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Prospects for agricultural water management
Under climate change and the related increased temperatures, evapotranspiration will increase and the river system will suffer during dry months because of the acute low-flow condition. Consequently, the dry season water demand for irrigated agriculture, salinity control, and stream flow maintenance will increase significantly leading to the escalating shortfall in water supplies. Greater cooperation is required between the riparian countries to augment dry season flows in the transboundary rivers so as to meet the increasing gaps in water availability.
The main source of irrigation water during the last decade has been groundwater. There is a question of risky over-dependence, and over use of this water resource, and related quality constraints and emerging environmental concerns. The NWMP (MoWR, 2001), therefore, focuses on short-term and long-term strategies for agricultural water management, such as: i) balanced conjunctive use of surface water and groundwater resources; ii) future growth of tubewell irrigation with FMTWs; iii) surface water conservation and rainwater harvesting using rubber dams in small and medium rivers; iv) new low-cost major irrigation schemes (gravity flow system); and v) larger floating pumps, particularly in the southeastern part of the country.
On-farm water management (OFWM) can be considered as a potentially useful measure to save irrigation water use per hectare and to expand the irrigation command area, mainly for STW and LLP irrigation within the coastal zone and other water crisis areas. Wherever feasible, tubewell irrigation should be integrated with domestic water supply. The RDA developed multipurpose low-cost DTW technology package is one that can be replicated in suitable areas.
According to NWP estimates in 1991, the expansion of irrigation coverage would reach its maximum potential limit by 2025. However, the rate of increase in water demand is expected to decline in response to demand-management practices such as conservation, water-use efficiency, recycling.
The strategy of water resources development has so far been centered on flood control and irrigation expansion to promote food grain production. Without denying the importance of food production and food security, it is now widely recognized that conflicts among alternative and competing uses of water are becoming sharper as the demand for water has been increasing. It is, therefore, necessary to formulate a long-term vision for IWRM to address the demands of all water using sectors in order to sustainably maintain the environment.
Some challenging issues that are related to agricultural water management, such as arsenic pollution, climate change, salinity, have been found in many locations. The impacts of which have already been discussed. As a result of reduced freshwater flows, caused by upstream abstraction of the Ganges water, the salinity front in the coastal areas of Bangladesh has already advanced. About 10 percent of the southwest region has experienced increased salinity in the wet season, which rises to 40 percent in the dry season (BWP, 2000).
The salinity problem adversely affects the availability of required irrigation water in this region. The possible solution to this particular problem could be to take advantage of the Ganges Water Sharing Treaty of 1996 between India and Bangladesh, which specified the amount of water to be released downstream of Farraka during the dry season. A major endeavour, to meet this end, is the Gorai Restoration Project. Moreover, assured in-stream flows in the Ganges have increased the potential for surface water augmentation in the southwest region after construction of the Ganges Barrage.