root zone (1 m) during
the freezing period.
It is reemphasized that significant amounts of water and salts
will only be transferred when the soil moisture content is in
adequate supply to support capillary flow and there is a source
of soluble salts. Of 200 drvland sites in Saskatchewan where
soil moisture changes have been monitored over winter during
the past 5 years. which includes soils ranging in texture from
sandy loam to heavy clay under fallow, stubble, and grass,
approximately 75 % have shown increases in moisture content
in a 300- 1000 mm depth increment because of freezing. As
would be expected, the incidence of significant amounts of
q
*
GRAY ANDGRANGER
CONDUCT IV IT Y
0.0
0. 0
2. 5
<mS/cm>
5.0
7.5
10.
T E M P E R A T U R E <OC>
C O N D U C T I V I T Y CHANGE <mS/cm>
-10
-5
0
5
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T E M P E R A T U R E <OC>
C O N D U C T I V I T Y CHANGE <mS/om>
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a
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50
75
S O I L MOISTURE
(a) NOV.
100
<%-vol>
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-50
-25
0
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S O I L M O I S T U R E CHANGE
(b) NOV.
8/83
50
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t o FEB.
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la
-25
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S O I L M O I S T U R E CHANGE
( c ) NOV.
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<X-vo1>
0 t o MAY 1 4 / 8 4
FIG.5. Profiles of soil moisture, electrical conductivity, and soil temperature in the fall and changes in soil moisture and electrical conductivity
from fall to spring measured in a silt loam at Outlook, 1983- 1984. The soil temperature profiles in (b) and (c) were measured on February 9.
1984 and May 14, 1984, respectively.
CONDUCT IV IT Y
<mS/cm>
S D I L MOISTURE
( a ) NOV.
9
<%-vol>
4/04
T E M P E R A T U R E <OC>
C O N D U C T I V I T Y CHANGE <mS/cm>
S O I L M O I S T U R E CHANGE
( b ) NOV.
4/84
t o MAR.
(X-vol>
4/85
T E M P E R A T U R E c0C>
C O N D U C T I V I T Y CHANGE <mS/crn>
S O I L M O I S T U R E CHANCE
( c ) NOV. 4 / 8 4
<X-v01)
t o MAY 28/05
FIG.6. Profiles of soil moisture, electrical conductivity, and soil temperature in the fall and changes in soil moisture and electrical conductivity
from fall to spring measured in a silt loam at Outlook. 1984 - 1985. The soil temperature profiles in ( b ) and ( c ) were measured on March 4, 1985
and May 28, 1985, respectively.
J
movement is higher on fallow than stubble, because the moisture content is usually higher, and in soils where there is a
water table at shallow depth. Unfortunately, up to this time it
has not been possible to establish "critical" limits of soil
moisture content and its distribution with depth, when measurable amounts of migration can be expected. In these regards,
the variations in temperature gradient with respect to time and
magnitude must also be considered.
Also, one should not automatically assume moisture migration as a liquid causes a transfer of salts. Movement of salts
depends on-the type and concentration of the adsorbed ions,
the exchange capacity of the soil, the mode of moisture trans-
port, and numerous other factors. Figure 8 shows profiles of
the soil moisture change over winter (Fig. 8a) and the electrical conductivity (Fig. 8b) measured in a loam near Saskatoon
(Saskatoon 85, Fig. 1) in the fall (November 9 , 1984), when
unfrozen, and in the spring (March 15, 1985), when frozen
throughout the depth of measurement. Figure 8a shows a general increase in moisture content over winter to a depth of
about 1340 mm. Most of the increase to 140 mm can be attributed to snowmelt infiltration caused by a midwinter thaw; the
increase below this depth was -58.5 mm. Conversely, Fig.
