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Irrigation and drainage
Evolution of irrigation development
Irrigation is considered to be a necessary precondition for the enhancement of agricultural production. The earliest approach to irrigation facilities was during 1960-1970 with the construction of large-scale multipurpose irrigation, flood control and drainage projects. To some extent, these projects were successful for flood control and protecting coastal areas from tidal bores and saltwater intrusion. But they played a minor role in irrigation development and only about 7 percent of the total irrigable area was covered by these very costly projects. Though the country has abundant surface water resources, particularly in the monsoon season, its flat deltaic topography and the instability of major rivers make large gravity irrigation systems both technically difficult and costly. On the other hand, during the dry season irrigation using surface water has become difficult or practically impossible owing to the limited availability of surface water. Therefore the use of groundwater for irrigation has become increasingly important.
The expansion of minor (small-scale) irrigation is a vital component of the Government’s agriculture strategy. Minor irrigation consists of low lift pumps (LLP: power operated centrifugal pumps drawing water from rivers, creeks and ponds), shallow tubewells (STW: with a motorized suction mode pumping unit), deep tubewells (DTW: with a power operated force mode pumping unit), manually operated pumps (MOP: extracting water from a shallow tubewell) and traditional systems. At the end of the dry season, the water level falls beyond the suction limit of the centrifugal pump. In these situations, it is possible to draw water by placing the STW in a pit, which is called a deep-set shallow tubewell (DSSTW) or a very deep-set shallow tubewell (VDSSTW). Where static water levels fall further (over 10.7 m), a submersible or vertical turbine (FMTW: force mode tubewell) is needed.
Between 1950 and 1987, public tubewells, regulations governing private installations and public monopolies of the supply of pumps, motors and other equipment constrained irrigation development. Since 1972, the emphasis has been on minor irrigation using low lift pumps and tubewells (STW, DTW and FMTW).
From 1979 to 1984, there was a liberalized expansion of minor irrigation, mainly with STW in the private sector. In 1982, about 1.5 million ha were under food crop irrigation. The rate of minor irrigation development slowed from 39 000 STW in 1984 to less than 5 000 in 1986. This was because of a number of reasons: private sector STW sales were limited, there was official concern over reported declines in groundwater levels where STW operated, an embargo on all diesel engines was imposed in 1985, and engines were standardized.
In 1991, the National Minor Irrigation Development Project (NMIDP) was established in response to the needs of farmers and the requirement for increased private sector investment in minor irrigation technologies. The project activity mainly concentrated on VDSSTW and FMTW technology, whereas irrigation using STW was mainly controlled by the private sector. In 1994, 665 VDSSTW and 32 FMTW had been constructed by farmers as a result of the promotional action of the project. However, there has been a general reduction in the area irrigated by wells because of aquifer drawdown, and there has been an increase in salinity intrusion particularly along the coastal areas in the southwest of the country.
Currently, the irrigation potential is estimated as 6.93 million ha. During 2006, there were 29 170 DTW, 1 202 720 STW and 107 290 LLP and the total irrigated area was an estimated 4.88 million ha; where groundwater and surface water coverage were 81 percent and 19 percent, respectively. In 2008 the national irrigation coverage was 5.05 million ha, where groundwater covered 79 percent and surface water covered 21 percent of the total irrigated area (Table 5 and Figure 3). Table 6 gives a summary of irrigation methods using surface water and groundwater during 2008 is presented in Table 5.
In 1993, the total area of wetlands was 3.14 million ha, of which almost 1.55 million ha were cultivated. Thus, total water managed area is an estimated 6.59 million ha.
Surface irrigation is the only technology used in large irrigation schemes. In 2008, the total area equipped for full control irrigation covered by large irrigation schemes (major irrigation) was an estimated 0.14 million ha (3 percent). Small irrigation schemes covered a total area of 4.91 million ha (97 percent) (Figure 4).
In 1992, the average cost of irrigation development for large surface water schemes operated by the Bangladesh Water Development Board (BWDB) was an estimated US$522/ha as reported by the Food and Agriculture Organization of the United Nations (FAO, 2007). At that time the operation and maintenance (O&M) costs of these projects was estimated at US$100/ha; but under the 1983 Irrigation Ordinance, BWDB collected only Tk250/ha (nearly US$6/ha) as O&M fees. The average cost of irrigation development in minor irrigation schemes including O&M was estimated at US$50/ha in 1990-1991; recently this has risen to US$113/ha (BBS, 2008).
