3.4 Rainwater harvesting for agricultural water supply

Technical description

This is the most common and one of the earliest methods of rainwater harvesting used in Bangladesh. In this method, an earthen bund with a height and width of 30 to 45 cm is constructed around a field. Rainwater fills the aman created within the bund; an aman is a monsoonal paddy grown in June/July and harvested in October/November. In Bangladesh, the monsoon season begins in June/July, or some times in August if the monsoon comes late. With water stored on the field for few days, the soil softens and is suitable for preparation using ploughs drawn by animal power or power tiller. If the field retains excess water, it may be flushed out by cutting a portion of the bund. Once the land has been prepared, aman seedlings are transplanted or seeds are broadcast spread. In hilly areas, this technology can be applied in valleys between hills or wherever there is level ground on which water may be stored for few days before planting.

Figure 8. Rainwater is Stored Prior to Land Preparation

Extent of Use

All farmers whose lands are fed by harvested rainwater during the monsoons make use of this technology for preparing their lands. Farmers, by experience, have learned to match the aman crop cycle with the rainfall pattern, and the cropping cycle may be delayed by a month or so if monsoon season starts late in any particular year. This method not only softens the soil but also assists in controlling weeds by inundating organic matter and exposing weeds, crop residuals and other organic substances to aquatic decomposition processes. The organic matter is then mixed into the soil, thereby supplementing soil nutrients. Nearly all types of farms, large and small, use this process. The rainwater satisfies about 78% to 97% of the water requirements for land preparation for aman crops; the other farms, about 3% and 22% of farms, that are unable to match their cultivation with the onset of the monsoon rain, bring water from ponds or khals, rivers, wells or other waterbodies to prepare the fields for aman crops. In saline areas, harvested rainwater is used to provide agricultural water for aman paddy cropping until the river water becomes sweet. In these areas, harvested rainwater meets around 50% of water requirement for the aman crop, with the balance of the water being obtained from river sources.

Operation and Maintenance

The bunds need to be repaired every year.

Level of Involvement

This technology is implemented at the household level and requires little external involvement.

Costs

One time construction costs incurred in the creation of the bunds around an acre of land amount to six days of hired labour, or, at the 1996 wage rate of $ 1.50/day during harvesting period, about $9.00. Locally available agricultural equipment can be used for the bund construction. Annual repair costs typically involve two day of family labour per year at an equivalent cost of about $2.50. Costs per acre may be calculated at between four and five days of family labour and from three to six days of hired labour, or about $7 per season on average.

Figure 9. Excess Water is Discharged Through the Bund.

Effectiveness of the Technology

The information obtained during the field survey in Bangladesh suggests that paddy yields decline in cases where rainwater was not used to prepare the agricultural lands for planting. Declines in yields ranged from 44.8% to 51.4% of the yields obtained in fields that were prepared using this technology. To achieve similar yield as obtained with this technology, farmers would have had to incur additional expense.

Suitability

This technology is suitable for use in all flat areas with adequate rainfall.

Advantages

The technology uses locally available materials and is inexpensive to implement.

Disadvantages

The principle weakness of this technology is that it is dependent on rainfall, which may be uncertain. This may delay crop planting with consequential reductions in yield. Where there is no outlet for draining excess water from the bunded field, flooding may damage or destroy the standing crop.

Cultural Acceptability

Being a traditional technology, rainwater harvesting for agricultural supply is well accepted.

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].