Irrigation Definition

Water is the most important element for the growth of plants. Different types of plants require different quantities of water at different times during their growing period. Water is supplied to the plants through direct rain or flood waters of the rivers which inundate large land areas during floods. As these are natural processes, there may be heavy rain and damaging the crops or creating scarcity of supplying water for the crops. So an artificial method is needed by which water can be collected and stored so that it can be used when necessary. This method of science is termed as “irrigation”.


Irrigation may be defined as the science of artificial application of water to the land in order to fulfill the water requirements of the crops throughout the crop period for the full nourishment of the crops. Nutrients to the crops may also be applied through irrigation.


Crop period: The time period between the instant of sowing seeds to the instant of harvesting the plants is called crop period.

Irrigation water should be supplied as soon as the moisture falls up to the optimum level. The quantity of water that produces maximum yield of the crops is termed as optimum water level. More or less than the optimum level results the reduction of the yield.

yield vs water depth graph
Fig: Yield Vs. water depth relationship

Irrigation History

Agriculture is the oldest way to serve food to the human. Ancient people from different regions used irrigation system to supply water in their agricultural lands. At that time irrigation was mainly the control of water flow. Mesopotamia and Egypt were the most successful user of irrigation system. A brief description about the history of irrigation is stated below.


Mesopotamia (at present Iraq and some parts of Iran) was familiar as the land of two rivers of Tigris (to the north) and Euphrates (to the south). As rains were seasonal in Mesopotamia, irrigation was extremely necessary for its agriculture. Another problem was that Tigris and Euphrates carried large amount of silt which caused floods. So, a system was developed for controlling the direction and flow of water. Canals and irrigation ditches were also built for supplying water to the fields. A brief idea about Mesopotamia irrigation system can be found from this YouTube link


Irrigation system of ancient Egypt was mainly based on river Nile. Water was supplied to the agricultural lands from Nile. Uncertain level of water flowing in the Nile was the main problem for Egyptian agriculture. During high and low level of water flow, flood and scarcity of water were very common. Small basins, canals were built to collect water for irrigation purpose. Different regions like Mexico, North America were also familiar with the irrigation system.

Approaches for Irrigation Scheduling

Irrigation scheduling determines when to and how much of water have to apply to the field. Scientist and farmers use several approaches for scheduling irrigation. These are as follows.

  1. Soil moisture depletion approaches

    For scheduling irrigation, a good criterion of available soil moisture in the root can be used. The amount of water that lies between the field capacity and the permanent wilting point is called the available moisture content.

    Classes and availability of soil water
    Fig: Classes and availability of soil water

    The moisture content near the wilting point is not readily available to the plant for subsequent use. Normally 75-80 percent of the available moisture is most easily extracted by plants which is termed as readily available moisture. To maintain crops growth properly, irrigation should be started when 50 percent of the available water is depleted in the soil root zone and it is continued until the optimum level is reached. Soil moisture can be measured by various methods such as gravimetric method, electro-resistance blocks, tensiometer etc.

  2. Plant basis

    Plant can be used as a guide for scheduling irrigation. Plant will reflect the deficiency of water by various symptoms such as curling or rolling of leaves and change in foliage color. All of these symptoms are the indication that plant requires water. Plant basis irrigation scheduling has only drawbacks that it does not give the quantitative estimation of the moisture deficiency.

  3. Climatological approach

    Climate is the main controlling factor for evapotranspiration. Climatological data is used for estimating the amount of lost water by evapotranspiration (ET). Irrigation is scheduled when ET reaches to a particular level. Same amount of ET or fraction of ET is supplied as irrigation water. There are different method of climatological approaches such as IW/CPE method and pan evapotranspiration method.

  4. Critical growth approach

    In each crops, there are critical periods or moisture sensitive periods. Generally crops growth stages at which moisture deficiency leads to irreversible yield loss are termed as critical periods. Whenever soil moisture depleted to the critical periods, irrigation must be scheduled. In case of limited water supply condition, irrigation is scheduled only for critical periods and irrigation is skipped for noncritical conditions.

  5. Plant water status itself

    The latest approach for irrigation scheduling is plant water status. As plant is a good indicator of available soil moisture and climate factors, plant water status itself can be used for irrigation scheduling. This approach is not yet so common for the lack of standard and low cost techniques to determine plant water status.

Irrigation Projects


Great Man-Made River

Great Manmade River

The Great Man-Made River (GMR) in Libya is the largest irrigation project in the world. It is the largest underground pipe network which supplies water from Nubian sandstone aquifer to the Sahara desert. Nubian sandstone aquifer is the largest fossil water system which will take hundreds of years to deplete. This project was started in 1984 with five different phases funded by Gaddafi-run government. It is in a stage of partial completion as three out of five phases has been completed.

Columbia Basin Projects

Columbia BasinColumbia basin project (CBP) is the largest irrigation project in the United States of America. Water is pumped from the Columbia River, carried to the main canals, stored in the reservoirs and fed into the lateral irrigation canals. Though Congress authorized this project in 1943, its construction started after World War II.