3.5 Rainwater harvesting for irrigation water supply

Technical Description

In Bangladesh, two types of agricultural water storages are observed. One type is used to store water on plots which are slightly inclined; water accumulates in the low lying portions of the fields or in a low lying areas nearby. Water stored using this method is used for both land preparation for aman paddy cropping and during the growing/milking stage of paddy farming when the water requirement is most critical. In Bangladesh, aman is transplanted in June/July and harvested in October/November. The other type of water storage involves the storage of rainwater in ditches or depressions located outside bunds or in land depressions beside road embankments. Water is carried from these depression to the fields in buckets, pitchers or traditional dhoon whenever it is needed. In some cases, if the waterbody is large enough, water may be used to irrigate a rabi crop or for fishing during the period of monsoonal inundation. In some upland areas of Bangladesh (North Central zone), rainwater is stored in low lying plots between two hills for use in times of necessary.

Figure 10. Stagnant Water on the Lower Slope of a Plot

Rainwater stored using this technology may be transferred between fields by overflowing successive plots and may be collected in the lowest fields in a given area. In the West-Central region, rainwater is harvested from lands situated at higher elevations and conveyed to storage ponds through culverts. In this region, because farm lands are situated at lower elevations than the storage ponds, stored rainwater may also be used for irrigating farm fields during the dry season. The specific location of the catchment area and its elevation relative to the fields to be irrigated are important factors in determining the potential for using these types of rainwater harvesting technologies.

Figure 11. Irrigation of Crops with Rainwater in Upland/Hilly Areas

Extent of Use

Rainwater storage in low lying portions of farms or in neighbouring plots is possible only when the locations and elevations of the plots are suited to such storage. No detailed data on the extent of use of this technology were identified, but the field survey suggested that, if such opportunities exist, farmers made use of these technologies. In upland areas, nearly all farms having access to rainwater stored in depressions within hills made use of such water for irrigating aman paddies. In saline areas, this practice was observed on lands located within polders or embankments erected to obstruct the intrusion of saline water. In these areas, there is a conflict between aquacultural (saline water-based shrimp culture) and agricultural (freshwater-based crop) production, which has led to violent confrontations. During the field visit, the murder of a local womens' leader, named Korunamoey Sardar and aged around 40 years, allegedly by shrimp growers has caused continued deep resentment among the local farmers and has strengthened the resolve of the local people to continue fresh water cultivation within the polder in defiance of the threats by shrimp cultivators.

Operation and Maintenance

The use of natural depressions does not eliminate the maintenance requirements. Repair work, such as repairing bunds, maintaining slopes, etc. involve about two person-days of family labour per year at an equivalent cost of $2.50, based upon a rate of $1.25 per day. Local agricultural equipment is used for this purpose.

Figure 12. Irrigation of Crops with Rainwater Using Traditional Methods

Level of Involvement

This technology is implemented at the household level, although it has been implemented sometimes at the community level.

Costs

Construction costs are minimal since this system of rainwater harvesting is practised only when the land is naturally sloped, when the farmer has a plot nearby with a low enough elevation, or when there is an existing ditch or depression available where rainwater can be stored.

Effectiveness of the Technology

Data from the North Central zone of Bangladesh shows that the use of this technology increases paddy crop yields by about 32%. In order to achieve similar yields as can be achieved through irrigation using rainwater harvesting techniques, farmers would have to incur additional expenses in transporting water from other sources. The cost of transporting water would be the equivalent of five to six person-days of family labour and one to two days of hired labour.

Suitability

This technology is suitable in areas where natural depressions are found and where there is adequate rainfall to produce runoff.

Advantages

The advantages of this technology are that it is a locally implemented technology, which can be used at little additional cost.

Disadvantages

The disadvantage of this technology is that rainwater cannot be stored for long time due to seepage losses. The method also lacks scientific rigour.

Cultural Acceptability

This technology is very well accepted and has no known cultural disadvantages.

Further Development of the Technology

This is a fully developed technology. However, development of criteria for sizing storage areas would make this technique more rigorous.

Information Sources

Mohammed Aslam, Saleh Ahmed Chowdhury, Alamgeer Faridul Hoque, and S.R. Sanwar, Intermediate Technology Group, House 32, Road 13A, Dhanmondi, Dhaka, Bangladesh, Tel. 880 2 811 934, Fax: 880 2 813 134, E-mail: [email protected].