Role of irrigation in agricultural production, the economy and society
In 2008, total harvested irrigated cropped area was estimated at 5.98 million ha, of which the most important crops are rice accounting for 4.34 million ha (73 percent), wheat 0.31 million ha (5 percent), potatoes 0.26 million ha (4 percent) and vegetables 0.24 million ha (4 percent) (Table 5 and Figure 5).
Improved irrigation water management (IWM) practices, increased use of modern variety (MV) seeds and fertilizers have made a major break through in achieving almost self-sufficiency for cereal crop production. Irrigation is mainly practiced in the dry season to cultivate Boro rice and wheat. Supplementary irrigation could appreciably increase transplanted Aman rice production by mitigating the effects of drought.
Irrigated paddy yield is moderately high, ranging from 3.85 to 4.75 tonnes/ha. During 2007-2008 total Boro rice production (including HYV, hybrid and local varieties) amounted to 18.67 million tonnes and the total rice (Aus, Aman and Boro together) was 31.67 million tonnes (DAE, 2009). The total irrigated rice production was about 58 percent of the country’s total rice production.
Status and evolution of drainage systems
Because of the low-lying topography, about 26 500 km2 or 18 percent of the country is inundated during the monsoon season each year. During severe floods the affected area may exceed 53 000 km2 or 37 percent of the country and in extreme events, such as in the 1998 flood, about 66 percent of the country is inundated. Floods are caused by overspills from main rivers and their distributaries, overspills from tributaries and by direct rainfall. Flood control works can reduce floods caused by the first two, but only drainage can have any effect on the latter two. The basic benefit of drainage is water control – supply as well as removal. The particular benefits can be: i) potential increase in cropped area with earlier drainage; ii) higher yields from transplanted Aman rice with early planting; iii) crop diversification in the wet season with better drainage; and iv) more control over crop calendars and patterns with control of the water regime.
In 1964, a master plan was developed for water resources development. This envisaged the development of 58 flood protection and drainage projects covering about 5.8 million ha of land. Three types of polders were envisaged: gravity drainage, tidal sluice drainage and pump drainage.
Flood control and drainage projects have accounted for about half of the funds spent on water development projects since 1960. They include:
- large-scale projects such as the Coastal Embankment Project (949 000 ha), the Manu River Project (22 500 ha), the Teesta Right Embankment (39 000 ha), the Ganges-Kobadak Project (141 600 ha), the Brahmaputra Right Flood Embankment (226 000 ha), the Chandpur Irrigation Project (54 000 ha), and the Chalan Beel Project (125 000 ha);
- medium-scale projects such as the Sada-Bagda, Chenchuri Beel and Bamal-Salimpur-Kulabasukhali projects implemented under the Drainage and Flood Control Projects (DFC I to DFC IV) and financed by the World Bank. These projects typically cover areas of 10 000-30 000 ha and involve flood control and drainage with limited irrigation development; and
- small-scale projects such as those implemented under the Early Implemented Project, the Small-scale Irrigation Project and the Small-scale Drainage and Flood Control Project.
During the National Water Plan Phase I and Phase II period (1986-1991) the Master Plan Organization (MPO) made a comprehensive assessment of the ongoing water resources development projects (large-scale irrigation projects, flood control and drainage (FCD) projects, and flood control, drainage and irrigation (FCDI) projects. It was noted that the performance of FCD and FCDI projects needs to be improved under the NWP. The FCD projects under this NWP strategy would focus on gravity drainage schemes in shallow to medium flooded areas, and submersible embankments in deeply flooded areas. After 1991, FCD projects were implemented under the Flood Action Plan (FAP) by the MoWR. This was a comprehensive plan for the progressive reduction of floods from major rivers in association with improved drainage systems. Under the existing Five Year Plan (Planning Commission, 2009), the Government approved 12 FC and FCD investment projects in the Annual Development Programme (ADP) (2009-2010).
In 1993, the total area of wetlands was 3.14 million ha, of which 1.55 million ha were cultivated and 1.38 million ha were drained by surface drains. In 1992, the average cost of drainage development was US$192/ha.
Different types of floods occur in Bangladesh. Of the total cropped area, about 1.32 million ha are severely flood-prone and 5.05 million ha are moderately flood-prone. The flood protected area in 1990 was an estimated 4.20 million ha.