SNOWMELT INFILTRATION TO UNCRACKED, CRACKED AND SUBSOILED FROZEN SOILS
D.M. Gray1, R. J. Granger2 and W. ~icholaichuk~
ABSTRACT
The effects of snowmelt infiltration characteristics of uncracked, cracked and subsoiled frozen soils on soil water augmentation by snow management practices are discussed. It is shown the potential for sig- nificantly increasing infiltration in uncracked or undisturbed soils by increasing snowcover accumulation is limited by their poor infiltration characteristics.
The average amount of infiltration to a naturally-cracked clay can be of the order of 7.5 times the amount to the same soil in an uncracked condition, depending on the snow water equivalent. Ripping a soil to a depth of 600 mm increased infiltration by a factor of 7.
The use of deep tillage practices of ripping and paraplowing for increasing the infiltration potential of frozen soils is reviewed.
'chairman and 2~esearchOfficer, Division of Hydrology, University of Saskatchewan, Saskatoon, and 3~ead,Watershed Research Section, National Hydrology Research Institute, Saskatoon, Saskatchewan. INTRODUCTION
During the past five years the Division of Hydrology, University of
Saskatchewan, has undertaken an extensive field measurement program in the
Brown and Dark Brown soil zones of Saskatchewan on soil moisture changes occurring between freeze-up in the fall and the disappearance of the sea- sonal snowcover. To date approximately 200 sites have been monitored on soils ranging in texture from sandy loam to heavy clay under fallow, crop stubble and grass. The findings from this research (see Granger et al,
1984; Gray et al, 1984; Gray and Granger, 1985) having direct application to the use of snow management practices for increasing soil water reserves in seasonally-frozen soils are:
(1) The average depth meltwater penetrates an uncracked, frozen soil
is approximately 260 mm (standard deviation = 100 mm) and upon infil-
tration the average ice content 0.f the layer, 0-300 mm, tends to a
limit of saturation (L) defined by the relationship: L = 8 + P 0.55(1-8 ) in which 8 is the average moisture (ice) content of the P P layer expressed as the degree of saturation in mm3/mm3 (decimal
fraction).
(2) The amount of snowmelt infiltration (INF) to an uncracked, frozen
soil which does not have an ice lense on the surface or at a shallow
depth is directly related to the snowcover water equivalent (SWE) and
inversely related to 8 . For cases where SWE>INF, the relationship P takes the form:
INF = 5(1-8 )SWEO.~~~ P in which INF and SWE are in mm and 8 is in mm /mm3. Equation 1 has a P correlation coefficient of 0.85. These findings (items 1 and 2) suggest that in uncracked soils the amount of snowmelt infiltration is strongly affected and limited by the ice content, or the available moisture storage capacity of a shallow depth of soil at the time of melt.
Figure 1 shows the family of curves defined by Eq. 1. They serve to demonstrate the dilemma in attempting to obtain substantial increases in soil water reserves through snow management practices on uncracked, frozen soils. It is emphasized that the following discussions concern solely the application of snow management practices for augmenting soil water reserves in amounts greater than may be expected from a "normal" snowcover. They do not consider the importance of an average snowcover in providing some water for crop growth, the role of snow as it protects a soil against wind ero- sion, the insulating effect of the snow on soil temperature extremes or how it serves other beneficial purposes.
The data in the figure show that the increase in infiltration per unit increase in snowcover water equivalent decreases with the amount of avail- able snow water, independent of the degree of saturation of the soil layer,
0-300 mm. However, the rate of decrease increases with 0 . Also plotted in P Fig. 1 are diagonal lines of the infiltration ratio (INFR), the ratio of the amount of infiltration (INF) to the snowcover water equivalent (SWE).
