At my Dundurn farm I now have three years of records of the water table level in my asparagus crop in the yard and at two locations in the annual cropped field. Now, you may wonder why I would bother you with asparagus data. I use my asparagus patch as a surrogate for perennial forage crops like alfalfa and to some extent for longer-season corn.
It is all about crop water use and the calendar. Most of the annual crops we grow have serious water use for only a few weeks — usually from mid June to late July. Alfalfa uses water from snowmelt to freeze-up. My asparagus patch starts sucking hard about mid June and carries on until near freeze-up.
It is all about a dry layer — or not. If there is no dry layer from soil surface to the water table and no crop sucking water then further rain will raise the water table.
The theory says that if there is no dry layer, an additional inch of rain will raise the water table by:
Six inches for a sandy soil,
Eight to nine inches for a medium textured loam/clay loam soils,
12 inches (a foot) for clay and heavy clay soils.
What I have learned in the past three years is that the theory works very well in practice. If I knew the soil was already full and rain was imminent I made sure to measure the wells just before the rain and a day or two after the rain. The theory was proved correct.
The past eight years
With the exception of 2015 and 2017, we have had rain enough for crops and sometimes excess to raise the water table. The 20 inches of rain in 2010 was 10 inches more than needed, which in theory would raise the water table by more than six feet.
In 2015 there was very little rain in May or June. By end of June my asparagus was growing like a horse so I decided to soil probe to see if watering was needed. When I found no dry soil I concluded the water table must be high so installed a well. Sure enough the water table was 1.8 m.
When the water table gets within a couple of metres of the soil surface, the soil will suck water up from the water table so some plant roots can access that water. This is Mother Nature’s irrigation from the underground. That process has saved our bacon in two of the last four years (2015 and 2017).
In years with little or no soil moisture reserve, 2015 and 2017 would have been complete wrecks.
Where are we now?
After the pea harvest in 2017 we had about three inches of rain. In the field, soil probing has shown that there is no significant dry layer even on hilltops and in about one-third or more of the field, the water table is close enough to be a factor. The graph below shows the water table measurements of a low elevation area for 2017.
With the water table at 1.5 m, water should not be a problem at that location in 2018. With much more snow the problem will be too much, not too little.
But, for the asparagus patch (read alfalfa) the well went dry in mid September (see graph below) and did not bounce back with 1.3 inches of rain in October. It will be interesting to probe in spring to see how far the snowmelt has wet the soil.
Areas with high water tables
As long ago as 1882 it was known that the Winnipeg-Portage la Prairie area of Manitoba had high water tables. John Macoun’s book, Manitoba and the Great Northwest, 1882, lists 48 shallow dug/bored wells in that area with water levels mostly from six to 16 feet.
If your area had many old shallow dug or bored wells, it is likely that the water table could play a role in keeping your truckers busy in the fall some years. As this column has stated ad nausea: water in the ground is like money in the bank. If that water includes root access to the water table it is like irrigation without the pivot.
Water in the ground is a fact not a probability. In farming we deal mostly with probabilities so we should grab a fact when it comes along.
Do you know where your water table is? Perhaps agronomists should be adding that service to their lists along with all the other high tech goodies.