When water is applied to the soil, it seeps down through the root zone very gradually. Each layer of soil must be filled to “field capacity” before water descends to the next layer. This water movement is referred to as the wetting front. Water moves downward through a sandy coarse soil much faster than through a fine-textured soil such as clay or silt.
If only one-half the amount of water required for healthy growth of your garden or landscape is applied at a given time, it only penetrates the top half of the root zone; the area below the point where the wetting front stops remains dry, as if no irrigation has been applied at all.
Once enough water is applied to move the wetting front into the root zone, moisture is absorbed by plant roots and moves up through the stem to the leaves and fruits. Leaves have thousands of microscopic openings, called stomates, through which water vapor is lost from the plant. This continual loss of water called transpiration, causes the plant to wilt unless a constant supply of soil water is provided by absorption through the roots.
The total water requirement is the amount of water lost from the plant plus the amount evaporated from the soil. These two processes are called evapo-transpiration.
Evapo-transpiration rates vary and are influenced by day length, temperature, cloud cover, wind, relative humidity, mulching, and the type, size and number of plants growing in a given area.
Water is required for the normal physiological processes of all plants. It is the primary medium for chemical reactions and movement of substances through the various plant parts. Water is an essential component in photosynthesis and plant metabolism, including cell division and enlargement. It is important also in cooling the surfaces of land plants by transpiration. Water is also a primary yield-determining factor in crop production. Plants with insufficient water respond by closing the stomata, leaf rolling, changing leaf orientation and reducing leaf and stem growth and fruit yield.
information provided by Alldredge Gardens