It can be observed that INFR also decreases with SWE for each 0 . In P other words, the data suggest there is a defined depth of snow water which may be accumulated on an uncracked, frozen soil in a given moisture con- dition above which the losses to evaporation and direct runoff per unit increase in SWE exceed the gains by infiltration. This can be demonstrated by making the assumptions: (a) on average a soil has a field capacity of EFFICIENCY . 1.9.8 .7 .6 -5
SNOW WATER EQUIVALENT
Figure 1. Interrelationship between infiltration (INF), snowcover water equivalent (SWE), ice content of the 0-300 mm soil layer at the time of melt (€Ip), and infiltration efficiency (the ratio of INF/SWE) for uncracked, frozen soils. The hatched area repre- sents the interaction for the available moisture range of a "normal" soil. about 50% saturation (8 = 0.50) and a wilting point about 25% saturation P = 0.25), and (b) a producer wishes at least 60% of the snow water to (eP infiltrate, the range in SWE-values which would satisfy these criteria is small (see shaded area of Fig. 1). If one further assumes an average density of snowcover in stubble of 250 kg/m3, the maximum depth of snow recommended for a soil in the dry condition would be about 325 mm; in the wet condition about 125 mm. Given the facts that throughout much of the semi-arid region of the province the average annual snowfall falls in the range 750 to 1200 mm and it is unlikely a producer would harvest a crop at a height less than 125 mm unless unavoidable, the strategy of expecting large increases in soil water replenishement and increased yields on undisturbed and uncracked soils from snow management has serious short- comings. This is not to suggest that stubble management practices, by assuring a reasonable depth of snowcover each year, would not have the effect of increasing the average long-term yield.
The discussions above accentuate the basic proposition of this paper, namely, significant increases in soil water augmentation by snow- melt infiltration to uncracked, seasonally-frozen, soils cannot be realized without altering their structure, by some mechanical or other means, so as to increase the number of macropores which will allow more infiltrating meltwater to percolate deep into the root zone. It is suggested that heavy, lacustrine clays which naturally crack on drying offer a high potential for benefiting from snow management practices.
The paper also discusses the use of two subsoiling practices, a Killefer plow (a deep, chisel subsoiler) and a Paraplow*, for increasing the infiltration potential of frozen soils. Because of the high energy costs associated with a deep-tillage operation designed to alter soil structure at depth, consideration is also given to the important questions: (a)
What are the optimum depth and spacing of the treatment to obtain maximum benefit at a minimum cost of installation? (b) How long will the effects of the structural changes on infiltration last? and (c) What increase in yield can be expected from the treatment? At present the answers to these questions are not known, nor are they likely to be in the immediate future. The discussions provide a cursory treatment of these subjects based on the findings of two years of studies by members of the Division of Hydrology, University of Saskatchewan and the Canada Agriculture
*Paraplow is a trademark of the Howard Rotavator Company. Research Station at Swift Current. They represent the current "state-of-
the-art" of research on the subject by these groups.
EXPERIMENTAL PROGRAM
The results reported herein were obtained from field studies conducted in the Brown and Dark Brown soil zones of Saskatchewan at Bickleigh (Bad
Lake), Kerrobert, Portreeve, Richlea, Saskatoon and Swift Current. The textures of the soils at the different locations were: Bickleigh - heavy, lacustrine clay (63% clay); Kerrobert - glacial clay loam (25% clay) and heavy clay; Portreeve and Richlea -heavy, lacustrine clays (70-74% clay);
Saskatoon - lacustrine silty clay (44% clay) and Swift Current - a Wood
Mountain loam (28% clay). In addition to the data collected on soils in their natural condition, infiltration measurements were also made on subsoiled plots. At Kerrobert, the subsoiling treatment involved ripping the soil with a Killefer plow. This subsoiler has been used extensively in the midwestern states of the USA to break-up hard pans underlying coarse and medium-textured soils to improve drainage. The unit used in the study consisted of a single, wheel-mounted cultivator shank approxi- mately 25 mm in thickness, 200 mm in width and 970 mm in length having a wedge-shaped chisel (approximately 75 mm in width) attached to the bottom.
