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Soil Water Losses
The rate of seedbed water loss when daytime temperatures reach the seventies is quite high. As comparative data a wet soil surface will lose water at a rate approaching evaporation from an open pan. A maximum air temperature of 40 F has a pan evaporation of .20 inches. While an air temperature of 78 F has a pan evaporation of .40 inches of water per day. In a soil with 2 inches of available water holding capacity/ft. these losses would be the equivalent of all the available water from 1.2 and 2.4 inches of soil.
Fortunately, seedbed water loss slows as the surface dries. It reduces to .02 -.05 inches/day over 2-3 days when a dry layer thick enough to form a protective cover develops. The dry soil depth necessary to retard water loss will vary with surface conditions. A quarter inch soil crust and three quarters inch of packed seed cover can have about the same moisture loss retarding effect. A .05 inch/day loss in a silt loam extrapolates to drying .3 inch of a packed soil covering a seed row. Technically this means you can seed two inches deep in a high moisture seedbed and have the seed laying in dry soil in 5 to 7 days.
Soils vary in water holding capability. Loam soils hold about 2.25 inches of plant available water per foot of soil. Three feet of dry soil can hold nearly 7 inches of water. This water is vital to good yields. Since not all rain is absorbed by soil, more than 7 inches of rain or snow is needed to wet 3 feet of dry soil. Long term average precipitation from August 1 to May 1 is 8.5 liquid inches including 2.5 inches in the form of snow.
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