Irrigation Systems
SELECTION OF IRRIGATION SYSTEMS
The irrigation can be accomplished by different methods: surface, subsurface, sprin-kler and drip [2]. With any of these means, it is necessary to select an irrigation system, before the design, the equipment specification and installation can proceed. In order to make a suitable selection of the system, one should carefully consider the capacities and limitations of all the potential alternatives. Tables 4.1 and 4.2 indicate the aspects that must be considered in the selection of an irrigation system. These aspects are listed as follows [9]:
1. The crop and related factors: Type, root depth, water consumption, develop-ment of diseases.
2. Soil characteristics: Texture and structure, depth and uniformity, infiltration rate, potential of erosion, salinity and internal drainage, topography and degree of irregularity.
3. Water supply: Source, quantity available and reliability, quality, solids in sus-pension, chemical analysis.
4. Value and availability of land.
5. Limitations and obstructions of flooding.
6. Phreatic level: Level of underground aquifer.
7. Climatic conditions: Flood and furrow systems are usually associated with long irrigation cycles (many days between irrigations), so while the evaporation for the first few days after wetting may be high, the soil surface soon reaches “air dry” condition, and evaporating for the remaining days of the cycle is essen-tially zero––the average evaporation over the entire cycle is often low.
8. Availability and reliability of energy.
9. Economic considerations: Investment of required capital, availability of credit and interest rate, durability of the equipment and annualized cost, inflation costs, crop yield and value.
10. Available technology.
11. Social considerations: Political and legal issues, cooperation of the habitants, availability and reliability of the manual labor, level of knowledge and special-ization of the manual labor, expectations of the government and inhabitants, desirable level of automation, potential damage by vandalism, and health is-sues.
Table 4.1. Conditions for the selection of an irrigation method.
Conditions Irrigation methods
Flood Furrow Sprinkler Drip
Topography Moderate to irregular Moderate Irregular Irregular
Soil permeability Good Good Excessive to good Excessive to good
Potential for erosion High High Low Low
Crop characteristics Sown by broadcast Sown in rows Crop value variable Crop value variable
Water flow requirements High High Moderate Low
Table 4.2. Comparison of irrigation methods.
TOPIC Flood Furrow Sprinkler Micro
Evaporation loss Low Low Medium Minimum
Wetting of the foliage High Medium High Minimum
Water consumption by weeds High High High Minimum
Surface drainage High High Medium Minimum
Control of irrigation depth Minimum Minimum Medium High
Crop yield per unit of applied water Minimum Minimum Medium High
Uniformity in the crop yield Little Medium Medium High
Soil aeration Minimum Little Little High
Interference of other tasks by the irrigation
method Low Low High Low
Application of fertilizers and pesticides through
the irrigation water Minimum Minimum Moderate High
Operation and labor cost Low Low Moderate High
Leveling of the land is required High High Low Minimum
Automation of the system Low Low High High
Energy requirements Low Low High High
Quality of water Minimum Minimum Moderate High
Use of filters Minimum Minimum Moderate High
Control of diseases and pests Minimum Minimum Moderate High
Capacities and Limitations Crop, soil and topography
Surface irrigation or irrigation by infiltration
There are different methods of surface irrigation for almost every crop. In some re-gions, hay crops are irrigated by the furrow method. The flood irrigation method is adequate for forage crops after an adequate leveling. Row crops are irrigated by the furrow method. The irrigation system works better when the soil is uniform because of good infiltration of the water. The length of a furrow is limited to 100 meters in a heavy textured soil. However it can go up to 400 meters in well textured soils. Furrow irrigation is adaptable in all soil types; however soils with fast or slow infiltration rates may need an excessive manual labor. Uniform and slopes of slight incline adapt bet-ter to the infiltration method of irrigation. The irregular topography and steep slopes increase the cost of leveling and reduce the length of a furrow. The deep plowing can affect soil productivity requiring a special fertilization. Steps, benches or terraces may be required to control erosion in the case of high flow rates.
Sprinkler irrigation
Almost every crop and soil can be irrigated with some type of sprinkler irrigation system, although the crop characteristics and crop height must be considered when the system is selected. Sometimes the sprinklers are used to germinate the seed and to cover soil. For this purpose, the light and frequent applications can be easily accom-plished with some type of sprinkler system.
Soils with infiltration rates less than 5 mm/h may require special considerations.
Sprinklers can be used in soils with depths too shallow to permit effective surface irri-gation. In general, the sprinklers can be used in any topography that can be cultivated.
Generally the leveling of the land is not required. The sprinkler system is designed (with some exceptions) so that it can apply to water at lower intensity than the infiltra-tion rate of the soil in order that the quantity of water infiltrated at any point depends on the intensity of the application and time of application and not on the infiltration rate of the soil.
Drip irrigation
Drip/micro or trickle irrigation is more convenient for vineyards, tree orchards and row crops. The principal limitation is the high initial cost of the system that can be very high for crops with very narrow planting distance. Forage crops cannot be irrigated eco-nomically with drip irrigation. Drip irrigation is adaptable for almost all soils. In very fine textured soils, the intensity of water application can cause problems of aeration. In heavy soils, the lateral movement to the water is limited, thus more emitters per plant are needed to wet the desired area. With adequate design, use of pressure compensating drippers and pressure regulating valves, drip irrigation can be adapted to almost any topography. In some areas, drip irrigation is used successfully on steep slopes.
