Water usage and productivity of Boro rice at the field level and their impacts on the sustainable groundwater irrigation in the North-West Bangladesh

Groundwater-irrigated Boro rice is produced on 83 % of the net cultivable area (NCA) in North-West (NW) region of Bangladesh. Falling groundwater levels in many parts of the region raised concerns about the sustainability of groundwater irrigation. It is generally accepted that, in the absence of a comprehensive scientific study, uncontrolled groundwater use for Boro rice cultivation leads to water wastage and low water productivity. Therefore, it is crucial to know the actual field-level water usage and, irrigation water productivity, which will help identifying options to sustain groundwater irrigation. This study provides a comprehensive account of these aspects of Boro rice cultivation based on field observations at 420 farmers’ plots in 2015–16 and 2016–17 seasons across seven sites in the NW region. Necessary data, including land type, soil type, rice varieties, seeding and transplanting times, grain and biomass yields pump discharge, and irrigation amount were recorded. Average water productivity was 0.67 kg m−3 and 0.64 kg m−3 based on total available water (rainfall plus irrigation) in the fields, 0.80 kg m−3 and 0.95 kg m−3 based on supplied irrigation, and 1.60 kg m−3 and 1.78 kg m−3 based on estimated actual crop evapotranspiration (ETc) during 2015–16 and 2016–17, respectively. These water productivities are the highest among the major rice-growing Asian countries indicating limited scope for improving farmers’ water management practices. Comparison of the actual water supplied to the field and the estimated requirements shows that farmers are, in general, very efficient in supplying water to rice. In shallow tube well (STW) sites, water supplied by the farmers was very close to actual requirements, but rice plots in DTW sites had some over application. The average total amount of water available in the field to grow one kilogram of rice was 1,606 L (L) in 2015−16 and 1605 L in 2016−17. The Average irrigation water supplied to the field was 1402 L kg−1 in 2015−16 and 1086 L kg−1 in 2016−17. However, not all water supplied to the rice plots are consumed by the plants. Actual crop evapotranspiration is the real water use and based on that only 661 L in 2015−16 and 584 L in 2016−17 were required to grow one kilogram of rice. Percolation and seepage water return to the underlying aquifer as return flow. So, the current government policy of so called ‘water savings’ by reducing pumping of groundwater is unlikely to have any major impacts on the sustainable groundwater irrigation in the NW region.