Abstract:
The study area (462 aq.km.) of the thesis is located to the north-west region of Bangladesh. The groundwater table is declining permanently in the study area due to over abstraction of groundwater for irrigation purposes. Mohananda is the only perennial river of the area. The available water of Mohananda river (if stored) can be a potential alternative source for irrigation in the area which can minimize the stress on groundwater. In the study, the impact of surface water irrigation on groundwater evaluated in a specified area (Surface Water Irrigation Zone, 7200ha). Specific objectives of the study area are calibration and validation of mathematical models and investigation of future groundwater level without and with rubber dam using the modelling tools.
For the study groundwater level, surface water level, discharge, river cross-section, bore log, aquifer test, rainfall, evaporation, land use and abstraction data have been collected from secondary sources. The hydrological and hydrodynamic model has been setup using Rainfall-Runoff Module (NAM) and Hydrodynamic Module (HD) of the MIKE 11 respectively. The groundwater model has been set up of 100m grid size using MIKE SHE. The coupled MIKE 11 and MIKE SHE model has been calibrated with observed surface water level and groundwater level data respectively. The coupled SW-GW interaction model has been simulated up to year 2030. Three options such as Option-0 (base condition), Option-1 (without Rubber Dam) and Option-2 (with Rubber Dam) have been formulated, simulated and evaluated to attain the study objectives.
After analyzing the collected data for the period of 2001-2014, it has been found that the annual rainfall varies from 900 mm to 2500 mm, the lowest dry season water level is 12.5 mPWD (average bed level 9-10 mPWD) for the Mohananda River, the maximum depth to groundwater table ranges from 5- 29m in dry season in the area.
The groundwater level decreasing rate in high barind area varies from 734 mm/year to 1100 mm/year and in low barind area 147mm/year to 221 mm/year over last 15 years. Increased abstraction and decreased precipitation caused groundwater declination in the study area. From the modeling study, it has been observed that, the lowest groundwater level remains around maximum 16 mPWD to minimum -2.0 mPWD in 2013 in the study area, the maximum declination of groundwater level is around 2m found for Option-1 in 2029, which is regained by surface water irrigation (Option-2). Besides, the flow from river to aquifer for option-2 is reduced by 35 % compared to Option-1. Due to surface water irrigation, the groundwater level increased adjacent to the Mohananda River especially in the surface water irrigation area. Thus, flow from river to aquifer decreased and flow from aquifer to river increased for Option-2. The groundwater level decreasing rate is 96 mm/year for option-1 where the rate reduces to 50 mm/year in option-2 in Surface Water Irrigation Zone. In addition, it has been observed that, the influence area due to surface water irrigation for year 2020 is 234 sq.km where it has been found 242 sq. km for year 2029.