Quantity and quality of water Surface irrigation
It is important that the water applied for irrigation reaches the end of the field relative-ly quickrelative-ly to obtain a uniform irrigation. The volume flow rate required varies from l5
to 300 liters per second. The furrow method is not well suited for leaching salts from the soil, since water cannot stay in the soil for a required time. However, flood irriga-tion with border strips is ideal for this situairriga-tion.
Sprinkler irrigation
Sprinkler irrigation adapts particularly well to a situation with high water table, since the sprinkler equipment can make the application with a determined volume of water.
Sprinkler irrigation generally requires the application of smaller quantity of water than flood irrigation.
Drip irrigation
With drip irrigation, the application of water is less intense during a period of time lon-ger than in other methods of irrigation. The most economic design would utilize water flowing throughout the crop area during almost all the day, every day, during peak pe-riods of water use. If the water is not available constantly, then it may be necessary to store water. Saline water can be used successfully, with special precautions. The salts tend to concentrate in the perimeter of the wet volume of soil. For longer time intervals between the irrigations, the movement of the water in the soil can be reversed return-ing the salts to the root zone. When it rains after a period of accumulation of salts, the normal irrigation must be continued until approximately 50 mm of rain have fallen to prevent the damage by salts. In the arid regions, where annual rain is insufficient (<of 300 to 400 mm) to leach the salts, the artificial leaching of salts may need more time, requiring the use of a complementary sprinkler or surface irrigation method.
Sand (hydro cyclone) filters and screen filters are used to remove suspended solids in the irrigation water. A media filter can remove organic clogging agents. Chemical treatment of the water may be required to control biological activity, to adjust the pH or to avoid the chemical precipitation that can obstruct the emitters. Suitable design and periodic maintenance of the system for the treatment of the water are vital for the successful use of the drip irrigation. Drip irrigation can operate at low operating pres-sures and at low discharge rates.
Crop yields Surface irrigation
High potential crop yield is possible with surface irrigation, especially with the border strip method. The yields must be 80–90% in the design for the storage systems, except with soils having a very high infiltration capacity. Reasonable irrigation efficiency for border strip varies from 70 to 85% compared to 65–75% for furrow irrigation. One must take into account factors such as the runoff and drainage water characteristics.
The engineer and the operator can control many of the factors that affect the irriga-tion efficiency. However, the potential uniformity of the water applicairriga-tion in a surface irrigation is limited by the variation of the soil properties, mainly the infiltration ca-pacity. Studies indicate that relatively uniform soils can have a uniformity of 80% in a single irrigation. Researchers have suggested that the calculations of uniformity of the surface irrigation based on the infiltration rate need to be reduced by 5%–10% due to variation of the soil properties.
Sprinkler irrigation
The sprinkler irrigation system can give performance as tabulated next page:
Manual or portable 65–75%
Properly designed and maintained drip irrigation is able to give high performance. The design efficiency can vary from the 90 to 95%. With reasonable care and maintenance, the efficiencies of systems operation can range from 80 to 90%. Where obstruction is a problem, the emitter flow efficiency can be as low as 60%.
Considerations of Manual Labor and Energy Surface irrigation
The flood irrigation requires minimum use of manual labor compared to other sur-face methods. Furrow irrigation can be automated to a certain degree. To reduce the requirements for manual labor input, furrow irrigation requires expertise to obtain high performance. The need for this expertise can be reduced with the use of higher cost equipment. Putting siphons or tracks are useful to measure the desired flow rate.
The surface irrigation method requires little or no energy, to distribute the water to the entire field. However the energy is required for taking the water to the field, and for pumping the underground water. In some cases, these energy costs can be substantial, particularly with a low efficiency of water application. Some manual labor and energy for leveling and preparing the land are always needed.
Sprinkler irrigation
The requirement for manual labor varies depending on the degree of automation and mechanization for the equipment to be used. Manual systems require the least degree of ability, but more hours of labor. On the other end, the center pivot and linear move systems require considerable operational ability and fewer hours of labor. The energy consumption related to the requirements of operating pressure varies considerably among all methods of sprinkler irrigation. The traveling sprinkler system with pro-gressive movement requires seven bars or more of pressure. Other systems can use of two to five bars, depending on the design of the sprinklers and nozzles.
Drip irrigation
Due to characteristics of low flow and relatively long irrigation set times, drip irriga-tion can be automated totally. Therefore, manual labor is needed only for inspecirriga-tion and maintenance of the system and the initial installation. The manual labor require-ments for maintenance are related to the sensitivity of the emitters to obstructions and the quality of the irrigation water. In a vineyard, for example, a worker can inspect and maintain approximately 20 hectares per day.
Generally, the drip irrigation method requires less energy than other pressurized irrigation systems. The operating pressure ranges from 0.5 to 1.5 bars. The pressure of the system fluctuate from approximately 2 (small flat land systems) to four bars (steep slopes and uneven lands).
ECONOMIC CONSIDERATIONS