2.1. Study Area Bangladesh has eight Hydrological Regions, which are mainly characterized by surface water flow processes and major rivers as boundaries. These regions are North West (NW), North Central (NC), North East (NE), South East (SE), South Central (SC), South West (SW), Eastern Hills (EH), and the active flood plains and char lands of the main rivers and estuaries. It is noted that although usually, the boundaries of watersheds or water region coincide with water divides, the hydrological regions in Bangladesh were delineated based on appropriate natural features for the planning of the development of their water resources. Accordingly, in defining the hydrological regions, the principles adopted were: (i) the entire country should be covered; (ii) the principal rivers and natural features should form the boundaries; (iii) the principal rivers themselves should form a region; and (iv) effective use should be made of previous studies. The Barind Tract and its surrounding areas have been divided into three major physiographic units and fifteen sub-units. The major units are the Barind Tract, Channel-floodplain complexes, and Himalayan piedmont plain (Tisa Fan). The elevation of the Barind Tract ranges from about 11–48 m above mean sea level (AMSL), the floodplain is about 8–23 m AMSL, and the Tista Fan is about 30–45 m AMSL. There is a wide variation in the sub-surface lithology of the NW hydrological region. The upper-most layer of Rajshahi area comprised a 2.5 to 35 m thick clay to silty clay layer, but in Pabna, the upper-most clay to silty clay layer varies from 1 to 20 m. The aquifers in these areas are generally unconfined. In the northern half of NW region (Rangpur area), the upper layer of soil comprised coarse and medium sand and gravel and the aquifers are unconfined. This study focused on the NW hydrological region, which covers the administrative divisions of Rajshahi and Rangpur. Rajshahi division, consisting of 8 districts and 70 upazillas, has an area of 18,174.4 square kilometers and a population of 18,484,858, and Rangpur division, which consists of 8 districts and 58 upazillas, has a population of 15,665,000. In the NW region, the mean maximum temperature is 32 0C in summer, mean minimum temperature is 10 0C in winter, annual average rainfall is 1927 mm, and potential evapotranspiration is 1309 mm.
Groundwater irrigation is crucial to the recent attainment of rice grain food security in Bangladesh. The NW region alone supplies about 35 percent of the irrigated Boro rice and about 60 percent of the wheat of the whole country. The use of groundwater in this region is the most intense; over 97% of the total area (85% of net cultivable area) was irrigated by groundwater during 2012–2013. The increased irrigation coverage associated with increased extraction of groundwater from the aquifers over the last three decades has contributed to decline in groundwater levels. Therefore, the NW region, especially, the Barind area, is of greatest concern over falling groundwater levels, which lead to a lack of access to water for drinking and irrigation in some areas. Sustaining groundwater irrigation in the NW region is, therefore, vital for the domestic water supply and future food security of Bangladesh. In this context, investigation of the groundwater table dynamics in the NW region to evaluate the nature of equilibrium of the groundwater resources is crucial, since this can dictate policies for future groundwater use in the region.
2.2. Data Collection and Preparation Bangladesh Water Development Board (BWDB) maintains an extensive groundwater monitoring database, which contains time series of groundwater table depth recordings of about 1200 monitoring wells across the country. Although some wells have GWT data from the mid-1960s onwards, most wells have weekly GWT data from 1985. The NW region has a total of 437 monitoring wells, and their water table data from 1985 to 2016 were analyzed in this study. First, by plotting the observed GWT data against time in scatter plots, the important features of the data, such as trends, seasonality, discontinuities, and outliers, were identified. The monitoring wells with more than 5 years discontinuous data and irrational or erratic distribution of water tables, and also the wells with identification problems, were discarded. In total, 350 monitoring wells had good quality GWT data and these were selected for trend analysis and critical GWT depth identification. Because of the large-size information, the geographical distribution of these monitoring wells is provided as supplementary material. From the weekly GWT data of the monitoring wells, annual maximum and minimum depths of GWTs from ground surface were identified and recorded in a MS Excel spread sheet separately for each well.
2.4. Water Scarcity Identification All suction-mode pumps, including STW and HTW, utilize atmospheric pressure to lift water. The standard atmospheric pressure is 1.034 kg/cm2, which is equivalent to 10.34 m of water column. Therefore, the maximum theoretical lift for suction-mode tubewells for abstraction of groundwater is 10.34 m, which reduces to 8 m due to frictional head losses in the piping system. A further reduction in suction lift occurs due to dynamic drawdown of GWT during the pumping period, which depends on aquifers’ water transmitting properties and pumping rate. We determined specific drawdown (drawdown per unit discharge) from discharge-drawdown data of 31 DTWs of 19 upazillas of the North-West region. The specific drawdown varied from 0.14 to 15.24 m/m3 /s, with a mean of 5.69 m/m3 /s. This implies that a dynamic drawdown of 0.14 to 15.24 m per cubic meter per second of well discharge will occur at the well face during pumping. The design capacity of most STWs in the North-West region is 0.5 m3 /s (14.3 L/s). Therefore, an average dynamic drawdown of 2.85 m will occur at the well face during the pumping period. The actual capacity of STWs is, however, smaller than the design capacity, and hence the dynamic drawdown will be smaller than the observed values. In this study, we used an average 2 m dynamic drawdown in calculating the suction lift limit of STWs. Thus, considering frictional head losses in the piping system and a dynamic drawdown of GWT (≈2 m) during pumping, practically, 6 m was considered as the critical depth (maximum suction lift) for pumping groundwater by the suction-mode pumps. However, the low-capacity suction-mode pumps, such as HTWs, do not create considerable drawdown of the GWT during pumping and they can operate up to 8.0 m suction lift or even a bit more in case of low frictional head loss. If GWT resides below the suction limit, it makes the suction-mode pumps inoperative.
The groundwater monitoring wells were categorized into three groups based on their GWT depths over the study period (1985–2016): (i) wells with GWTs below critical depth (6 m) during the whole year, (ii) wells with GWTs below critical depth for certain months of the year, and (iii) wells with GWTs above critical depth during the whole year. The locations (up to village level) of these three categories of monitoring wells were identified to reveal the conditions of groundwater availability of the places from suction-mode pumps, especially for domestic water supply.