Rainwater harvesting for crop cultivation in the coastal saline area of Bangladesh

Soil is the prime natural resource on which the existence of mankind depends and, therefore, the optimum utilisation of soil needs adequate attention. Maintaining soil's good health for high productivity on sustainable basis is a Herculean task in a country like Bangladesh, where the soils and the agro-climatic conditions are extremely variable.

Bangladeshi farmers are not well aware of the problems, and pressures on land resources are increasing at a very fast rate due to high rate of population growth. The landmass of the coastal zone of Bangladesh still remains under-utilised and the intensity of cropping is much less than the national average. 

As the traditionally cultivated areas in the country are under constant pressure and already over utilised, it is of prime importance that in order to increase agricultural production in underutilised soils like that of the coastal zone should be appropriately utilised. 

The knowledge on the soils of the coastal region in respect of their extent, distribution, characteristics as well as their potentials are extremely important not only for land use but also for better prosperity.

The coastal zone represents a transition from terrestrial to marine environments and vice-versa. It comprises not only shoreline ecosystems, but also the upland watersheds draining into coastal waters, and the near shore sub-littoral ecosystems influenced by land-based activities. Functionally, it is a broad interface between land and sea that is strongly influenced by both.

The coastal zone has distinct agro-ecological conditions for crop production in Bangladesh. Of the 2.85 million hectares of coastal and off-shore areas, about 0.83 million hectares of arable land are affected by varying degrees of soil salinity.

Coastal saline soils vary widely in nature of salinity, depth of groundwater and its fluctuations along with seasonal variations in the salinity of surface water.

In the coastal zone, lands are single cropped with traditional varieties of T Aman rice in the monsoon when soil salinity comes down at a tolerable level. Recently, farmers are growing some other crops in some areas due to construction of embankment to prevent tidal surge of seawater. 

T Aman rice is affected by water logging due to inadequate drainage of rainwater. Cultivation of high yielding rice varieties is vulnerable in low-lying areas due to submergence by rainwater. 

Farmers cannot cultivate rice due to scarcity of fresh water for irrigation during the Boro season. Ground water near surface is saline and is not fit for irrigation. Groundwater from the deep aquifer at depth of 213 to 274 metres having salinity level below 0.5 dS per metre, which is suitable for irrigation but uneconomic due to high installation cost of deep tube well.

Most of the areas in the coastal region remain fallow during Rabi (dry) season. Few farmers cultivate pulses, potato, sweet potato, vegetables, oilseeds, groundnut and chilli using residual moisture in a limited scale in Rabi season. 

An alternative source of water to be suitable for irrigation may be helpful in this regard. An attempt has been made to study the possibility of using surface water for Rabi crop cultivation in the coastal area. 

Irrigation from surface water sources is becoming difficult because of unavailability of water. Most of the surface water bodies dry up in the irrigation season such as in Rabi season. 

Moreover, river flows in the country are gradually decreasing in the dry season due to gradual withdrawal of water in the upstream rivers by the neighbouring country. Generally, saltwater balance is maintained in the coastal zone due to upstream river flow and protected the lands from saltwater intrusion, which is not maintained. 

Salinisation of crop lands is increasing day by day. In this situation, conservation of rainwater in a reservoir in the monsoon period and utilising it in Rabi (dry) season may be a most suitable option for non-rice crops and vegetable cultivation in the coastal region. 

About 0.4 million hectares of land is cultivated for pulses and production is about 0.3 million metric tonne, according to Bangladesh Bureau of Statistics report 2005. And, pulse production needs to be increased to fulfil the protein deficiency. 

On the other hand, total vegetable production in Bangladesh is about 6.7 million metric tons which is not sufficient to fulfil vitamin deficiency of the people of Bangladesh. But to fulfil the vitamin deficiency, enough vegetables could be produced in coastal areas in Rabi season (December to April) to meet existing need of the country.

In this context, Bangladesh Rice Research Institute (BRRI) conducted a field experiment in a moderately saline soil at Sonagazi, Feni to study the feasibility of farm pond for irrigation of Rabi crop. Farm pond received rainwater and runoff from pre-defined farm land. Stored water was used for irrigation of chilli in Rabi season in the specified land. For comparison, rainfed chilli was also cultivated. Water salinity of ponds was less than 0.63 dS per metre during irrigation which was not harmful to chilli cultivation. Peak salinity levels of rainfed and irrigated cropped soils were 34-37 per cent and 42-44 per cent lower respectively, than fallow soil salinity level which generally varied from 1.0 to 5.2 dS per metre. 

Dry season cropping caused a lowering in topsoil salinity and ploughing fallow land during the season was also found to lower the salinity build-up. The salinity of upper soil layer (0-15 centimetres) came down at the range of 1.20 to 0.69 dS per metre in wet season (July to October) due to leaching of salts by rainfall. 

Farmers can cultivate rice in Aman season without any salinity hazard. In Rabi season, three irrigations were applied to chilli at 20 days interval from the date of transplanting during the growing period. Yield of chilli responded significantly to the application of irrigation. Irrigated chilli production was 30 per cent more compared to that of rainfed chilli. 

A farm pond storage simulation (FPSS) model was developed to simulate the water storage in the farm pond. The FPSS model optimised seepage and percolation losses for pond which was found 2.44 millimetres per day per metre. 

The model also showed that approximately 19 per cent of rainfall becomes runoff from the command area which could be stored for supplemental irrigation of Rabi crop. The estimated crop evapotranspiration was found 74 per cent of that of potential evapotranspiration as determined by pan evaporation method. 

Annual cost of pond was Tk3,416. The pond is economically profitable for chili irrigation in Rabi season. The command area was optimised for a given size of pond using long term rainfall and evaporation data and fitted to the normal distribution and optimum command area for desired probability of non-failure were chosen from the distribution. 

With these data, farm pond length-command area relationship was developed. From these relationships, the lengths of square-sized pond with depth of 2.25 metres having 1:1 side slope was estimated to be 32.30 metres for a non-failure probability of 80 per cent for a hectare farm land. 

The area occupied by pond was about 12 per cent of the command area. In the study area, 'irrigated chilli-rainfed T Aman' rice cultivation was found more profitable than that of 'rainfed chilli-rainfed T Aman' rice cultivation, the benefit cost ratio of 1.3. 

Therefore, rainwater harvesting in coastal saline areas using farm pond for irrigation is profitable for growing vegetable crops in Rabi season. 

In the coastal zone, about 3,47,671 ponds are scattered comprising of about 37,530 hectares of land, according to a The Bangladesh Bureau of Statistics report. 

Due to the lack of proper maintenance, a large number of ponds have already been abandoned. Re-excavating these large number of abandoned ponds and using the FPSS model, large amount of excess rainfall that occurs during June-September is drained out as surface runoff, this excess rainwater can be conserved in the re-excavated ponds or ditches in the crop field for Rabi crop irrigation; the ditches may also be used to drain excess water. 

Fishes can be reared in these ponds and ditches for household consumption as well as for commercial purpose. Similarly, open channels of various sizes catch rainwater but the storage of rainwater for off-season use becomes difficult due to absence of appropriate control structures.

Suitable control structure across the canal at the tail end can conserve rainwater, which could be used for Rabi crop irrigation. Also, the barren road-side ditches or borrow pits may be made deeper and wider enough to store large volume of rainwater for vegetable and pulse crops irrigation in the Rabi season. 

Hamidur Rahman Molla, former principal scientific officer, Bangladesh Rice Research Intitute