A number of different versions of surface irrigation are practiced. Furrow irrigation is employed for irrigation of row crops. For field crops, border check irrigation is the most common, but other versions are also in use, their selection depending on die slope (or lack of it) at die site, die crop grown, die availability of water, and die degree of control required over water flow.
Figure 5.1 Furrow irrigation of cotton. In this example, water is siphoned from the supply channel to the head of the bay.
Furrow irrigation.Crops are established in raised beds, enabling water to flow down defined furrows. Successful irrigation requires water to soak laterally into the bed as well as downward into die root zone. Furrow spacing is therefore partly determined by the hydraulic conductivity of die soil and subsoil, but also by preferred crop row spacing and machinery requirements.
Figure 5.2 Below, Furrow irrigation of grapes, using above ground low-pressure delivery pipe. Below right, Furrow irrigation of citrus, from buried pipe and risers in this orchard.
Water is admitted to the head of me furrow by a siphon in the case of water delivered in open channels. Low pressure gated pipe or fluming is also available, and this occupies much less land area man channels (some can also be buried). It is well suited to high value crops where land area may be limited, or where seepage from a channel would be excessive.
The length of furrow is determined by me slope along the furrow and soil infiltration characteristics, since these determine both the travel speed of the wetting front as it proceeds down the furrow (and therefore the time taken to reach the end of the furrow, and the time water is in contact with the soil) and the amount of water entering the profile. These are characteristics specific to each site. Other factors include the amount of earthmoving required to create the desirable slope for the particular soil type, the shape of the field, and a preference for uniform furrow lengths in each field to assist crop management.
Furrows are normally constructed down the slope, but occasionally furrows are located on the contour to enable this method of irrigation on steeper slopes (usually for orchards and plantations).
For annual crops, the field is cultivated after harvest, and beds are reconstructed before the next planting. This can have long term detrimental effects on soil structure, organic matter levels, and activities of soil organisms. Some farms have established permanent beds; ie the beds and furrows are always retained. This requires a re-think of crop management techniques, and machinery requirements, but significant improvements in crop performance have been reported. Improvements are due not only to better soil quality in the beds, but also due to improved timeliness of operations.
Border check irrigation.This method is used extensively for a wide range of grain and fodder crops and pastures. A series of bays are formed within each field, running down the slope. The borders of each bay are formed by constructing small earthen check banks (hence border check). Water is admitted to the head of the bay using syphons, gates or pipes in or over the wall of the supply channel. It quickly spreads across the head of the bay, and a wetting front proceeds down the bay.
Like furrow irrigation, the length of the bay is determined according to the slope and soil infiltration characteristic. The cross slope of a bay is limited to 25 mm (preferably less) so this will partly determine the width of bay. Check banks are a possible nuisance to machine operations, so it is preferably to have bays as wide as possible. If they are too wide, uneven watering will result.
Naturally, the surface of the bay must be built and maintained at the correct, uniform gradient. Unevenness in die bay surface will create uneven watering and poor drainage, reducing crop performance. This is noticeable just after irrigation (or moderate rainfall) where water may pond on die lower parts of die bay for too long.
Where bay cross slope is a problem, die surface of die bay can be corrugated. This helps guide water down die bay, providing a more even watering, without interfering with crop management.
Figure 5.3 Border check irrigation. In this example, water is admitted to the bay by а pipe in the wall of the supply channel.
Where cross slopes are excessive, check banks can be constructed on the contour rather than down die slope, provided die vertical interval between adjacent banks is 25 mm or less. This will tend to give uneven watering, and the bay width is likely to be uneven. From a technical and management point of view, it may be preferred to landscape the area to give uniform width and grades to the field.
Other versions of surface irrigation.On flat sites where drainage is difficult at the end of irrigation, the borders of irrigation bays can be formed by ditches instead of check banks.
For irrigation of rice, level bays are formed and flooded, water flowing continuously from bay to bay during the irrigation season. Hillside flooding is occasionally practiced for irrigation of pasture, where a ditch delivers water approximately on the contour above the irrigated area. A temporary check is placed in the channel, water builds up in the channel and floods over the side.
Figure 5.5 Earthmoving under way using a tractor drawn scraper. The operating height of this is adjusted in response to a laser signal. The mast on the machine carries the laser receiver; the laser emitter is mounted on a tower near the centre of the field.