Soil water and winter wheat prospects
Parts of Kansas received up to 6 inches of rain on August 24-26, while other parts missed most of that rain. All areas remain in a prolonged, severe drought. Where it rained several inches, how deep into the profile will moisture go? Where it did not rain, or did not rain very much, how much precipitation will be needed at this point to provide enough moisture to make a wheat crop.
Filling the profile with water
Most soils in central and western Kansas are loam, silt loam, or silty clay loam in texture. In general, soil profiles of these textures have potential to hold about 2 inches of available water per foot of soil depth. A 4-foot profile will hold about 8 inches of available soil water.
To fill the profile to that depth will take more than 8 inches of rainfall, however. If you follow the math, you might conclude that a 6-inch rain would moisten a loam, silt loam, or silty clay loam soil to a depth of 3 feet. But not all the rain that falls gets into the soil because of runoff. And not all of the rain that infiltrates the soil remains there because of evaporation, transpiration from weeds, or drainage as the profile becomes wetter.
As a general rule, about 80 percent of the first inch of rain gets into the soil and remains there. The next inch of rain in a single rainfall event is a bit less efficient. In a 2-inch rainfall event, about 1.5 inches of water could typically be expected to remain in a silt loam soil – about 75 percent intake efficiency. This is under reasonably good soil surface and rainfall conditions.
Runoff is affected by many conditions such as soil roughness, residue cover, soil surface sealing, rainfall rate and amount, soil slope, soil texture, soil compaction, and initial soil water content. If surface runoff is increased by those negative factors, then the infiltration efficiency would be less than the 75 percent value for the 2-inch rain.
Evaporation will work to deplete the soil of water after a rainfall event. In the 5 to 7 days after a rainfall event, total evaporation would likely be from about 0.15 to 0.5 inches – with evaporation being increased by certain conditions, such as tillage, reduced residue cover, high temperature and wind speed, and low humidity. If weed growth is present, that will obviously further reduce the stored soil water.
Using those general figures, here’s how much rainfall it would take to fill the profile of a loam, silt loam, or silty clay loam soil that is at the lower limit of available soil water to the 4-foot depth, using an example of 2-inch rains occurring at 5 to 7 day intervals.