Influence on soil water.
Covered or under drains take not only surface water, but also remove free water from the soil beneath down to nearly the level of the bottom of the drains, and thus increase the area of film water. Removing the free water enables the soil to absorb more readily rain water falling on the surface and therefore checks surface wash and the gullying of fields.
Influence on soil ventilation.
Lowering the free water allows a deeper penetration of air and, therefore, a deeper root development and enables crops to better resist dry periods.
Influence on soil temperature.
Lowering the free water in the soil influences soil temperature:
By diminishing the amount of water to be heated.
By checking evaporation.
By letting warm showers sink down into the soil.
By increasing ventilation and therefore permitting the circulation of warm air in the soil.