Soil water and winter wheat prospects

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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

* Net amount of water remaining in soil after a 2-inch rain, followed by 5 to 7 days of no rain: 1.0 inch (if 0.5 inch of evaporation) to 1.35 inch (if 0.15 inch of evaporation)

* Number of 2-inch rainfall events occurring every 5 to 7 days needed to reach the target of 8 inches of available soil water: 6 (if 0.15 inch evaporation) to 8 (if 0.5 inch of evaporation)

Total amount of rainfall needed to fill the profile of a silt loam soil: 12 to 16 inches, occurring in 2-inch events every 5 to 7 days over a 6-week period (12 inches if 0.15 inch of evaporation per rain or 16 inches if 0.5 inch of evaporation per rain). This assumes a rather optimistic infiltration efficiency of 75 percent.

Coarser-textured soils that have little to no available water will also need considerable rainfall to fill the profile. A coarser-textured sandy loam soil has a smaller available water holding capacity (about 1.5 inches per foot of depth) than the loam, silt loam, and silty clay loam soils. So it takes less water to fill the profile of a sandy loam soil with available water than it does a silt loam soil. With our example for the silt loam soils, we gained about 1 inch per rainfall event if 0.5 inches of evaporation or about 1.35 inches per rain if 0.15 inches of evaporation. Assuming similar conditions for the sandy loam soil, to fill the sandy loam soil profile to the 4-foot depth would require about 9 inches of rain if 0.15 inches of evaporation after each 2-inch rain or 12 inches of rain if 0.5 inches of evaporation after each 2-inch rain. 

Relative importance of available soil water and in-season precipitation

It is unlikely the dry areas of Kansas will receive sufficient rains during the next 6 weeks to fill the soil profile with available water for this year’s wheat crop. A full soil profile at planting time is not required for a decent wheat crop. However, increased available soil water at planting does improve greatly the odds of getting a good wheat crop. In-season precipitation and available soil water at planting are both important in determining the ultimate yield of a wheat crop.

click image to zoom The following table is based on results from 30 years of research data collected at the K-State Southwest Research-Extension Center at Tribune. The wheat yields listed were calculated from equation 3.5, table 3, page 1361 of “Yield—Water Supply Relationships of Grain Sorghum and Winter Wheat”, L.R. Stone and A.J. Schlegel, 2006, Agron. J. 98:1359-1366. Wheat yields were calculated in response to both available soil water at emergence and total in-season precipitation.

In the above table, keep in mind that 2 inches of available soil water is equivalent to having moisture to a depth of one foot in a silt loam soil, since a silt loam soil holds about 2 inches of available soil water per foot. Likewise, 4 inches of available soil water means a silt loam soil is moist to a depth of 2 feet. In a sandy loam soil, 2 inches of available soil water would be moisture to a depth of roughly 1.33 feet.

The chart shows the influence of available soil water and in-season precipitation at producing long-term yield results. Having water in the fall is critical for germination, emergence, stand establishment, and vigor. Precipitation during winter is closely related with yield potential, providing for winter survival and increased soil water at the beginning of spring regrowth. Water in spring is normally most effective at increasing wheat yields if received at about boot through head extension, providing for decreased water stress at flowering and grain development.

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