It generally is difficult to devise accurate formulas specifying the water needs of a given plant. Such needs are determined by a number of variables that include climatic factors such as sun exposure, as well as changes in temperature, humidity, and wind speed, all of which affect the evaporation rate of the water given to the plants. Other factors affecting a plant's need for water are the depth of its root system, the stage of its growth, soil composition, and maintenance procedures such as fertilizing and pruning.
A number of irrigation techniques are available for watering plants. Each has its own distinctive characteristics, and certain procedures need to be followed to ensure that each irrigation method is used optimally to allow for the growth of healthy, low-water consuming plants.
This method is simple and easy to implement. However, half of the water provided using flood irrigation might be lost as a result of evaporation, runoff, or wind distortion. Flood irrigation also might give plants more than their needs of water and faster than their ability to absorb it. To minimize water runoff resulting from flood irrigation, it is advised to flatten the irrigated area and to create a ditch around each plant.
This method does not require much effort and is suitable for irrigating areas of various shapes, whether rectangular or circular. However, it requires constant maintenance, and about a third of the water provided by spray irrigation is lost before it reaches the plant as a result of wind distortion or evaporation. Therefore, limit the use of spray irrigation to lawns, which require the equal distribution of water over relatively wide areas. Also, spray the water as close as possible to ground level to reduce water loss due to wind distortion.
In most cases, this is the most effective and water-saving irrigation method. It involves moving water in plastic pipes connected to the water source. The pipes are placed above the ground, or buried under the soil near the plants' root systems. Water slowly flows out of the pipes through openings or valves that control the rate of water output, and these openings and valves are placed near the plants so that the water easily reaches the plants' root systems. The slow rate of water output gives the plants adequate time to absorb the water.
One of the most important advantages of drip irrigation is that it saves water. Very little of the water administered by drip irrigation is lost to runoff, evaporation, or wind distortion. It is the most suitable method for irrigation in arid areas, or ones characterized by high winds or strong slopes. Another advantage of drip irrigation is its flexibility. The number and location of openings and valves on the irrigation pipes, as well as the rate of water output can be adjusted whenever the need arises. Drip irrigation also does not require much effort, but the components of drip irrigation systems need regular maintenance.
When designing your garden, place plants with similar water needs in proximity to each other so as to irrigate them using the same drip irrigation line. Also, develop a schedule that trains your plants to consume less water. This usually consists of widely spaced but deep applications. Such a schedule encourages the roots to expand and extend deeper into the soil in search of water, and therefore increases the plant's drought tolerance. Generally, irrigate trees with generous amounts of water given at widely spaced time intervals, but irrigate ground covers and plants that have shallow root systems with smaller amounts of water given at more tightly spaced time intervals. Also, irrigate so that water reaches a soil depth of 50 - 60 cm for trees, 35 - 40 cm for shrubs, and 15 cm for ground covers.
Irrigate during the early morning hours since the temperatures are lowest at that time of the day and evaporation therefore is minimized. If that is not possible, irrigate in the evenings, when temperatures begin to decrease.
Modify your irrigation schedule as the seasons change and your plants grow. Since plants need less water in the winter, decrease the amounts of water given to them - and possibly even stop irrigation - during that season. Also, keep in mind that drought tolerant plants need less water as they mature.
Pay attention to the quality of the soil in your garden and make sure that it includes a mixture of topsoil and organic fertilizers. Such soils retain moisture and provide plants with their nutritional needs. Also consider the use of moisture-retaining materials to minimize the need for irrigation. These can be placed either on top of the soil or mixed in it. One such material is pumice stone. This material is extracted in Jordan - where it is known as "touf" stone - and can be obtained in various sizes. It contains cavities that hold moisture and also provide breathing space for the soil. Other moisture retaining materials include polymers that absorb water when coming into contact with it to form a gel-like substance. This water consequently is stored in the soil for extended periods of time.
It generally is difficult to predetermine the amounts of water that a plant requires. Consequently, one needs to inspect the plants and the soil in which they are located on a regular basis to ensure that each plant receives its exact needs of water.