Study on groundwater recharge in greater Sylhet area

Abstract

The greater Sylhet area comprises four districts namely Hobiganj, Moulvibazar, Sylhet and Sunarnganj situated in the northeastern part of the country. The desired expansion of inigated agriculture in the area depends on groundwater development. The aquifers underlying the area are mostly semi-confined or leaky in character. Existing rocks and gas pockets make the aquifer conditions complicated. The present study focuses on a preliminary assessment of groundwater recharge in the greater Sylhet area fulfilling its objectives such as to review the previous studies on the assessment of groundwater resource potential of the area and determination of groundwater recharge based on recent water table data. The method of estimating annual recharge from annual fluctuation of water table and specific yield has been utilized in this study. Recharge for each upazila of the study area has been calculated using data on water table fluctuations and specific yield collected from North East Minor Irrigation Project (NEMIP) of Ministry of Agriculture. The ultimate objective of NEMIP was to increase agricultural production by introducing specific changes in agricultural practices through development of groundwater resources by minor inigation technology. In order to popularize the use of various minor inigation equipments especially STW for extraction of groundwater resources with a view to increase the production of various diversified crops during dry season, Ministry of Agriculture with financial assistance of Asian Development Bank designed a project entitled North East Minor Irrigation Project (NEMIP) in the greater Sylhet area for the period from 1992-93 to 2001-02. As per NEMIP provision, Ministry of Agriculture mainly completed 1600 test bore drills to prepare a data base on the availability of aquifer conditions and conducted 28 pumping tests to determine aquifer characteristics in the project area. From the analysis of bore log data and long pumping tests it was found that the aquifers underlying the greater Sylhet area are mostly semi-confined or leaky in character. From this study it was also observed that the high capacity shallow tubewells (STW) may not be suitable at many places and it may happen that STW might dry up after prolonged pumping. In view of such a situation, the NEMIP installed 194 piezometers in the area to gather infonnation about the static water level and prepare a database so that suitable sites can be easily identified for installation of different types of tubewell with different capacities. Fortnightly water table data at tlle piezometers are being collected since 1995. But, these data have not yet been analyzed to assess groundwater development potential of this area. As such, this project work was taken up to study the groundwater table fluctuation and to assess the groundwater recharge in the greater Sylliet area. The previous studies undertaken to assess the groundwater resources potential in the study area have also been critically reviewed in this study. A total of 157 piezometers have been selected and water table recorded at these piezometers for the period from 1995-2001 has been collected from the district offices ofNEMIP. Groundwater hydrographs for the selected piezometers have been prepared and a contour of groundwater table in the dry season of 1999 has been drawn. The annual fluctuations in groundwater level for all piezometers have been worked out. Annual recharge for each upazila has been calculated from the values of the fluctuation of groundwater level and specific yield. The values of specific yield were considered from the values reported in the Aquifer Parameters Study by NEMlP (1997). A comparison of recharge values for the study area with those estimated by Karim (1987) was perfonned. The results show that the annual recharge to groundwater ranged from 20.4 to 946.5 mm with an average of 264.0 mm in the greater Sylliet area. The wide variation of annual recharge occurred due to the wide variation of specific yield in the area. The recharge in the area increased gradually with time. The groundwater table fluctuation varied from 0:7 to 3.4 m in the study area. The contour show that the depth of groundwater level ranged from 2.5 to 7.5 m on 15 April 1999.