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SOIL, WATER AND PLANT RELATIONSHIPS
The rate at which water soaks into the soil is called the "infiltration rate". The rate at which we apply water through irrigation, or the rate that water falls during a rain storm is called the "application rate". Surface runoff occurs when the application rate of the irrigation/rainfall is greater than the infiltration rate of the soil.
Infiltration rates and application rates are both measured in terms of inches of water applied per hour, or "inches per hour".
For example, a standard field sprinkler system using 7/64" nozzles and running at 50 psi applies water at about .2 inches of water per hour. (That is, for every hour the system runs, an equivalent depth of about .2 inches of water is sprayed onto the ground.) If the soil's infiltration rate is greater than .2 inches/hour, then all water applied by the sprinkler system will soak in. If it is less, then you will see water standing on the surface or running off.
Figure SWP-4 is a graph of infiltration rate versus time during an irrigation. The infiltration rate at the start of the irrigation(the left side) is very high, while at near the end of the irrigation(the right side) it has dropped greatly. The straight horizontal line represents the application rate of a sprinkler system that could have been used during the irrigation, high and low rate. It is straight because the application rate of the sprinkler system doesn't change. It pumps out the same amount of water all through the irrigation.
FIGURE SWP-4: Sprinkler Infiltration Rates
The infiltration rate is higher than the application rate at the start of the irrigation. Thus, all water applied by the sprinklers soaks into the soil. However, by the end of the irrigation, the infiltration rate has dropped below the application rate. Now the sprinkler system is applying more water than the soil can soak in. You will see standing water or runoff occur.
If you see excess surface runoff with sprinkler (or trickle) systems, then either the system is being run too long per set or the design of the system is not matched to the soil (it applies water too fast). Note that sprinkler systems do not spray water on all parts of the field at equal rates. Because of this you may see standing water close to sprinkler heads and not further away during an irrigation. If this occurs continually check your spacings or the length of your sets.
Last updated September 2000