To choose an irrigation method, the farmer must know the advantages and disadvantages of the various methods. Farmer must know which method suits the local conditions best. Unfortunately, in many cases there is no single best solution: all methods have their advantages and disadvantages. Irrigation systems are often designed to maximise efficiencies and minimize labour and capital requirements.
Most effective management practices are dependent on the type of irrigation system and its design. A large number of considerations must be taken into account in the selection of an irrigation system. These will vary from location to location, crop to crop, year to year and farmer to farmer.
Broad guidance and several important criteria in the selection of suitable irrigation method are discussed:
In general, these considerations will include the compatibility of the system with other farm operations, economic feasibility, topographic and soil properties, crop characteristics and social constraints.
Surface, Sprinkler or Drip Irrigation:
Suitability of surface, sprinkler or drip irrigation depends mainly on the following factors:
1. Compatibility,
2. Natural conditions,
3. Type of crop,
4. Type of technology,
5. Previous experience with irrigation,
6. Required labour inputs, and
7. Costs and benefits.
1. Compatibility:
Irrigation system for a field or a farm must function alongside other farm operations such as land preparation, cultivation and harvesting. Use of large mechanised equipment requires longer and wider fields. Irrigation systems must not interfere with these operations and may need to be portable or function primarily outside the crop boundaries (surface irrigation systems). Smaller equipment or animal powered cultivating equipment is more suitable for small fields and more permanent irrigation facilities.
2. Natural Conditions:
Natural conditions such as soil type, slope, climate, water quality and availability, have the following impact on the choice of an irrigation method:
i. Soil type:
Sandy soils with low water storage capacity and high infiltration rate need frequent but small irrigation applications, in particular when they are also shallow. Under these circumstances, sprinkler or drip irrigation systems are more suitable than surface irrigation. On loam or clay soils, all three irrigation methods can be used, but surface irrigation is more commonly found. Clay soils with low infiltration rates are ideally suited to surface irrigation.
When different soil types are found within one irrigation scheme, sprinkler or drip irrigation are recommended as they will ensure a more even water distribution.
ii. Slope:
Sprinkler or drip irrigation are preferred over surface irrigation on steeper or unevenly sloping lands as they require little or no land leveling. An exception is rice grown on terraces of sloping lands.
iii. Climate:
Strong wind can disturb spraying of water from sprinklers. Under very windy conditions, drip or surface irrigation methods are preferred. In areas of supplementary irrigation, sprinkler or drip irrigation may be more suitable than surface irrigation because of their flexibility and adaptability to varying irrigation demands on the farm.
iv. Water availability:
Due to higher water application efficiency with sprinkler and drip irrigation than surface irrigation, these methods are preferred when water is in short supply.
v. Water quality:
Surface irrigation is preferred if the irrigation water contains much sediment, to prevent clogging of drip or sprinkler irrigation systems.
If the irrigation water contains dissolved salts, drip irrigation is particularly suitable, as less water is applied to the soil than with surface methods.
3. Type of Crop:
Surface irrigation can be used for all types of crops. Sprinkler and drip irrigation, because of their high capital investment, are mostly used for high value cash crops, such as vegetables and fruit trees. Drip irrigation is ideal for irrigating individual plants or trees or row crops such as vegetables and sugarcane. It is not suitable for close growing crops like wheat, rice, sorghum, groundnut, pulse crops etc.
4. Type of Technology:
In general, drip and sprinkler irrigation are technically more complicated methods. Purchase of equipment requires high capital investment. Equipment maintenance requires high level of know-how. Purchase of spare parts may require foreign currency.
Surface irrigation systems, in particular small scale schemes, usually require less sophisticated equipment for both construction and maintenance (unless pumps are used). Equipment needed is often easier to maintain and less dependent on the availability of foreign currency.
5. Previous Experience with Irrigation:
Choice of an irrigation method also depends on the irrigation tradition within the region or country. Introducing a previously unknown method may lead to unexpected complications. It is not certain that the farmers will accept the new method. The servicing of the equipment may be problematic and the costs may be high compared to the benefits.
It will be easier to improve the traditional irrigation method than to introduce a totally new method.
6. Required Labour Inputs:
Surface irrigation often requires a much higher labour input – for construction, operation and maintenance – than sprinkler or drip irrigation. Surface irrigation requires accurate land leveling and regular maintenance. Sprinkler and drip irrigation require little land levelling; system operation and maintenance are less labour intensive.
7. Costs and Benefits:
Before choosing an irrigation method, an estimate must be made of the costs and benefits of the available options. On the cost side, not only the construction and installation, but also the operation and maintenance should be taken into account. These costs should then be compared with the expected benefits (yields). It is obvious that farmers will only be interested in implementing a certain method if they consider this economically attractive.
