Soil water and winter wheat prospects
Parts of Kansas have been in a prolonged, severe drought. Where soils are extremely dry, wheat producers may wonder how much rain will be needed at this point to fill the profile – or at least 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. 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.
* Target amount of available soil water in 4 feet of silt loam soil: 8 inches
* Amount of water infiltrating into the soil profile from a 2-inch rain: 1.5 inches
* Amount of soil water lost to evaporation in the 5 to 7 days after the rain: 0.15 to 0.5 inches