Probabilistic approach for water budget of Ganges-Padma, Brahmaputra-Jamuna and Surma-Meghna (GBM) river basins of Bangladesh

Abstract

Water budget is a very important aspect in discussing the environment. In the present study a picture of total water budget in Bangladesh has been shown. In the probabilistic water budget equation, precipitation and surface runoff are taken as total inflow in the catchment area. Evaporation and soil moisture has been taken as total outflow. The difference between the volume of inflow and volume of outflow has been taken as total volume of surface water loss in the river basin in the water budget in one year. It can be said that the ground water potential of the Ganges basin is roughly about forty times the flow of water in the river Ganges. Three major river basins of Bangladesh e.g. Ganges-Padma, Brahmaputra-Jamuna and Surma-Meghna (GBM) control almost 50% of the total agricultural area of Bangladesh. Bangladesh is facing climatological challenges. From current study it has been found that total loss of water in Ganges – Padma river basin was 2.31*1013 m3 in 2003. Loss of water was reduced by 13.8% in 2004. The average depth of rainfall increased in one year (2004 - 2005) is 1.45%. Total loss of water was reduced by 1.34% in 2005. In Brahmaputra-Jamuna river basin the average depth of rainfall increased in one year (2003- 2004) is 20.93%. Total loss of water was increased almost twice in 2004. In this basin, surface runoff in 2004 was lower by 3.37*1013 m3 in 2005. Total losses in the Brahmaputra – Jamuna river basin were reduced by 39.78% in 2005. In the Surma – Meghna river basin, total loss of water in 2004 was 1.23 times less than in 2003. But, the losses were increased by 19.05% in 2005. From the groundwater level assessment, it has been found that overall groundwater level lies 1.35 m to 20.25 m below the ground in six divisional headquarters. Groundwater level in Khulna is considered to be most shallow, which lies 1.35 m to 1.6 m below the ground whereas in Dhaka groundwater is 16 m to 20.25 m below. Depth of groundwater has been classified in six different classes according to availability of water in pre monsoon, monsoon, post monsoon and winter. A factorial design was done to see how temperature, evaporation and soil moisture, extraction of groundwater affect the groundwater table. It is very important to figure out the mutual relation among precipitation, ground water, surface water. Impact of climate change is pronounced on water budget. It has been observed that the average rainfall is below 50 mm during the months of January, February, November and December in twenty one districts in ten years (1996 – 2005) while during June, July and August more than 400 mm of average rainfall was observed with 95% confidence level. Vulnerable slope of groundwater has been chosen where the slope of groundwater is steepest in spite of heavy rainfall. Slope of groundwater has been classified in seven different classes according to steepness. From the current study it has been found that in 1998, the groundwater slope was very gentle in Bogra and Dhaka. These two cities were taken because of the difference in topography. Flood and drought are two extreme cases of surface runoff. Maximum discharge had been taken for calculating the return period of flood and minimum discharge had been taken for return period of drought. Three decades (80’s, 90’s and 2000-2007) were taken into consideration on the availability of data of discharge. There was an increase in surface runoff in Padma, Meghna and Jamuna in 2005, and the discharge for ten years return period of flood was higher for the decade 2000-2007. The specific flood and drought prone areas could be depicted if the flood and drought data were available.