How much water does a plant need?
An irrigation system should provide plants with their water needs, but not more. Yet, it is difficult to accurately calculate the water needs of a given plant as such needs are determined by a number of factors. Climatic factors play an important role since part of the water provided to a plant is lost to evaporation before the plant can make any use of it. The rate of this evaporation is affected by a host of factors including sun exposure, temperature, humidity, and wind speed.
A plant’s water needs also are determined by microclimatic factors. For instance, plants facing southern and western exposures need more water than ones facing eastern exposures, and plants facing northern exposures generally need the least amount of water. Also, plants located in shady and protected areas usually need less water than those exposed to the sun or winds.
Also, the stage of growth of a plant dictates the amount of water it needs. A young plant requires frequent irrigation, but as it matures, it will require more widely spaced but deeper irrigation.
Drought tolerant plants usually have deeper root systems than water consuming plants, and therefore can seek water deeper in the soil. Trees have deeper root systems than shrubs, and shrubs have deeper root systems than groundcovers. A good way to identify the rooting depth for a given plant and the moisture of its soil is through using a soil probe or screwdriver. Insert it into the soil after irrigation, and push it until it hits dry soil and stops. This will give you an indication as to how deeply your plants are being watered.
Soil composition also affects a plant’s water needs, as soils differ greatly in their ability to store water. Soils that include a mixture of topsoil and organic fertilizers retain moisture well and also provide plants with their nutritional needs. Note that most agricultural soils in Jordan are clay loam and therefore have a higher water holding capacity than sandy soils. Adding organic materials to the soil mix will further increase its water holding capacity.
Maintenance procedures such as fertilizing and pruning help determine a plant’s water needs. Providing proper maintenance for plants and the soil will result in savings in the amount of water that the plants consume. [very vague paragraph]
To help determine how much water drought tolerant plants usually need, keep in mind that trees need supplemental irrigation to get established, especially if planted after the rainy season. The establishment period is the length of time needed for a plant to clearly show root and foliage growth. During the first year, a tree usually needs to be irrigated in the amount of 20 – 25 liters of water two times a week. During its second year, it needs to be irrigated in the amount of 40 liters once a week. Beginning with the third year, when trees usually get established, some trees need to be irrigated in the amount of 50 – 60 liters once a month (e.g. Pomegranates), and some do not require any supplemental irrigation (e.g. Cypresses). Usually, native trees (e.g. Carob) do not need supplemental irrigation. On the other hand, trees with flowers or crops need supplemental irrigation after their establishment to achieve optimal results.
Ornamentals should be irrigated during their establishment period, which may extend over two to three years. After establishment, some require no irrigation (e.g. Agave), and others require irrigation once every week, two weeks, or every month.