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The (Very Wet) Prairie Soil Moisture Map

We all know 2010 was an exceptional year for rain, especially in parts of Saskatchewan. So when it came time to making the soil moisture map for freeze up 2010, some extra thinking was required.

The Very Dry, Dry, Moist and Wet categories are the same ones I set up in 1978 and have stood the test of time. Note that the categories relate to the depth of the wetting front for various soil textures and the amount of available water that provides. Maps were made by simple soil probes to map the wetting front and using rain records to determine where probing was required. (See the January 24 issue ofGrainews for a discussion on the wetting front.) Grid sampling wasn’t necessary. Grids are for those who don’t understand what they are mapping.

In previous years, rainfall records from about mid-August to November 1 were used to determine where to probe to map the refilling of the soil profile after the crop had sucked up all the summer rain. It was all possible because Saskatchewan’s Ministry of Agriculture had about 250 weekly rain records available in the weekly crop report. No fancy technology needed — farmers dumped the gauge every morning and every week sent the results off to Regina.

But this year was different. In addition to well-watered Manitoba, big chunks of Saskatchewan had an “irrigation” year with no pivots necessary.

When the November 2009 Soil Moisture Map was made this column said: “I have thought for a long time that Manitoba should have a different approach to the soil moisture map question. In the Palliser triangle of Saskatchewan and Alberta we are always holding our breath and looking wistfully at the sky to see if we can sneak out enough rain to grow some kind of a crop. In the Interlake and Red River Valley of Manitoba they would like to have a big umbrella to run off the excess rain. They need a category painted purple which signifies soil profile at Field Capacity Plus and water table dangerously close to the soil surface.”

Well, the summer of 2010 brought rain to parts of Saskatchewan to make us summon Noah with advice on building an ark. It was obvious to me that a new approach was needed. Therefore, to make this map the main data piece used was total rain April 1 to October 31, 2010 and a new category was established.

So in addition to Moist and Wet, I’ve added Super Wet. This category refers to soils where the crop rooting zone has moisture in excess of the normal field capacity and the water table is dangerously close to the soil surface. To map Super Wet I used the areas that had received two feet or more of rain.

Please note that soil and aquifer situations should also be considered in defining Super Wet, not just total rain. In some areas, like Nipawin, Sask., and the Assiniboine Delta, Souris and Oak Lake Aquifers of Man., internal drainage to the aquifer will continue all winter long (the springs will run like mad) and the water table will drop a lot. Where no such internal drainage conditions exist, the water table drop over winter will be small. See the February 7 issue of Grainewsfor info on water tables and wells.

In the information gathering stage of developing this moth, I discovered that Alberta had still prepared a Soil Moisture Map as of November 1, 2010 and it was on the website. The Alberta Soil Moisture Map is a computer model with no field checking — a bit scary — but the map seemed to make sense when rain data was compared. Neither Saskatchewan or Manitoba prepared a map, but I sure think there is something to map and here it is.

In western Saskatchewan and Alberta the total rain was mostly 15 inches or less. If we look back at the map of November 1, 2009 we see that there was very little stored soil water. So it took most of the 15 inches to fill up the profile and grow the crop, with little left over for reserve for next year.

But in most of Saskatchewan and all of Manitoba there was very little room for storage so we see the big surplus as of freeze up 2010.

The most rain was actually the Quill Lakes area of east central Saskatchewan. Anecdotally, I heard of areas with as much as 40 inches of rain, but the biggest official total I can find is RM306 (Invermay) in Saskatchewan with a total of 915 mm (yes, three feet) of rain from April 1 to October 31, 2010.

Not all of eastern Manitoba had two feet of rain, but much of the Interlake and southern Manitoba had a significant rain event from October 25 to 27. In those three days, Arborg had four inches and Fisher Branch 3.4 inches. Before that, rain had time to drain away, but late in the year the ground would be frozen so not a good omen for water intake when snow melt arrives. Manitoba is busy figuring out who gets on the Ark come spring.

To map the moist category (Green) in Alberta and Saskatchewan, I used the areas that had less than 17 inches of rain from April 1 to October 31. The crucial piece of information that gave confidence to the Green zone was a few soil probes in the Kindersley- Rosetown area by John Ippollito of Saskatchewan Ministry of Agriculture. Note that the Green category is defined as including isolated areas of Blue.

In choosing crop insurance coverage levels I have used soil moisture as a guide. When soil is bone dry, I select 80 per cent coverage and when soil profile is fully recharged I use 50 per cent coverage. Maybe those at the opposite end of the scale, in the Grey (Super Wet) category may wish to select 80 per cent coverage.

I keep thinking that some year the digitally savvy young generation will figure out how to make a three province Soil Moisture Map and this old fossil can retire his four coloured pencils (five this year). In 1978, I and U of S colleagues made the very first Stubble Soil Moisture map, and I have been at it ever since.

In preparing this map I discovered that many good things are happening with weather and climate data but some fine tuning is required and a few very serious warts need to be fixed. That will be the subject of a future column.

ACKNOWLEDGEMENTS

I wish to thank the Alberta Agriculture website for the provincial Soil Moisture Map, Daphne Cruise of Saskatchewan Ministry of Agriculture and David Waldner of the Climate Unit of Agriculture and Agri-Food Canada for special maps. A special thanks to John Ippollito, Saskatchewan Ministry of Agriculture at Kindersley who made a few crucial soil probes in that area, and Elaine Moats of Saskatchewan Ministry of Agriulture. Nothing like a few facts to clear up muddy waters — pun intended! Thanks also to Dale Tomasiewicz of Agriculture and Agri-Food Canada, Carberry, Man., Graham Phipps of the Manitoba Water Stewardship and Andy Nadler of Manitoba Agriculture, Food and Rural Initiatives for specific data.

J.L.(Les)Henryisaformerprofessorand extensionspecialistattheUniv.ofSask.and farmsatDundurn,Sask.Healsorecentlyfinished asecondprintingofHenry’sHandbook ofSoilandWater,”abookthatmixesthe basicsandpracticalaspectsofsoil,fertilizer andfarming.Leswillcovertheshippingand GSTforGrainewsreaders.Simplysenda chequefor$50toHenryPerspectives,143 TuckerCres,Saskatoon,Sask.,S7H3H7,and hewilldispatchasignedbookposte-haste

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