When pulled through the soil it produces a slit (rip) to a depth of about
600 mm; however, fracturing of soil by the chisel can extend to depths
>800 mm. Because the equipment had only a single shank the rips were on the wheel spacing of the tractor, Q 1.85 m. At Saskatoon and Swift
Current some plots were "paraplowed". The Paraplow has a shank or "bottom" with physical features similar to a moldboard plow. However, rather than inverting a furrow it shears, lifts and fractures the soil leaving a standing stubble or trash cover on the surface. The width and depth of
cut of each "bottom" is usually in the range of 300-500 ma. At Swift
Current the treatments consisted of ripping and "paraplowing" to an
average depth of 300 mm and 350 m, respectively. The subsoiler was operated with three shanks spaced 1-m apart; the "Paraplow" also had
three bottoms which allowed for loosening of a 2-m wide strip of soil in each pass.
Estimates of snowmelt infiltration were obtained from soil moisture measurements made prior to and following (as soon as possible) the disap- pearance of the snowcover. At all locations except Swift Current changes
in soil moisture were monitored with a two-probe gamma density meter.
This system provides non-destructive sampling of a volume of soil approxi- mately 50 mm wide, 250 mm long and 20 mm thick. With the method, 50-mm diameter plastic access tubes spaced 305 rnm apart are installed vertically
into the soil. A radioactive source is inserted in one tube and detector at the same depth in the other. A measurement is then taken of the number of photons striking the detector in one minute. The density of the soil can be calculated knowing the intensity of the source and the attenuation coefficients of gamma radiation for soil and water. By assuming the mass of the soil between the tubes on successive measurement dates remains constant i.e., no major structural changes occur in the soil profile, changes in attenuation of the gamma radiation can be attributed to changes in the mass of water contained in the volume. The
equivalent moisture change is calculated from the readings assuming a density of water equal to 1000 m3/kg. Measurements of soil dry density and moisture content taken at the time the access tubes are installed allow one to establish the moisture profile on any measurement date.
In dry soils, which were initially cracked due to the withdrawal of water for crop growth, it was observed that additional cracking could occur deeper in the soil profile due to dessication by freezing. However, once the soil was frozen the cracks remained stable. Also field measure- ments have shown that movement of water laterally from a crack into the adjacent frozen soil is a relatively slow process and the cracks do not close rapidly. These details are important to the assumption that changes in density monitored by the two probe system in a cracked soil during the period of snowmelt infiltration accurately reflect changes in moisture.
Measurements of moisture changes at depth with a neutron probe also supported the assumption.
Measurements were taken at 20-mm increments of depth to 1 m and at
40-mm increments between 1 m and the bottom of the access tubes. Repeat- ability tests with the equipment in the field produced an error of estimate of about k 2.5 mm in a 1-m profile. The equipment was extensively tested and calibrated to operate reliably to temperatures of -20°C.
At Swift Current the changes in soil moisture were obtained by gravimetric sampling at 300 mm intenrals to a depth of 1.2 m. Six sites for each replicated treatment were established. The dry density of the soil being established from soil cores taken from the sites.
The depth of snowcover was measured routinely and the snowcover water equivalent obtained either directly from nuclear or gravimetric density measurements. Some locations were equipped with a Nipher-shielded precipitation gauge and these measurements were used to update snowcover data. Several sites were also equipped to monitor the ambient air temper- ature and soil temperatures at different depths. RESULTS AND DISCUSSION
Infiltration into Uncracked, Cracked and Subsoiled Soils
Figures 2 and 3 compare the infiltration amounts and patterns in two
soils under stubble in uncracked or undisturbed and cracked or subsoiled conditions. In the figures it should be noted that Figs. 2b and 3b are soil moisture changes obtained from measurements taken across the crack or "rip" and Figs. 2c and 3c are changes measured perpendicular to the rupture at a distance between 150-450 mm. The subsoiling treatment was a
"Killefer" plow and the measurements were made on a plot which was located in the "upper" slope position and protected against flow along the lines
(rips) by plastic liners inserted to a depth of about 1.25 mm. It is
(1) On an uncracked or undisturbed soil the amount and depth of infiltra-
tion are limited. At both Richlea (Fig. 2a) and Kerrobert (Fig. 3a)
approximately 27 mm of meltwater infiltrated to depths of 375 and
185 mm, respectively. The corresponding snowcover water equivalents
at the sites were 78 mrn and 194 mm.
