|"Dry" Soils||Infiltration and Runoff||Wetting and Saturation|
|Water Holding Capacity||Percolation and Drainage||Engineering and Bearing Capacity|
Infiltration is the movement of water from the soil surface into the soil. As long as water the rate of water addition to the soil is slower than the rate of infiltration, water will move into the soil, and there will be no runoff. In the pictures to the right, water is being added to the sponge slowly, and all the water is moving into the sponge.
The far right image is nearing its holding capacity. As water infiltrates the sponge, several forces simultaneously pull on it. The dominant forces in this example are gravity, capillarity, and matric (force of attraction of the water for the fibers of the sponge).
Initially, the matric and capillary forces keep most of the water near the point of entry. As the sponge wets, however, potential energy gradients develop in the sponge, resulting in downward movement of water. The gravitational (large) pores near the top of the sponge begin to fill. These pores are not able to retain water against gravity as the capillary pores can, so water moves down through the sponge due to the pull of gravity.
The same thing happens in soils. The infiltration rate of soils is affected by many things, including texture, surface structure, surface cover (bare soil, growing plants or plant residues), hydraulic conductivity (rate of water movement through the soil), and the presence of limiting layers below the soil surface (compacted layers, bedrock, etc.).
When water is added to the sponge faster than the infiltration rate, runoff develops. In these images (right) water is seen flowing across the surface of the sponge, and cascading off the sponge into the water in the pan.
When water is added to the soil slowly in precipitation or irrigation, all the water can infiltrate the soil. When the irrigation rate is excessive, or the precipitation comes rapidly, the infiltration rate is exceeded, resulting in runoff.
Agricultural management practices are designed to keep water on the field, in other words, to minimize runoff. Sound water conservation practices protect the soil surface structure and leave cover on the soil surface to increase the infiltration rate and reduce the potential for runoff. These management practices occur in all agricultural management systems, though for different reasons.
In humid climates, keeping water in the soil for growing crops is not an issue, however, keeping soil in the field is. Soil erosion accompanies runoff, so humid regions attempt to limit runoff to decrease soil losses from water erosion.
Semiarid and arid regions use similar practices to store and conserve water in the soil because it is not possible to grow a crop on growing season precipitation alone.