There are five basic weather factors which influence the plant water requirements. There are daily variations in the amount of water used by the landscape because all of the weather factors vary from day to day. Measuring these weather factors provides information which can be used to estimate the amount of water needed. It is important to remember that these factors are not constant, but are continually changing.
The five factors are:
Sunlight (Solar Radiation)
Sunlight provides most of the energy used in the evaporation of water form the soil and in the process of plant transpiration, which is the transformation of water in the soil to the water vapor 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 form the landscape. Wind is measured in terms of its velocity, direction and duration and may be influenced by trees, shrubs and nearby structures such as buildings and fences.
Temperature influences the rate 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 of the air. Lower humidity (dry air) provides a greater differential between the moist leaf surface and the air, which increases the rate of water loss from the plant. Humidity is typically quantified in terms of relative humidity (relative to completely saturated air).
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 total rainfall that is actually stored in the root zone, or effective rainfall. Some rain water is held in mulch or turf thatch or it runs off and does not reach the soil. Depending ton the intensity and duration of the rainfall, and the water content of the soil prior to the rain, some water may percolate below the root zone and be lost.