(2) Inf iltration to "cracked" and "ripped" soils is substantially greater
than to undisturbed soil. The increases in amounts to these fractured
soils were 68 mm and 147 nun respectively. It is interesting to note
that the amount of infiltration to the "crack" of 94 m was greater
to interflow between interconnecting cracks or surface runoff from
areas adjacent to the crack entering the opening directly. The
phenomenon is not uncommon in these soils and points out the dif-
ficulty in obtaining reliable estimates of the average soil moisture
content over a field under these conditions.
Only 36 mm of infiltration occurred to the site located adjacent
to the crack (Fig. 2c), an amount comparable in magnitude to that
observed at the "uncracked" site. Most of the water entered the
crack directly, therefore the net increase in infiltration due to
soil cracking over a field would depend directly on the density,
interconnection and physical dimensions of the cracks.
The subsoiled soil, which was "ripped" to a depth of 600 mm,
showed significant increases in infiltration to a depth of 1 m (Fig.
3b). Also, as shown in Fig. 3c, there was a significant moisture
increase in the site adjacent to the "rip" which extended below the
depth of moisture measurement. At the beginning of melt the frost
depth was between 1200 and 1300 mm; at the end of melt, when the
last moisture readings were taken the soil at an undisturbed site
had thawed to a depth of 'L 250 mm. The pattern showing an increase
in moisture change with depth can be attributed to several factors;
the increase in hydraulic gradient, the higher soil temperature and
the longer infiltration or percolation opportunity time at depth.
Table 1 lists representative values of the ratio of the amount of snowmelt infiltration into "cracked" and "subsoiled" soils to the compara- tive amount into an uncracked or undisturbed stubble for different ranges of snowcover water equivalent. For the cracked sites and those subsoiled with the Killefer plow the soil moisture changes were measured across the fracture. In interpreting these data it is important to re-emphasize that the amounts of meltwater infiltration to cracked and subsoiled soils can be either greater to or less than the snowcover water equivalent because most sites were not protected against interflow or direct runoff.
Hence, the magnitude of the ratio can vary widely, as demonstrated by the ranges for the "cracked" soils, which were obtained from 31 sites.
Despite the limitations, the data show some interesting aspects of the in£iltration characteristics of "naturally-cracked'' and subsoiled soils.
They support the subscance of previous discussions indicating the amounts of meltwater infiltration to "fractured" soils are greater than to the same soil in an undisturbed condition. As expected there is a trend for the average ratio to increase with the snowcover water equivalent reach- ing maximum values in the range of 6.5-7.5. This trend emphasizes the need to employ snow management practices in combination with a tillage treatment in order to obtain maximum gains in soil water augmentation.
The fact that the ratios for the "ripped" sites at Swift Current and
Kerrobert differ significantly can be mainly attributed to three factors: the average depth of rip at Kerrobert was 600 mm compared with 300 mm at
Swift Current; different subsoilers having different shanks were used, and the soil moisture measurements made on the plots at Swift Current were taken midway between the rips.
The data in Table 1 also indicate that "paraplowing" also enhanced snowmelt infiltration, but in much smaller amounts than obtained by
11ripping". These findings suggest that "ripping" is a more effective method of increasing the infiltration potential of a frozen soil than Table 1. Ratio of the amount of snowmelt infiltration in a cracked or subsoiled soil to the amount in the same soil in an uncracked or undisturbed condition for different ranges of snowcover water equivalent .
