These rules of thumbs could have been included in my book, Henry’s Handbook of Soils and Water.
Rule 1: Water and nutrient use by dept
A while back Gordon Kent, Riverhurst farmer and former winter instructor in Agricultural Engineering at the University of Saskatchewan asked me why Henry’s Handbook did not have the 40, 30, 20, 10 rule of thumb. I was sure it had been included but as usual Gordon was right.
It goes this way: if we separate the crop rooting depth into quarters the percentage of total crop water and nutrient use will be: 40 per cent, 30 per cent, 20 per cent and 10 per cent for the quarters.
If we assume a rooting depth of four feet, the water use will be:
- 40 per cent from the first foot;
- 30 per cent from the second foot;
- 20 per cent from the third foot; and,
- 10 per cent from the fourth foot.
Rule 2: Water table rise from one inch of rain
When a soil is at field capacity moisture content from the surface to the water table and more rain falls the result is a rise in the water table.
But, by how much you may ask?
A one inch rain on a soil at field capacity will raise the water table as shown in the table.
For many of our good ag soils, and additional inch of rain will bring the water table up about eight inches.
In 2010 at my Dundurn farm, we had at least 10 extra inches of rain, which would bring the water table up by about seven feet. No wonder the combine was getting stuck. By the way, when you pull out a stuck combine and see water in the ruts, that is the water table at that location.
Chapter 6 of Henry’s Handbook explains how it all works and Figure 6.1 on page 107 shows it in living color. What is missing is the above rule of thumb. So, here it is — sorry it is a bit late. †