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close this book Using water efficiently - Technological options
close this folder Water use efficiency
View the document What efficiency are we talking about?
View the document What are current levels of water use efficiency in irrigation?
View the document Factors affecting irrigation water use efficiency
View the document Water use efficiency in the urban sector: Definitions
View the document Factors affecting urban water use efficiency: Examples

Factors affecting irrigation water use efficiency

Many factors affect WUE in the irrigation sector. They include seepage, percolation, soil depth and texture, evaporation and evapo-transpiration, design of irrigation structures and their operation and maintenance, and management skills. At various efficiency levels, climate and rainfall patterns, size of irrigated areas, and methods of water application also play important roles.

SEEPAGE AND PERCOLATION losses reflect irrigation water losses from unlined and poorly lined distribution canals, ditches, and from crop fields. In the Bas-Rhone region of France, main canals are entirely lined and well maintained. This results in a high network efficiency of 75-85 percent. In Pakistan, losses in conveyance systems are high. About 25 percent of the supplies diverted from rivers is lost in the canal system through seepage and evaporation before it reaches distribution inlets. From the inlets, losses through secondary watercourses have been measured at 2040 percent. As a result, only 45-60 percent of the supplies diverted from rivers is actually delivered to the fields (Murk, 1991). In Kyrghyzstan, seepage and leakage losses in the distribution system are also considerable. Only 24 percent of the canals are lined, resulting in a network efficiency of 55 percent (Le Moigne, 1992b). Seepage losses are sometimes reused elsewhere in the basin. This aspect will be discussed in Chapter V.

SOIL DEPTH AND TEXTURE can make a significant difference in efficiency levels. Two extreme examples are the Gezira scheme (Sudan) and East India. The Gezira irrigation system has an extremely high network efficiency of 93 percent (Plusquellec, 1990). Although the design of the minor canals is a contributing factor, the high efficiency is due mainly to the nature of the soil. The soil is highly impermeable and significantly reduces leakages from the system. These factors account for an overall efficiency level of 70 percent. In some areas in East India, soils are shallow and rice irrigation is performed over hard-rock areas. These effectively prevent water losses and lead to high field efficiency levels of about 85 percent (Frederiksen, 1992). Frederiksen's study also shows that water applications needed for rice production on heavy clay soils can be only a quarter of those on light textured soils. Canals passing through coarse materials, common in alluvial fans, can lose huge quantities of water.

EVAPORATION AND EVAPO-TRANSPIRATION losses are associated with open canals, irrigated fields and crop growth. In Egypt, the annual evaporation losses from irrigation canals are estimated at 2 billion m³ (Abu Zeid, 1991). In Jordan, the high evaporation rates and seepage losses from open irrigation canals in the Jordan Valley are one of the main causes of water losses of up to 58 percent in the agricultural sector (Abu Taleb, 1991). The study by Abu Taleb shows that, if these losses are effectively reduced, the quantity of water savings could reach 50 million m³ per year. Cyprus has a high network efficiency of 95 percent (Van Tuijl, 1992), due to complete pipe conveyance systems distributing water to the sprinkler and drip irrigated fields. The average on-farm efficiency is estimated at 70 percent, and overall efficiency 66 percent. The systems have successfully prevented losses from both seepage and evaporation.

FAILURES IN DESIGN OF IRRIGATION STRUCTURES contribute greatly to inefficient water use. Many systems were designed to meet only limited objectives, and are not suitable for modern agricultural practices. Technical constraints to these systems often limit the possibility for improvement through better management, such as in some areas of Ethiopia (Abate, 1991), where many canals in the small districts in the highland areas are unprotected against erosion. The headworks of canals are often washed away when floods occur.

Poor land leveling has been a constraint to proper on-farm water management. For instance, many areas in Upper Egypt that were converted to perennial irrigation after construction of the Aswan High Dam are not properly leveled. Fragmented land and small and separate holdings limit establishing efficient irrigation methods. Surface irrigation systems are used in most cultivated lands of the Nile Valley. The overall water use efficiency of individual farms is generally low. Farmers apply excessive irrigation water to reach areas at higher elevations. As a result, water which is not consumed by plants infiltrates and recharges groundwater or flows into the drainage system (Abu Zeid, 1991). Although downstream users along the Nile reuse a large part of the drained water, excess irrigation water leads to salinity problems by raising groundwater tables.

The main cause of high water losses in the irrigation systems of Kyrghyzstan is the poorly designed structure of distribution canals (Le Moigne, 1992b). As a result, the facilities for water control are underdeveloped. Most gates, manually operated, do not function because of poor maintenance and vandalism. Joints between units are often missing. By contrast, the main canals-particularly those downstream of large storage dams--are better designed and more advanced, with remote monitoring and automatic control. Maintenance of the equipment is of a high standard. Clearly, the appropriate design of irrigation systems is a prerequisite for effective operations and management.

