SALINITY MANAGEMENT IN A RIVER SYSTEM IN SOUTHWEST REGION OF BANGLADESH

Abstract

Bangladesh's Southwest Region (SWR) is severely affected by salinity intrusion. The situation has now reached a level where salinity seriously hampers irrigation, domestic as well as industrial water supply. High salinity in the region is associated with the decreasing upstream freshwater flow. The Gorai is the only remaining major spill channel to the Ganges that supplies freshwater to the region. The Gorai drains into the Bay of Bengal mainly by the Gorai-Nabaganga-Atai-Rupsa-Kazibacha-Pussur river system. It is observed that the dry season flow in the Gorai, during the period 1975-1998, has been declined from about 150 cumec to zero cumec. In fact, since 1989, the offtake of the Gorai fully dried up during the dry season and completely cutting off the supply of fresh water to SWR. Due to shortage of freshwater flow and ongoing groundwater utilization leading to increase of surface water salinity, ground water salinity and soil salinity in the region. It was revealed that salinity pattern .showed both seasonal and spatial variation. 'Salinity starts to rise from the month of November and reaching its maximum value in April and early May following the trend of low flows in the Gorai river. During dry season (Nov-May), salinity of the river system ranging from 300 μmhos/cm to 50,950 μmhos/um and wet season (June-Oct), it was about zero to 48,820 μmhos/cm. The river system showed a north-south gradient of salinity level. The study deals with the management of salinity and its intrusion processes in the Gorai-Nabaganga-Atai-Rupsa-Kazibacha-Pussur river system for the period 1975-2002. The study is based on analysis of flow and salinity data at selected stations (GRB, Bardia, Khulna, Mongla and Hironpoint) on the river system. From the study, it was observed that there was a decreasing trend of Gorai discharge at GRB and displayed increasing trend of salinity level at the study stations on the river system. Salinity build up condition in the river system was non-linear and the condition was higher at Khulna and lower at Hironpoint. The management of salinity was envisaged by augmentation of freshwater flow in the river system to keep the salinity level within tolerable range for different uses of water. MIKE 11 model was used to evaluate various options of freshwater augmentation. It was observed that for keeping the salinity level within permissible limits for irrigation and drinking purposes from Gorai offtake to Khulna, i.e., about 247 km of upper reach of the river system, the minimum flow in the Gorai at GRB should be 105 cumec and 165 cumec respectively without changing other existing condition of the river system. . It also emerged from this study that further studies are required for better understanding the process of salinity intrusion and its management in the Southwest region of Bangladesh. To do so, improvement in techniques should be applied for data collection, analysis and modelling work.