Since a constant supply of water in the soil is necessary for plant survival and growth, the irrigation engineer is concern with how water moves in a soil, how much water a soil can hold and how much of it is available to plants. Generally, the finer the soil particles and larger the amount of organic matter, the more water a oil holds.
Movement of Water in the Soil:
When the water first enters or makes its downward movement through the soil surface. It is called infiltration of water and the rate at which water and the rate at which water is penetrating the surface of soil at any given instant is called the infiltration rate, usually measured in cm/hr. infiltration rates may be ranged from 2.5cm/hr to about 25 cm /hr hydraulic conductivity values.
2. Permeability or Hydraulic Conductivity:
It is the quality of soil that enables it to transmit air and water and is measured in cm/day. Hydraulic conductivity depends on the properties of the soil. A soil that has high porosity and coarse open texture has high hydraulic conductivity values.
The downward movement of water through the soil due to force of gravity is termed as percolation. The percolation water goes into the soil unit it meets the free water table. On the one hand, due to rapid percolation, there is practically no danger of soil suffering from bad drainage, but on the other hand, there is a possibility of the dissolved plant nutrients like calcium and magnesium being carried into deep into roots of common field crop. In sand soil or open textured soil there is a rapid loss of water through percolation.
4. Capillary Movement:
Once flow due to gravitational force has ceased the water moved in the form of thin or capillary film from a wet regions to dry region.