A. Description of the Study Area
1) Site Location and Observation Wells The north-western region of Bangladesh is mainly occupied by Pleistocene deposits (Morgan and McIntire, 1959). It covers the district of Rajshahi, Nator and Bogra. This region is situated between 24 o 22´ to 24 o 51´ North and 89o 18´ to 89o 22´ East. The present study is however based on part of a configuration of three districts (Rajshahi district only, comprising 08 sub-districts called ‘Upazila’). The water-table data were collected from the Bangladesh Water Development Board (BWDB), which is the department responsible for water-related records. From the monthly records, maximum depth to water-table was taken for analysis.
2) Topography and Hydro-Geological Conditions The topography of the region is characterized by two distinct landforms: (a) The Barind tract – dissected and undulating, and (b) The floodplains. The elevated Barind tract is characterized by less infiltration due to clayey and semi to impermeable Barind clay with excessive surface runoff. Morpho-stratigraphically, the region is subdivided into three geological units: (1) Barind clay residuum – overlies and developed on Pleistocene alluvium, (2) Holocene Ganges flood-plain alluvium, (3) Active channel deposits of the Ganges and major distributaries (modern alluvium). The lithology types include alluvial sand, alluvial silt, Barind clay residuum, and Marsh clay and peat (Alam et al., 1990). Hydrogeologically, the area covered by semi-impervious claysilt aquitard of the Recent-Pleistocene period (thickness 3.0 – 47.5 m) is characterized by single to multiple layered (2 – 4) aquifer system of Plio-Pleistocene age (thickness 5.0 – 42.5 m) (Jahan et al. 2005).
3) Rainfall Pattern of the Area The annual rainfall at the study site varies from 843 mm to 2241 mm. Approximately 83% of this rainfall occurs during the months from May to September, which is noted as monsoon season. The yearly rainfall fluctuates considerably, with mean, standard deviation and coefficient of variation of 1532 mm, 294 mm, and 19 %, respectively.
B. Pattern of Water-Table Fluctuation Long-term patterns of water-table are presented graphically to explain the dynamic behaviour of groundwater table. To analyze the long-term trend, the yearly maximum depth to water-table was used.
C. Studies of Water-Table Trend To detect and estimate trends of the water-table, the "MAKESENS" software is used. It utilizes the non-parametric Mann-Kendall test for trend detection, and the non-parametric Sen's method for detection of the magnitude of the trend (Salmi et al., 2002). Mann-Kendall test compares the relative magnitudes of sample data rather than the data values themselves. One benefit of this test is that the data need not conform to any particular distribution, and thus are applicable for both monotonic and non-monotonic trend.