Subsoiled b d SWE~ Cracked 300 nunc 600 mm Paraplowed mm Range Mean Range Mean Range Mean Range Mean < 30 4.5 1.1 1.1-1.9 1.5 30-50 1.2-6.7 2.9 .7 .7 50-70 2.1-7.1 4.1 70-100 2.7-9.4 4.7 1.8 6.9 1.5 100-150 3.0-17.1 7.5 .8 6.3-7.3 6.8 1.2 > 150 4.2-8.5 6.7 a SWE = snowcover water equivalent.
Measurements of soil moisture changes made with the crack located between source and detector.
C Ripped to depth of 300 mm on 1-m spacings; loam soil at Swift Current. Measurements of soil moisture changes taken at 300 ram intervals to a depth of 1.2 m.
Ripped to a depth of 600 mm on 1.85-m spacing with a Killefer plow; clay loam and clay soils at Kerrobert. Measurements of soil moisture changes made with the rip located between source and detector.
11paraplowing". However the finding should be treated with caution since the absolute values of the differences in soil water enrichment between treatments could not be evaluated because of the different moisture measurement procedures used.
Depth and Spacing of Rips
Ripping a soil not only increases the water storage capacity and allows easy entry of meltwater to a frozen soil but also permits the water to penetrate to a greater depth. During the post-melt period most of the water is not lost to evaporation and is retained for crop use.
Gray et al. (1985) have shown that the amount of snowmelt infiltration (INF) is directly related to the depth of penetration (d). Figure 4 is a scatter diagram of these variables for uncracked, cracked and soils
subsoiled with a Killefer plow. As shown by the data the trend is for
INF to vary directly with d. The curve enveloping most of the points is described by the equation: INF = 0.2d. A practical application of the findings is that it provides a simple, direct means for obtaining a rough estimate of the maximum amount of infiltration from measurements of the depth of penetration of the infiltrating water. That is, if following the disappearance of the snowcover a producer were to probe his field and find an average wetted depth (usually the depth to the frost layer) of
200 mm, the maximum amount of infiltration is likely of the order of 40 mm (0.2 * 200). On average, the amount will be closer to one-half this quantity; i.e. INF = O.ld. In cracked fields both the cracked and uncracked parts must be probed and the values weighted in proportion to the relative area each represents so as to obtain an average value for infiltration.
Although the above discussion makes the case for placing the "rips" as deep as possible, the selection of the "optimum" depth-of -installation must take into consideration other factors such as the rooting depth of the plant and the energy required to install the "rips", which will directly affect the cost of the treatment. With respect to the first the main objective would be to obtain an uniform distribution of water through- out the root zone between the lines. Norum and Gray (1970) studied the movement of water from unlined mole lines which were constructed with the
Killefer plow and used for subirrigation. They show that the moisture distribution pattern is strongly affected by the texture and anisotrophy of the soil. In heavy-textured soils lateral movement predominates; in light-textured soils significant movement in the vertical direction can E E 0 Crackod Soil w X Subsoiled Soil * Uncrackad Sol 1
Depth of Penetration. d
Figure 4. Scatter diagram showing the relationship between snowmelt in£iltration (INF) and the depth of penetration of infiltrating meltwater (d) in uncracked, cracked and soils subsoiled with a Killef er plow. occur, in amounts which lead to appreciable losses of water below the root zone. In addition to the effects of the hydrophysical properties of soils, the soil moisture distribution and other factors on moisture movement in unfrozen soils, redistribution in frozen soils is also affected by the soil temperature regime at the time of melt and the thaw process.
Figure 2d shows the moisture changes during the two-week period (Mar.
