Flood irrigation is used where the topography is flat and level. This method is often used for drilled or broadcast crops, such as hay, pasture, and small cereal grains. Orchards and vineyards are also sometimes flood irrigated.
The land must be graded and leveled for flood irrigation. The amount of grading needed depends upon the topography, cropping system, and cost of grading. A uniform downslope of 0.1 to 0.4 percent3 is used for most soils and crops, with little or no cross-field slope. Permanent or temporary levees are constructed running downslope, with a border disk forming ridges that divide the field into strips or checks, preferably not over 10 to 20 meters (33 to 66 ft) wide and 90 to 300 meters (300 to 900 ft) long.
Water from an irrigation pump or canal is turned into the supply, or head, ditch at the higher end of the field. It is released or siphoned into one or more checks and allowed to flow slowly downslope, spreading evenly and uniformly over each entire check as it advances toward the lower end. Ponding, excessive percolation, and inadequate wetting of the soil in different areas in the field should be avoided. Designing and operating such a system efficiently requires considerable experience, skill, and knowledge.
Furrow irrigation
Furrow irrigation is a modification of flooding-water is confined to furrows rather than wide checks. Water is used more efficiently with furrows than with flooding because the entire surface is not wetted, thus reducing evaporation losses.
Furrow irrigation is frequently used for row crops, or chards, and vineyards. The length of furrow varies from 30 m (100 ft) for small gardens to 450 m (1,500 ft) for field crops, but lengths of 90 to 180 m (300 to 600 ft) are more common. Long furrows cause greater loss of water because of deep percolation and excessive soil erosion at the head of the field.
Furrow spacing is determined by the plant row spacing. One irrigation furrow is generally provided for each crop row. The furrow spacing can be 60 to 180 cm (2 to 6 ft), depending upon the type of crop and wetting characteristics of the soil.
The depth of the furrow should be such that the water can be controlled. Water should flow in the furrow for sufficient time to allow it to percolate into across the bed, although not directly wetting the surface of the bed. For most row crops and orchards, furrow from 20 to 30 cm (8 to 12 in.) deep provide the necessary control. Furrows from 10 to 15 cm (4 to 6 in.) in depth are better for small-seed crops.
Uniform crop maturity, necessary for mechanical harvesting, is easier to achieve with furrow than with flood irrigation. Furrow irrigation applies water more uniformly than flood irrigation and improves uniformly of crop development.
J.McMahon, Margaret. 2002. Harman’s plant science. Pearson education: new jersey.