Surface irrigation

Surface Irrigation
Surface irrigation is one of the most common irrigation systems in the world and in our country. According to statistics, more than 95 percent of the country’s agricultural lands are currently irrigated using surface irrigation methods. Despite the wide application and considerable complexity of this system, it has not received sufficient attention from researchers and users. Based on numerous studies, the current efficiency of surface irrigation in the country is estimated to be less than 35 percent.
Surface irrigation is the oldest and most widely used method of irrigation in agriculture. It can be classified into several types; however, the most prominent classifications are as follows:
Basin irrigation
Border (strip) irrigation
Furrow irrigation
Flood irrigation
The differences among various surface irrigation methods are mainly nominal. For example, a border or basin system can be converted into a furrow system. Flood irrigation is conducted in such a way that water simply flows over the land surface without any regulation to ensure uniform water distribution across the field. Therefore, no effort is made to control demand or uniformity. If control of this type of irrigation is desired, it must be converted into one of the border, basin, or furrow irrigation systems. The stages of irrigating a field are shown in the figure.
Components of the Irrigation System
Water enters the field either from a reservoir or freely by gravity, or it is pumped from groundwater sources to the field surface. Excess water exits through drainage systems. For each component involved in water conveyance, distribution, and drainage on the farm, proper design, operation, maintenance of structures, and water control and regulation are required.
Stages of Surface Irrigation
Four general phases are identified in surface irrigation:
Advance phase
Storage and moisture distribution phase
Depletion phase
Recession phase
These phases are illustrated in the figure. The advance phase begins when water enters the field and continues until the water reaches the end of the field; this duration is called the advance time. From the end of the advance phase until the inflow is cut off is known as the wetting or storage phase. During the storage phase, the water required by the farthest parts of the field—which had less infiltration time during the advance phase—is supplied. After the inflow is cut off, water either drains from the upstream end of the field or infiltrates into the soil, and the soil surface becomes exposed; this stage is called the depletion phase. Once the soil surface becomes exposed at the upstream end of the field, the recession phase begins and continues until the surface water completely disappears. In fields with gentle slopes, the depletion and recession phases occur almost simultaneously across the entire field.
Irrigation Phases
Criteria for Selecting a Surface Irrigation Method
To select an appropriate surface irrigation system for a given region, the following factors should be evaluated:
Costs of the system and its components
Size and shape of the field
Water availability and the infiltration rate of the field soil
Conditions and accessibility of water from the source (timing of water access, water conveyance, water transport, and duration of water delivery)
Climate
Cropping pattern
Historical background and experience
Consideration of land leveling equipment
Adapted from the book Surface Irrigation: Design by Empirical and Semi-Empirical Methods
By Dr. Seyed Hassan Tabatabaei, Dr. Payam Najafi, and Engineer Soudabeh Golestani