8b shows that the changes in conductivity with depth in the
period are small and generally fall within the repeatability of
C A N J EARTH SCI. VOL. 23, 1986
CONDUCTIVITY
S O I L MOISTURE
TEMPERATURE C°C>
C O N D U C T I V I T Y CHANGE (mS/cm>
(rnS/crn>
(X-vol)
S O I L M O I S T U R E CHANGE
( b ) NOV. 1 0 / 8 3
( a) NOV. 1 0 / 8 3
(%-vol>
t o MAR. 1 3 / 8 4
TEMPERATURE C°C>
C O N D U C T I V I T Y CHANGE (mS/cm>
S O I L M O I S T U R E CHANGE
( c ) NOV. 1 0 / 8 3
t o MAY
(%-vol>
lE/BA
FIG.7. Profiles of soil moisture, electrical conductivity, and soil temperature in the fall and changes in soil moisture and electrical conductivity
from fall to spring measured in a silty clay loam at Saskatoon. 1983- 1984. The soil temperature profiles in (b)and ( c )were measured on March
13, 1984 and May 16, 1984, respectively.
correspond to levels near or above field capacity and would
allow capillary movement; and (3) the increases in conductivity between 1050 and 1600 mm suggest liquid flow.
1
SOIL
M O I S T U R E CHANGE
<%-Val.
<a>
>
2
CONDUCTIVITY
<mS/cm)
Cb)
FIG. 8. Prnfiles of soil temperature. soil moisture change, and
electrical conductivity measured in a loam at Saskatoon, 1984 - 1985:
(a) soil temperature on March 15, 1985 and soil moisture change
from November 9, 1984 to March 15, 1985 and ( b )electrical conductivity on November 9, 1984 and March 15, 1985.
measurements taken on extractions from the same soil sample;
the exceptions are the increased readings in the 1050- 1600
mm soil layer. The lack of association between the salt and ice
distributions suggests either that moisture was transferred primarily as a vapor or that the movement of ions was resisted by
their electrokinetic bonding to the soil particles. The latter is
considered the more plausible cause because (1) the electrical
conductivity, hence ion content, of the soil was low (initially
falling in the range of 0.5-0.7 mS/cm or <65 pg/g); (2) the
soil had an average cation exchange capacity of 15.2
mequivt l00g (3) the-moisture content of the soil profile in the
fall was in the range of 42-60% of saturation, which would
Salt and water movement during thawing
The findings above demonstrate the potential for substantial
quantities of salts and water to migrate in response to freezing.
Automatically, they raise a question with respect to the quantities of these elements retained in the root zone of a crop after
the soil has thawed. On the one hand, water retained in the
zone could benefit plant growth; on the other, salts may lead to
the degradation of yields and eventually to salinization.
Following the disappearance of the seasonal snow cover, the
soil moisture profile of a Prairie soil changes because of the
withdrawal of water from the surface in response to evaporation and evapotranspiration demands, thawing of the soil followed by percolation of water downwards, and infiltration of
precipitation (rain and (or) snow). A frozen soil thaws from
both the top and bottom, with the rate of thaw being highest
near the soil surface. Usually there is no pronounced downward movement of water until the frozen zone becomes isothermal at O°C, and in some years on the Prairies this may not
happen until 45 -60 days following the disappearance of the
snow cover. It is during the early part of the thaw sequence that
the greatest potential exists for those salts, which have migrated during the freezing cycle, to move to the soil surface
because (1) following snowmelt the moisture levels of the
shallow depth of soil near the surface are high; (2) on both
stubble and fallow lands evaporation is high (as compared with
evapotranspiration); for example, Gray et al. (1984) reported
an average ratio of evaporation to the sum of snowmelt infiltration plus precipitation in the postmelt period of 1.31; (3) salts
that have migrated over winter are nearest the surface; and (4)
percolation of water in the frozen profile is low.