Surface irrigation is by far the most widespread irrigation method. It is normally used when conditions are favourable: mild and regular slopes, soil type with medium to low infiltration rate and sufficient supply of surface or groundwater. In the case of steep or irregular slopes, soils with a very high infiltration rate or scarcity of water, sprinkler and drip irrigation may be more appropriate.
Basin, Furrow or Border Irrigation:
Important factors determining which surface irrigation method (basin, furrow or border (strip) irrigation) is most suitable are briefly discussed. It is not possible to give specific guidelines leading to a single best solution; each option has its advantages and disadvantages.
Factors to be taken into account include:
1. Natural circumstances (slope, soil type)
2. Type of crop
3. Required depth of irrigation application
4. Level of technology
5. Previous experience with irrigation
6. Required labour inputs.
1. Natural Circumstances:
Flat lands, with a slope of 0.1 per cent or less, are best suited for basin irrigation: little land leveling will be required. If the slope is more than 1.0 per cent, terraces can be constructed with considerable land leveling.
Furrow irrigation (short, near horizontal furrows) can be used on flat land and on mildly sloping land with a maximum slope of 0.5 per cent. On steeper sloping land, contour furrows can be used up to a maximum land slope of 3.0 per cent. A minimum slope of 0.05 per cent is recommended to assist drainage.
Border irrigation can be used on sloping land up to 2.0 per cent on sandy soil and 5.0 per cent on clay soil. A minimum slope of 0.05 per cent is recommended to ensure adequate drainage.
Surface irrigation may be difficult to use on irregular slopes as considerable land leveling may be required to achieve the required land gradients.
All soil types, except coarse sand with an infiltration rate of more than 30 mm per hour, can be used for surface irrigation. If the infiltration rate is higher than 30 mm per hour, sprinkler or drip irrigation should be used.
2. Type of Crop:
Many crops (maize, sorghum, trees etc.) can be grown in basins/check basins. Those that cannot stand a very wet soil for more than 12-24 hrs should not be grown in basins/ check basins.
Furrow irrigation is best used for irrigating row crops such as maize, cotton, vegetables etc. Border irrigation is, particularly, suitable for close growing crops such as groundnut, fingermillet etc. but it can also be used for several other row crops.
3. Required Depth of Irrigation Application:
When the irrigation schedule has been determined, it is known how much water has to be given per irrigation. It must be checked that this amount can indeed be given, with the irrigation method under consideration.
Field experience has shown that most water can be applied per irrigation application when using basin irrigation, less with border irrigation and least with furrow irrigation. In practice, in small scale irrigation projects, usually 40-70 mm of water are applied in basin irrigation, 30-60 mm in border irrigation and 20-50 mm in furrow irrigation.
This means that if only little water is to be applied per application, as for on sandy soils and a shallow rooting crop, furrow irrigation would be most appropriate. None of the surface irrigation methods can be used if the sand is very coarse (if the infiltration rate is more than 30 mm hr-1).
If, on the other hand, a large amount of irrigation water is to be applied per application, as on a clay soil and with a deep rooting crop, border or basin irrigation would be more appropriate.
Above considerations have been summarised in Table 8.1. Net irrigation application values used are only a rough guide. They result from a combination of soil type and rooting depth. For example: if the soil is sandy and the rooting depth of the crop is medium, it is estimated that the net depth of each irrigation application will be in the order of 35 mm. Last column indicates which irrigation method is most suitable. In this case, medium furrows or short borders.
4. Level of Technology:
Basin irrigation is the simplest of the surface irrigation methods. If the basins are small, they can be constructed by hand or animal traction. Their operation and maintenance is simple.
Furrow irrigation – with the possible exception of short, level furrows – requires accurate field grading. This is often done by machines (tractor drawn implements). The maintenance- ploughing and furrowing – is also often done by tractors.
Short, level furrows – also called furrow basins – can, like basins, be constructed and maintained by hand.
Borders require the highest level of sophistication. They are constructed and maintained by machines. The grading needs to be accurate. Machine operation requires a high level of skill, organisation and usually foreign currency.
5. Previous Experience with Irrigation:
If there is no tradition in irrigation, the most simple irrigation method to introduce is basin/check basin irrigation. The smaller the basins, the easier their construction, operation and maintenance.
If irrigation is used traditionally, it is usually simpler to improve the traditional irrigation method than it is to introduce a previously unknown method.
6. Required Labour Inputs:
Required labour inputs for construction and maintenance depend heavily on the extent to which machinery is used.
In general, it can be stated that to operate the system, basin irrigation requires the least labour and the least skill. For the operation of furrow and border irrigation systems, more labour is required combined with more skill.