27 - Apr. 10, 1984) following melt measured adjacent to a crack in the heavy clay at Richlea. On Apr. loth, the soil had thawed to a depth of
700 mm. Three features are evident: (a) a loss of moisture occurred from the surface layer, which can be largely attributed to evaporation; (b) the moisture content of the profile increased by about 22 mm (despite the evaporation loss), which can be attributed to lateral flow from the crack; and (c) redistribution occurred in both lateral and vertical directions; if the crack is considered the source, the relative ratio of lateral to vertical movement would be of the order of 1.5 to 1, suggesting that a frozen soil thawing from the surface favors lateral movement. In summary it can be stated that little is known about the redistribution of melt- water infiltration in cracked and ripped soils during the thaw process and the subject warrants further study.
With respect to energy requirements, Norum and Gray (1970) found the energy required to rip a soil with the Killefer plow can be directly related to the cube of the depth of installation, independent of soil texture. The finding suffices to conclude that the lines should be placed at the shallowest depth required for good infiltration and mois- ture distribution.
Despite the complexity of the problem the data in Fig. 3 can be used to obtain a first approximation of the depth and spacing of the "rips1'.
Assuming a hypothetical root-zone depth of 1 m, the results (Fig. 3c) suggest that the "rips" should be placed at a depth less than 600 mm, perhaps in the range of 400-500 mm. An initial guesstimate of an approp- riate spacing of the lines for clay loam and clay soils, based on Fig. 2c and analyses of soil moisture changes on cracked and subsoiled soils at other sites, suggest a range between 1 and 1.5 m.
It is recommended that the lines be placed on the contour or as near as possible. In the first year of the treatment the surface water reten- tion capacity between the lines, is increased as a result of extrusion or uplifting of soil during construction, in the range 130-200 nun. Durability
As stated earlier, an important factor which will govern the accept- ance of any deep tillage treatment for enhancing the snowmelt infiltration potential is the "life-expectancy" of the treatment effects. The authors are unaware of any field data on this subject for frozen ground and the following discussion is limited to a review of visual observations and other field measurements made in the past two years. Figure 5 compares snowmelt infiltration in the spring of 1985 to an undisturbed clay loam with that measured at an adjacent site which had been subsoiled with a
Killefer plow in the fall of 1983. It is evident from these data that some of the beneficial effects of the subsoiling treatment on the infiltra- tion potential were retained over the eighteen-month period following construction. The snowcover water equivalent on the undisturbed soil was
194 mm and on the "ripped1' stubble 89 mm, whereas the amounts of snowmelt infiltration at the two sites were 27 and 95 mu, respectively. The infiltration patterns to 1st and 2nd year lines (Figs. 3b and 5b) are difficult to compare because of the large difference in snow water at the two sites. That is, the average snowcover water equivalent on the lines constructed in the fall of 1984 was 185 mm, almost double the 89 rum covering the lines installed in the fall of 1983. This difference would account for the differences in the amounts of infiltration at the two sites and in part would explain the greater depth of penetration of meltwater in the newer lines. In addition, it was visually evident that in the eighteen-month period since construction the rip had closed along its depth and there was consolidation of the "fractured" soil at depth.
Both factors would reduce the hydraulic conductivity of the older line. Soil Moieture Change <%-Val.> Undi mturbod Submoi led 1883 a>
Figure 5. Soil moisture changes due to snowmelt infiltration in an undis- turbed and subsoiled clay loam at Kerrobert, Saskatchewan, dur- ing spring of 1985. (a) Undisturbed and (b) Subsoiled, 1983: line installed with a Killefer plow to a depth of 600 mm in the fall of 1983; rip was centered between source and detector.
It was also observed during installation of the plastic barriers used to control interflow on the plots, that the 2nd year rips pulled
into a heavy clay, which was located in the lower slope position, had completely collapsed. This failure was expected because of the low shear strength of these soils, especially when they are wet. In any event it is unlikely that "ripping" practices would find wide use under similar
soil and topographic conditions.
Another feature noted in the field, especially in a dry summer such as experienced in 1984, was the "rips" tended to promote soil cracking.
These cracks, which ran in both transverse and parallel directions to the rupture would further enhance the infiltration potential of a frozen soil.