LACK OF WATER CONTROL DURING NIGHT AND WEEKEND IRRIGATION is another problem in many developing countries. The study by Abu Zeid (1991) shows that, in Egypt, the average conveyance losses between main canal intakes and distribution outlets was 25 percent. That between the distribution outlets and fields was 11 percent. The combined effect leads to a network efficiency of 67 percent. The main reason for these losses was that farmers abstained from night irrigation. Irrigation networks were designed to operate for 24 hours a day. Thus, considerable amounts of water were drained wastefully at night, when irrigation was not practiced. As a result, some farmers faced water shortages during the day. A conservative estimate for Ethiopia shows that it is possible to increase the current irrigated area by 20-40 percent by reducing irrigation water losses during nights and weekends (Abate, 1991). In Sudan, the original design and operational concept of the Gezira scheme adopted night storage systems (Plusquellec, 1990). By adjusting water releases at the headworks according to demand, it was possible to reduce excessive water losses. Due to various reasons (see following section), the night storage system was not used for a period of time. It was re-introduced by the Government after revising the design of the minor canals (Zaki, 1991). The new system not only reduces operational water losses, but also reduces siltation in the minor canals downstream.

WEAKNESSES IN MANAGEMENT means poor implementation of water control regulations and operation rules, and inadequate maintenance. It is an important factor explaining water losses in the irrigation sector. Inadequate O&M has caused severe deterioration of irrigation canals in many countries. The two Lam Pao projects in Thailand are examples of losses due to poor maintenance of irrigation diversion structures (QED, 1990). The two projects showed lower than expected efficiencies (28 percent instead of the 55-58 percent estimated at appraisal). The main reason for water losses is seepage from the main canals. Although the canals were lined, cracks and breakages occurred all over the canal linings because of failures in maintenance and inadequate weed cleaning in the tertiary system. As a result, there was little difference in seepage losses between lined and unlined canals. The same is true for some project areas in the Philippines (AST, 1991). In Egypt, for nearly 25 percent of existing canals, the actual widths exceed the design widths due to degradation and the misuse of canal banks. This has consequently changed water levels and canal discharges (Abu Zeid, 1991).

The regulations for managing water systems are often inadequately designed to meet variable supplies and demands. In Sudan, for instance, irrigation management operates on the basis of 'upstream control'. The Ministry of Irrigation controls the delivery of water to the heads of minor canals. From there on, field inspectors have the responsibility for supervising the rotational delivery of water to the fields. Farmers or farmer organizations handle the on-farm water management. This division of responsibility has been problematic. Farming programs, which determine crops, cropped area, rotation and cropping intensity, often have not been reflected adequately in the water delivery programs (Zaki, 1991).

CLIMATE PATTERNS AND EFFECTIVE RAINFALL affect irrigation water use efficiency. Reviewing previous definitions, the actual irrigation requirement, Vm, is the crop water requirement minus effective rainfall. Under-irrigation or over-irrigation in different seasons artificially affects efficiency levels.

The Philippines Upper Pampanga River Integrated Irrigation System (UPRIIS) is a typical example. Table 4 shows the overall efficiency, Eo, during both seasons for three continuous years. Eo is higher in the dry season. In the wet season, Eo is low due to high rainfall. There were apparently not enough incentives for farmers to save excess water from the run-ofriver system. In fact, project staff reported that during wet seasons farmers complained more often about flooding from uncontrolled river flows and high rainfall of all than about water shortages. The low efficiency level of 20-30 percent reflected more the virtual absence of a need to use river flows and rainfall effectively, than the actual technical inefficiency in the system. Underirrigation during dry seasons also artificially increased efficiencies.

Table 4 Overall Efficiency for Two Seasons (Philippine UPRIIS projects)

 

1986

1987

1988

Wet season

23.3

32.5

28.0

Dry season

54.6

46.9

52.0

Source: OED report, 1990

A similar phenomenon has been seen in areas of Lam Pao in Thailand and in the Panuco basin in Mexico (QED, 1990). In some project areas, high rainfall occurs in the wet season and low cropping intensity is practiced during dry seasons. The average overall irrigation efficiencies in those areas is below 30 percent. In Thailand, the estimated overall irrigation efficiency varied widely, from 8-51 percent in the wet season, and from 17-70 percent in the dry season (Vadhanaphuti, 1991), depending on the physical condition of the infrastructure and the availability of water.

Under these circumstances, a distinction should be made between water diverted and water pumped or released from reservoirs. If water is released at the expense of a storage or reservoir, pumping costs and delivery operations, it will affect the operational efficiency of these facilities. Will surplus water cause problems of drainage, flooding, water logging, and salinity in downstream areas? Alternative indicators need to be used to measure water use efficiency in such cases.

METHODS OF WATER APPLICATION are an integral part of optimal water use. There are many references on WUE levels under different application methods. Syria is an example where the technique of basin (flood) irrigation is widely practiced. This method can cause water losses of more than 50 percent (Bakour, 1991). Irrigation network efficiencies are 60 percent in most of the agricultural schemes of the country. Of the total water use of currently 10.3 billion m³ in the agricultural sector, more than 4 billion m³ is lost every year. Excessive irrigation without well-designed drainage networks causes a rise in groundwater levels, leading to increased salinity and lower agricultural productivity. In Yemen, the spate irrigation method is widely practiced. According to a study by Van Tuijl (1992), the overall WUE is 20 percent, much lower than the developing country average. Although spate irrigation has a low efficiency, it is a commonly practiced method to economically capture flood waters for irrigation. It also recharges the groundwater aquifer, from which the water is pumped for reuse in irrigation.