Snowmelt Infiltration to Uncracked, Cracked and Subsoiled Frozen Soils

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 infiltration 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 available 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 condition 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 <rnrn> 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 snowmelt 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 approximately 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.

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