When we made the first-ever Soil Moisture Map in 1978, it was early days in the quest to get rid of as much summerfallow as possible. Those with Henry’s Handbook can check out page 109 to see that first map.
The legend has stayed much the same. The objective of the map was to let farmers know where stubble was already full of water at freeze-up and to suggest practices for soils that were very dry (red) or dry (yellow). For the very dry category, we said, “Stubble cropping is a high risk unless all possible snow conservation measures are implemented and/or growing season rain is high.” For the dry category, we said, “Snow conservation measures should be implemented.”
Which brings us to our “Snow Job.” The November 1, 2020, Soil Moisture Map shows a big swath of very dry (red) through much of south and central Saskatchewan and southeast and east-central Alberta. The dry category extends all the way into Manitoba.
At my Dundurn farm near Saskatoon, the last recorded rain of any account was July 8, 2020. In years gone by, when I started to grumble about drought, my good neighbour Henry Block (1940-2011) always said, “A drought always ends with a rain.”
However, in 2020, the drought actually ended with a snow — a big snow — and blow on November 7-8. It was over a foot of snow and it fell on completely dry topsoil. That got me thinking about snow and what it might do for us.
The big snow of 2012-2013
I was also reminded of the big snow year we had in the winter of 2012-2013. Late winter snowstorms left supergrid roads blocked, requiring our municipality to obtain a rotary snowplow to open roads.
That snow in my asparagus patch (see photo at top) would have been about the equivalent of five inches of water — enough to raise the water table by about four feet. By 2015, the water table was at two feet in the asparagus patch and I seeded the inter-rows to barley to use up water.
Snow as a water source for crops
The effect of snow on spring soil moisture has been a subject for field study since the 1940s. When we started preaching extended crop rotations and getting rid of summerfallow, the issue of snow was top of mind.
We dug out old data from the 1940s at the Scott Research Farm that showed some advantage to snow management. I recall a short course at Kindersley where I presented some of that data and suggested farmers should try out the idea. A prominent farmer quietly informed me, “Out here, we do not get much snow.” As years rolled on, and I pounded winter pavement to short courses in southwest Saskatchewan, I realized he was right.
In addition to the older literature from the 1940s to 1980s, I now also checked out Google for recent data and soon had a very recent MSc thesis from the University of Calgary.
What did the many years of research say?
There were many experiments regarding ridging or V-plowing snow in early winter to catch snow as it blew by to provide more water for the next crop. Some of those experiments happened using old, open tractors, so it was a cold, miserable job. While there were a few success stories, for the most part it did not work.
Long stubble and variable-height swathing
Experiments from the Scott Research Farm found standing stubble in a continuous crop rotation did provide enough extra soil moisture to reduce the risk of continuous cropping. PAg or CCA keeners might want to check out Kirkland and Keys, 1981, Canadian Journal of Plant Science, Vol. 61, pages 241-246. If you have a canola field with 12-plus inches of stubble, that is probably the best snow catcher for this year.
The practice of variable-height swathing was also effective at trapping snow, but it was never widely practiced.
Effect of soil moisture at freeze-up on snowmelt
This is the key piece of information relevant to our big, very dry area as of freeze-up 2020.
A good study on this topic was from Mandan, N.D., on a fine sandy loam soil. They found more snowmelt infiltration and less runoff when the soil went into freeze-up dry. If there were fall rains or freeze-thaw cycles through the winter that wet up the surface to a foot or more, runoff increased and occurred early in the spring.
Speaking of runoff, it had long ago been my observation that even in fairly flat fields the sloughs seemed to fill up even before the snow had all melted. I went out to a field near Saskatoon I had combined for many years, so I knew the lay of the land. It was early spring and there were still six to 12 inches of snow over most of the field.
I selected an area and cleared away the snow to reveal the frozen topsoil. With a special soil auger that would penetrate the frost, I ended up with a hole about six inches deep. Within a short time, the hole filled up with water. The snow was melting and the water was running under the snow and on its way to the nearest slough.
Several other studies in west-central Saskatchewan in the 1980s also concluded dry soil at freeze-up was the big variable that allowed snow to contribute to the next crop.
Fast-forward to 2017
A Google search of snowmelt infiltration led to a recent MSc thesis from a student at the University of Calgary who also dealt with snowmelt infiltration. It was a hydrology study, but the principles are the same. It also showed dry surface soils were a prerequisite to rapid snowmelt infiltration and, especially, where large pores were present.
Large pores can be from roots, earthworms and cracks, especially in heavy clay soil. Zero-till is also important, as tillage will disrupt the large pores.
Farmers on heavy clay soil know all about cracks in dry years. They let a lot of water in, but usually not over a big percentage of a field.
When it comes to getting some water for a crop from snow, common sense prevails. If a soil is dry at freeze-up and stays that way, snow might be a big help. That is the situation we have so far in much of the very dry areas.
In hilly land, like my Dundurn farm, much of the snow ends up in the bush around sloughs and the farm shelterbelt. To find out what has happened, it is necessary to use the soil probe to follow the wetting front after the snow is gone. A soil moisture probe (half-inch rod) will do to determine how far down the wetting front is, but caution is needed. I always use the Backsaver Probe or a soil auger to be able to distinguish dry soil from frozen soil.
Farms where agronomists have installed and interpreted soil moisture probes for the 2020 crop will know what the situation was at the time the probes were removed. However, soil probing should be done in spring, so you can determine what water-driven yield is available as the seed and inputs go in the ground.
In some situations, snow could well add enough to ensure good germination and hold the crop until big water use starts in June, but above-average rain will be needed to get an average crop. Beware the other side of the average.
At the time of writing (December 21, 2020), the drought has returned. We had the big snow and blow November 7-8, but very little since then.