Before you look at the map take a close look at the legend. This year’s soil moisture map is made the same way as the map you saw in these pages last year.
This year’s map is based on the depth of moist soil — that is, it’s a map of the “wetting front” as fall rains proceed, or not!
The depth of the wetting front varies with soil texture. In the early stages of fall rains, all soils have the same available water. Sandy soils “fill up” first, but when all soils are “full” clay will hold about twice as much moisture as sandy soil.
The Stubble Soil Moisture Map as of freeze-up (November 1) is made by first mapping the fall rain.
Stubble soil moisture map
I started making this map in 1978. It was possible only because Saskatchewan Agriculture had a network of Crop and Weather reporters in each of the almost 300 RMs. They dumped the rain gauge each morning and sent in a report each week. Not high tech, but it worked.
I made the first map because I noticed that one probe in a quarter section was as good as a hundred probes. I made a preliminary map based on rainfall for a start date selected each year. That map was “truthed” by soil probing only where there was uncertainty.
The assumption has been that the crop used all that was available and we started each fall with a clean — dry — slate. I always thought Manitoba was a special case, and I had trouble relating to the large soil water values measured in Manitoba.
In Saskatchewan, mapping the wetting front worked well. But in the past three years most of Saskatchewan has had “irrigation” years from rain alone. Since 2009, pivots I drive by at Dundurn have been moved mostly to farm under.
David Waldner of Agriculture and Agri-Food Canada (AAFC) kindly provided the map of cumulative rainfall for April 1 to October 31 for 2010, 2011 and 2012. Areas from Swan River, Man., to Yorkton, Sask., have had five feet of rain.
That map is quite accurate in Alberta and Manitoba where there are lots of stations — less so in Saskatchewan where there are fewer automatic recording stations. That limitation is being overcome with the inclusion of Weatherfarm stations.
In addition to high rainfall we have also had a lot of snowfall, except for 2011-12. The excess water, above that needed by crops, goes to raising the water table. If the water table comes within the root zone, the crop lives off the capillary (suction) zone above the water table. I have observed that with soil probing on my Dundurn farm the past three years. See my column on “sub-irrigation” in the February 11, 2013 issue of Grainews.
You will notice that in the Meadow Lake area of northwest Sask., I have added a fifth category — “super wet.” In this area, the water table is within the root zone. Stuck combines at harvest are a good measure of that. And, there may be local areas elsewhere that have the same condition.
Generally, I have less confidence in this map than other years’ maps, but the general pattern should provide a framework for thought.
It takes a village
My sincere thanks to the sources of information for this map:
- the Alberta Agriculture website for their soil moisture map;
- Ken Panchuk, Saskatchewan Agriculture for the knowledge shared;
- Shannon Friesen, Saskatchewan Agriculture, Weyburn, for accessing specific maps from Sask Geomatics;
- Marla Riekman, Manitoba Agriculture, Food and Rural Initiatives, for the soil moisture map and rainfall map she provided in a poster at the Manitoba Agronomists Conference; and,
- Last, but not least, thanks to Dave Waldner, AAFC (National Agroclimatic Information Service), Regina, for the three-province three-year cumulative map included with this article, and for other special maps I request from time to time. Dave is quite adamant that he is only one cog in a very big wheel, but he always treats me with respect and provides anything I request. I have great respect for the young digitally “hip” folks that can do things this old fossil can only dream about. †