Water requirementsDefinitions

Sunlight provides moist of the energy used in the evaporation of water from the soil and in the process of plant transpiration, which is the transformation of water in the soil to the water vapour that exits the plant through leaf surfaces. �Sunlight is measured in terms of its intensity and duration. �It is influenced by cloud cover, altitude, and the shading of nearby plants, structures or terrain.

Wind moves air across leaf and soil surfaces and increases the amount of water vapor removed from the landscape. �Wind is measured in terms of its velocity, direction and duration. �It may be influenced by trees, shrubs and nearby structures such as buildings and fences.

Temperature influences the rates of transpiration from the plant and evaporation from the soil. �Higher temperatures result in more rapid removal of water from the landscape.

Humidity is a measure of the water vapor content in the air. �Lower humidity (dry air) provides a greater differential between the moist leaf surface and the air. This increases the rate of water loss by the plant. �Humidity is typically quantified in terms of relative humidity (relative to completely saturated air).

The sum of water lost from the soil surface (evaporation) and water used by plants (transpiration) is called evapotranspiration. ��There are many factors that affect the rate of ET, including plant species, weather factors and the amount of water available to the plant.

Reference ET (ETO) is a useful way to estimate the amount of water used by the plants and evaporated from the soil. �ETO is usually calculated with information from a weather station. �This ET number is then used to determine how long irrigation runtimes should be.
ETO is expressed in terms of a depth of water per unit of time (i.e. inches per day or millimeters per day). �ETO is a useful reference point for irrigation runtime calculations because it represents a specific rate of use in response to local weather conditions.

Rainfall influences the amount of irrigation water needed. �When rainfall occurs, the proper response is to postpone irrigation until the soil has dried down to a reasonable level. �One of the difficulties in working with rainfall is estimating the fraction of the total rainfall that is actually stored in the root zone. �The amount of rainfall that is actually stored in the plant root zone is effective rainfall. �Some rain water does not reach the soil profile because it runs off or it is held in mulch or turf thatch. �Some water may percolate below the root zone and be lost. �This will depend upon the intensity and duration of the rainfall, and the water content of the soil prior to the rain.