On the paraplowed plots at Swift Current a check of soil densities before and after treatment indicated a reduction from 1260 to 1060 kg/m3. Eighteen months later (after two winters) the average density of the plots
had increased to 1140 kg/m3. From two years of data it was observed that
the practice, when applied between strips of tall standing stubble, did
not appear to reduce the depth of snow trapped. The practice did appear
to have a beneficial effect on enhancing snowmelt infiltration on two
plots under different snow management treatments. Infiltration measured
on 2nd year "paraplowed" plots was 54 IIIIR and 160 rnm compared with 48 mm
and 129 mm on the respective "check" plots.
Grain Yield
Yields of spring wheat taken at Kerrobert on parallel, adjacent, 0.1
ha plots in 2nd year stubble were 1143 kg/ha on undisturbed stubble and
1412 kg/ha on the subsoiled area. Both areas received the same fertilizer
treatment. A grain sample taken from an adjacent field seeded on fallow
yielded 1614 kg/ha.
SUMMARY
The paper summarizes the results of field studies on the snowmelt
infiltration characteristics of frozen soils and their application to the
use of snow management practices for increasing soil water reserves. It
is demonstrated that the poor infiltration characteristics of uncracked and undisturbed frozen soils put in question the strategy of expecting
large increases in soil water from stubble management practices. For
these soil conditions the amount of snowmelt infiltration is directly related to the snowcover water equivalent and inversely related to the
soil moisture (ice) content of the 0-300 mm soil layer at the time of melt. Based on an empirical relationship between these variables and
assuming a hypothetical soil having a wilting point and field capacity of 25% and 50% of the porosity respectively, and a 60% infiltration efficiency, the depth of snow recommended for the dry condition is of the order of 325 mm and for the wet condition 125 mm. Snow covers of greater depth would likely result in the losses to evaporation and runoff exceeding infiltration.
Data are presented which show that "naturally-cracked" heavy clays and "ripped" subsoiled soils offer high potential for soil water augmenta- tion by snow management practices. On average, water intake to the frac- tures was in the order of 6.7 to 7.5 times the amount of infiltration to undisturbed stubble, depending on the available snowcover water equivalent.
The need for combining a snow management practice with a subsoiling prac- tice to obtain optimum soil moisture recharge by the tillage operation is emphasized.
A preliminary assessment, based on two years of data, of the depth, spacing and durability of rips is provided.
The practice of "paraplowing" for increasing the infiltration poten- tial of seasonally-frozen soils is discussed. Although not as effective as deep ripping, this tillage operation increased infiltration in the 1st year in the range of 1.1 - 1.9 times that to an undisturbed soil.
ACKNOWLEDGEMENTS
The writers wish to acknowledge the financial support provided the study by the Farmlab Program, Saskatchewan Department of Agriculture;
Natural Sciences and Engineering Research Council; and Water Research
Support Program, Inland Waters Directorate, Environment Canada.
REFERENCES
Granger, R.J., D.M. Gray and G.E. Dyck. 1984. Snowmelt infiltration to frozen Prairie soils. Can. J. Earth Sci. 21(6):669-677. Gray, D.M. and R.J. Granger. 1985. Snow management practices for increas- ing soil water reserves in frozen Prairie soils. In Watershed Manage- ment in the Eighties (E. Bruce Jones and Timothy J. Ward, eds.) Am. Soc. Civil Eng., New York, NY. pp. 256-263.
Gray, D.M., D.I. Norum and R.J. Granger. 1984. The prairie soil moisture regime: fall to winter. In The Optimum Tillage Challenge (G. Hass, ed.) Proc. Sask. Inst. Agrologists, Univ. Sask. Printing Services, Saskatoon, Sask. pp. 159-208.
Norum, D.I. and Don M. Gray. 1970. Unlined mole lines for irrigation. Trans. Am. Soc. Agr. Eng. 13(5):661-663, 668.