Surface irrigation is the application of water by gravity flow to the surface of the field. Either the entire field is flooded (basin irrigation) or the water is fed into small channels (furrows) or strips of land (borders).
Worldwide there are approx. 3.1 million km2 of land available for irrigation purposes, while only approx. 2.6 million km2 are utilized. With about 95% share of the total irrigation worldwide, surface irrigation is by far the most widespread irrigation method. It is normally used when conditions are favorable: mild and regular slopes, soil type with medium to low infiltration rate, and a sufficient supply of surface or groundwater.
Surface irrigation is widely utilized and therefore a well-known system which can be operated without any high-tech applications. In general, it is more labor intensive than other irrigation methods. Proper design of surface irrigation systems takes into account the soil type (texture and infiltration rate), slope, and levelness of the field, stream size, and length of run. Leveling the fields and building the water ditches and reservoirs might be expensive, but once this is done, costs are low and the self-help capacity is very high.
Basins are flat areas of land, surrounded by low bunds. The bunds prevent the water from flowing to the adjacent fields. Basin irrigation is commonly used for rice (paddy) grown on flat lands or in terraces on hillsides. Trees (e.g. citrus, banana) can also be grown in basins, where one tree is usually located in the middle of a small basin.
Furrows are small channels, which carry water down the land slope between the crop rows. Water infiltrates into the soil as it moves along the slope. The crop is usually grown on the ridges between the furrows. This method is suitable for all row crops and for crops that cannot stand in water for long periods (e.g. 12-24 hours).
To manage the irrigation with the traditional breach system, the farmer has to open and close the embankment of the water-conveying channel. This is the most common method of releasing water from a channel, but it can also be the most damaging. Not only is it difficult to control the discharge, but there can be serious erosion of the channel embankment. If other more controllable methods are available, then these should be used in preference to this.
Borders are long, sloping strips of land separated by bunds. They are sometimes called border strips.
Irrigation water can be fed to the border in several ways: opening up the channel bank using small outlets or gates or by means of siphons or spiles. Siphons are small curved pipes that deliver water over a ditch bank. Spiles are small pipes buried in the ditch bank. A sheet of water flows down the slope of the border, guided by the bunds on either side. When the desired amount of water has been delivered to the border, the stream is turned off. However, if the flow is stopped too soon, there may not be enough water in the border to complete the irrigation at the far end. If it is left running for too long, the water may run off the end of the border and be lost in the drainage system.
Border irrigation is best suited to the larger mechanized farms as it is designed to produce long uninterrupted field lengths for ease of machine operations. Borders can be up to 800 m or more in length and 3 - 30 m wide depending on a variety of factors.
Border slopes should be uniform, with a minimum slope of 0.05% to provide adequate drainage and a maximum slope of 2% to limit problems of soil erosion. Deep homogenous loam or clay soils with medium infiltration rates are preferred. On heavy clay soils, border irrigation may cause problems because of the low infiltration rates (basin irrigation is more suited on these soil types). Close growing crops such as pasture or alfalfa are preferred.
Advantages and Disadvantages
Level surface irrigation (water is ponded on an enclosed level field and allowed to infiltrate in basins, borders, or furrows) has some distinct advantages which can be summarized as follows:
- Irrigation management is very easy and does not require modern technology and can largely build on local traditional knowledge;
- Adapts well to small land holdings and does not require high financial input;
- Adapts easily to flat topography and can function without outlet drainage facilities;
- Works well with short-term water supplies;
- Irrigation allows full utilization of rainwater and can achieve high application efficiencies;
- Adapts well to moderate to low infiltration rates and allows easy leaching of salts.
Disadvantages of level surface irrigation include:
- Requires level land to achieve high efficiencies (maximum land elevation fluctuation should not be greater than half the applied irrigation depth);
- Soils with high infiltration rates require small field sizes, which interferes with mechanization.
- Difficulty to apply small irrigation quantities, excess water is difficult to evacuate, particularly during times of excess rainfall;
- Plants are partly covered with water sometimes over extended periods (in low infiltration rate soils);
- Small basins require extensive delivery channels and are not easily adaptable to tractor mechanization.
Graded surface irrigation (water is fed into the high end of a field and is allowed to run slowly to the low end) shows the following advantages:
- Requires low capital and energy costs;
- Allows irrigation on sloping land (as is found in many irrigated areas);
- Allows irrigation of long fields with relatively small flows;
- Is applicable to soils with moderate to fairly high infiltration rates;
- Field drainage of excess water is made possible.
Disadvantages of graded surface irrigation include:
- A high degree of management and water control is required to achieve high irrigation efficiencies;
- High irrigation efficiencies require uniformly graded and shaped land;
- With moderate to slow infiltration rates, long irrigation times are required.
- Except for soils with high infiltration rates, a drainage outlet must be available from every field to dispose of tail water and rainwater;
- Labor intensive.
A special and rarely use approach is the subsurface irrigation - Water is directed to the subsoil (crop root zone) in the area to be irrigated. The water is used to artificially control the groundwater table, and is normally delivered through perforated pipes buried in the ground. In some Latin American countries, porous clay pots are buried in the ground and filled with water that slowly seeps into the subsoil to moisten the roots of crops.
- Renner, J. (n.d.): Global irrigated area at record levels, but expansion slowing, Worldwatch Institute, November 2012 – assessment based on FAO data 2009.