The permeability rate is a measure of how fast applied irrigation water moves through the soil. If an irrigator applies water at a rate exceeding the ability of the soil to accept it, the water will pool on the surface and eventually will run off, causing a non-uniform irrigation application and soil erosion. The major factors affecting permeability rate are texture and density. As long as the irrigation application rate does not exceed the permeability rate, runoff will not occur.
Heavier Coachella soils are the only soils that may be subject to runoff at typical landscape sprinkler application rates (0.5"-2.0"/hr). This problem is more severe on compacted and sloping areas. Management of this problem is particularly important during the initial seeding and subsequent overseeding because the runoff will carry the grass seed away. The best response to sprinkler irrigation runoff is to break up the sprinkler runtime when irrigating areas exhibiting slow permeability rates.
The soil infiltration rate is a related measure that determines the rate at which the soil surface accepts water. The infiltration rate is a dynamic variable that initially changes rapidly over time. Soils usually accept water rapidly at first and then at a reduced rate until the permeability steady state rate is reached. The major factors affecting the infiltration rate are soil texture, plant cover and organic matter, thatch, compactness and slope.
The amount of available water held in the active root zone is an important factor in determining an efficient irrigation schedule. If irrigation water is applied in excess of the water holding capacity of the soil, the extra water percolates below the active root zone and is lost.