Once the soil has thawed, drainage occurs in response to
gravity, and water leaches salts downward. The extent and rate
of removal of salts from the crop root zone are largely a function of the soil moisture content, the drainability of the soil,
t
r
TABLE1. Changes in amount of exchangeable
ions in a I m profile with time in a silt loam at
Outlook, November 4, 1983 - May 28, 1985
Date
Exchangeable ions
(tlha)
Nov. 4, 1983
Feb. 9, 1984
May 14, 1984
Nov. 4, 1984
Mar. 4, 1985
May 28, 1985
aCalculated from the following empirical expression
derived for the soil: EI = -215.37 + 376.3EC +
29.9ECL,when: EI = exchangeable ions (Na, Ca, Mg,
K, CI, and SO,) (pgig) and EC = electrical conductivity (saturation) (mSicm).
and the ion exchange capacity of the soil particles. As shown
in Fig. 5c, in the well-drained silt loam the decrease in electrical conductivity corresponds closely to the decrease in moisture content; in the silty clay loam at Saskatoon, the pattern is
similar, except there is some indication that the exchangeable
ion content in the layer of silty clay (700-800 mm) may have
increased slightly because of over-winter migration.
Table 1 shows the cyclic nature of seasonal changes in the
total exchangeable ion conent in a 1 m depth of the silt loam at
Outlook during the period November 4 , 1983 - May 28,
1985. The data illustrate an accumulation during winter followed by a depletion during spring and summer. The large
decrease of 15.7 t/ha between March 4 and May 28, 1985 can
be largely attributed to leaching by 21.2 mm of snowmelt
infiltration and infiltration from two storms in late April and
early May that produced 89 mm of rainfall. Similarly, the
reduction in salts during the summer of 1984 is attributed to
rainfall infiltration and one surface imgation application from
which it is estimated there was 160- 190 mm of infiltration.
The authors wish to caution the reader on the use of singlepoint data, such as the amounts of exchangeable ions given in
Table 1, as spatially average values, as they are site specific
and based on single-point measurements. Significant moisture
fluxes have been monitored at 20 different sites at Outlook in
the past 5 years, and in some years they differed appreciably
between sites. Likewise, the amounts of salt movement at
different sites may show wide field variability because of spatial differences in the moisture fluxes and other factors affecting the process.
Clearly, however, the findings illustrate the salt regime is
highly dynamic and responds to annual cycling of the soil
moisture regime and that changes occuning during winter are
important to the process. Any model developed either to explain salinization or to forecast soil salinity from soil moisture
migration must consider, in addition to the chemical and hydrophysical factors of the soil, changes in climate, land use,
and other factors (for example, imgation, drainage) with time
throughout the year.
Summary
The paper presents the results of a field study on the movement of water and salts due to soil freezing. It is shown that
large fluxes of water can migrate to the freezing front and
upward into the frozen soil. These fluxes are largest in lighttextured soils having a water table at shallow depth; however,
fluxes can also be substantial in medium- and heavy-textured
soils when there is sufficient soil moisture present to allow
capillary movement. Evidence is presented that shows that the
soil moisture profile developed by the freezing action suggests
coupled vapor and liquid flow. In the surface layer of soil, to a
depth of 300-400 mrn, moisture moved primarily as a vapor;
in the soil below, transfer occurred primarily as a liquid. In the
"zone" of capillary flow the moisture (ice) content of a silt
loam rapidly reached a level of 80 -85 % pore saturation, and
measureable upward migration into the frozen soil ceased. In a
silty clay, movement into the frozen soil occurred over most of
the freezing period, with the ice content reaching 93% pore
saturation.
Soluble salts can be transported upward by water migrating
in response to the freezing action. A close association was
found in the profiles showing changes in ice content and electrical conductivity in a medium-textured silt loam. In a finegrained soil the association was less clear and attributed to the
adsorption of ions from the migrating water by the clay particles. Maximum increases in exchangeable ions of 11.9 and
15.7 t/ha in a 1 m depth, caused by soil freezing, were measured in a silt loam and a silty clay loam, respectively.
The results emphasize the importance of the freezing process
in the transport of salts and the potential for the phenomenon to
contribute to soil salinization and degradation. Further research is needed on the dynamics of the process and the interactions of climate, land use, soil drainability, and other factors
as they effect changes in the salt content of a soil profile with
time.
Acknowledgments
The writers wish to acknowledge the financial support provided the project by the Farmlab Program, Saskatchewan Department of Agriculture; the Natural Sciences and Engineering
Research Council of Canada; and the Water Research Support
Program, Inland Waters Directorate, Environment Canada.
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