Flood irrigation is an ancient method of watering agricultural fields by allowing water to flow over the entire land surface. Also known as surface irrigation, this technique relies on the natural force of gravity to move water across the field. Controlled flooding saturates the soil to a depth sufficient for crop growth, making it one of the simplest methods in use today.
The Underlying Principles of Water Movement
Water is delivered to the field’s highest elevation point, often through canals, ditches, or gated pipes. Once released, the water moves across the soil surface as a thin sheet, known as the advance phase, driven by the gravitational slope. Precise land leveling is a prerequisite for effective flood irrigation, ensuring uniform sheet flow.
As the water advances, it simultaneously begins to penetrate the soil surface, a process called infiltration. The rate at which the soil absorbs the water dictates the speed of the advance phase and the duration the water must remain on the field to ensure adequate moisture storage in the root zone.
Vertical water movement is determined by the soil’s physical properties, such as texture and structure. Fine-textured clay or loam soils have slower infiltration rates compared to sandy soils, requiring water to remain on the surface longer. Managing the flow rate and duration is necessary to prevent excessive water loss from runoff or deep percolation.
Controlling Water Flow Common Application Methods
Various methods structure the field to control the water’s path, maximizing uniformity and reducing water loss based on crop type and topography. These modifications are designed to maximize uniformity and reduce water loss based on the crop type and field topography. The three primary methods are basin, border, and furrow irrigation.
Basin irrigation is used for orchards, pastures, or crops requiring standing water, such as rice. The field is divided into small, level plots enclosed by earth dikes or berms to create a ponding effect. Water is introduced quickly to cover the area and remains until it has fully infiltrated, with the barriers preventing surface runoff.
Border irrigation involves dividing the field into long, sloping strips separated by parallel earth dikes or ridges. Water is released at the high end and flows as a sheet down the gentle slope, guided by the borders. Used for close-growing crops like hay or small grains, the strip design ensures water reaches the far end before excessive infiltration occurs.
Furrow irrigation is employed for row crops like corn, potatoes, or vegetables, where water is confined to small channels dug between the crop rows. Water flows down these furrows, and moisture moves laterally through capillary action to wet the soil where the plant roots are located. Weting only a fraction of the total field surface helps minimize water loss through evaporation.
Evaluating Flood Irrigation Efficiency and Suitability
Flood irrigation’s primary advantage is its simplicity and low initial capital outlay, requiring minimal infrastructure beyond water delivery channels and field preparation. It is highly energy-efficient because it relies on gravity, eliminating the need for pressurized delivery systems. This makes it accessible for farmers with limited financial resources or reliable electricity.
However, the method is the least water-efficient of all irrigation types, with typical application efficiencies ranging from 50 to 60 percent. Water is lost to deep percolation or surface runoff that escapes the field. This inefficiency is a major concern in regions facing water scarcity, as it can deplete water resources more rapidly than other techniques.
Flood irrigation requires a high level of labor for field preparation, especially precise land leveling, and for manually controlling water flow during application. It is highly dependent on suitable topography, working best on flat or gently sloping land that allows for controlled water movement. Despite these limitations, it remains suitable in areas with an abundant and inexpensive water supply, or for specific crops like rice adapted to ponding conditions.