Soil water and winter wheat prospects
* 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.
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