How Much Rain Do We Need to Catch Up?

Average rainfall in central Illinois is about 36 inches.  At least for now we are below average.  So how much will we need to go into next year in good shape?  Do we need to make up all we are behind this year? 

This simplified Water Budget from US Geological Survey shows what happens to 36 inches of rainfall in a year.  The illustration says 25 inches is lost to evaporation, but that is not exactly correct.  Actually most of that is evapo-transpiration, or what is used and lost by plants.  Oddly enough, that is just about what it takes to grow a good corn crop in a year.  Very little water evaporates once it gets into the soil.  The summer fallow system of raising a crop every 2 years in the west illustrates that point pretty well.   If the soil is saturated, water moves into the ground water.  The USGS illustration says about 3 inches of water does this.  The rest is lost to runoff, but we only get runoff if rain falls very fast or if the soil is saturated.  Other sources of runoff are impervious surface like roads and roofs.

So how much water do we need to recharge the soil?  I calculated how much water a good soil in my area like Herrick can hold.
 

Herrick Available Water
Depth	H2O/in.	in.
7	0.24	1.68
8	0.24	1.92
20	0.17	3.4
37	0.2	7.4
72		14.4	Inches of water

Most of the good soils in Illinois can hold about the same amonnt as Herrick.  Some a little more and some a little less.  So we need around 15 inches of water to recharge our subsoil and be ready for next year's crop. Average rainfall is 3 inches per month.  some months more and some months less, so about 5 months of average rainfall will help a lot.  We are a little more than 7 months from planting so we could get more rain than we need for next crop year if we are just average.  Keep in mind that 15 inches of rain would put us about 10 inches shy of what is needed for a good crop.  That is why we need rainfall during the growing season as well. 

Now if you need ponds filled or wells recharged, you need more water than the crop will need.  A few months a little above average would be good. In 1989 it took us until about mid-April to get tile flowing and ponds filled again.  Every little bit helps, but we need to be patient as things might look a bit dry all winter.