In surface irrigation systems, the area to be covered with water is sloped away from the supply channel so that the water will flow over the entire area with the water moving both across the surface to be irrigated and filtering down to the root bases of the plants in the field. Among the variations of surface irrigation are the techniques of furrow irrigation, border strips, basins, and wild flooding.
Furrow irrigation has the advantage of allowing the crops to be tended shortly after watering periods. The system is useful for crops that are grown in rows that can be separated by furrows (shallow ditches) along the rows. The furrows are usually dug along the line of the slope, but sometimes they run perpendicular to it. The problem with cross cutting the furrows on a slope is that it may collapse during irrigation periods from the force of the water.
The preferred method of supplying water to the furrows is to siphon water from a main source and carry it through plastic or aluminum pipes set in a main ditch at the head of the field. Another ditch at the end of the furrows collects excess water and runs it along to lower lying fields. The best incline for furrow irrigation is one from 0-5% slope. Crops are usually planted on the rise between the furrows, but sometimes trees are planted at the bottom of the furrow. Since there is less water surface in this method, evaporation of water is less than in surface flooding.
In pastures where there are crops that grow closely together border strips may be used. In this system a main ditch is constructed along the highest end of the slope and banks, called checks, which can be built as much as 70 ft (21 m) apart. Water is then siphoned from the main ditch onto the strips where the crops are grown. Sometimes the banks are replaced with border supply ditches, which allow more control over the release of water. This system is often used in research studies. In hilly areas contour ditches are built that follow the contour of a hill. They are carefully graded to control the flow of water.
For landscapes, gardens, and the watering of individual trees, the use of basins may be a suitable method of irrigation. The area to be irrigated is surrounded by banks (checks) and then watered from a main source along a high point in the basin. A drain is also placed along the major depression of a basin to allow water to run off. This system is easy to build since it requires very little movement of Earth. It is usually built around the natural contours of the area. Where the land is extremely steep, an adaptation of basin irrigation called terracing can be used. Here basins are created in a step fashion along the slope of the hill. At the end of each basin step a check is built. Basin watering is not generally recommended for flat ground.
While wild flooding is still practiced, it is not recommended by agricultural engineers because the water distribution is uneven and can lead to high saline contents in the soil and to waterlogging. The crop yields are consequently unpredictable.