Rainfall slowed harvest last week, but no one is complaining. Ninety-five percent of the corn crop has already been harvested and soils statewide desperately need moisture. As a result of the recent rainfall, topsoil moisture levels have improved to 31% very short, 38% short, 29% adequate, and 2% surplus. Subsoil moisture improved and is now rated 60% very short, 34% short, 6% adequate, and 0% surplus.
Subsoil moisture is the first indication of next year’s crop, says Iowa State Extension Climatologist Elwynn Taylor. We’re still facing dry conditions, and there’s a 22% chance that next year’s growing conditions will be as bad those we faced in 2012.
The good news is there’s a 60% chance that conditions will be better in spring 2013. Autumn rainfalls can go a long ways toward replenishing soil moistures. Taylor says 1 inch of rain equates to more soil storage in the fall and spring versus in the summer because (1) not as much moisture is being lost to warm soils and evaporation; and (2) no moisture is being taken up by growing plants.
Another reason why fall rains “goes further” is there are fewer drying winds. For example, the drying rate for April 15 was 0.2 inches and .14 of an inch for Oct 15. It took 5 days to dry 1 inch of water in April and 7 days in October.
Rainfall over a longer period of time, which allows the moisture to soak into the ground, is more advantageous than a hard rainfall that runs off the surface. It takes 2 inches of soaking rain to replace the moisture in one foot of soil, although this varies slightly depending on soil type. During the 2012 growing season, roots used the water down to 8 or even 10 feet in some places, so now we need 16 to 20 inches of soaking rain to replace those water levels. Fortunately, the rainfall we’ve received over the past two weeks have been gentle rains for the most part. We assume 70% of the rain soaks in, so 10 to 12 inches of rain over a long period could conceivably recharge 5 feet of soil.
Thanks to Taylor’s insights, we’re singing the praises of autumn rain. We’re also doing the rain dance because we’re still facing dry conditions here. Hopefully, we’ll be singing the rain sometime soon!