## 4 Groundwater Management
### Water Table Lowering  
The geomorphology of Bangladesh can be divided into four regions, namely, tableland (consisting of Pleistocene plain and Pleistocene upland), flood plain, deltaic, and hill tract [48]. While flood plains dominate in most of the northwest, northeast, north-central, and south-eastern part, tableland is observed in some parts of the northwest and north-central region. The south-western part is under deltaic plains with some exceptions for the presence of coastal plains in the coastal areas of Bangladesh [48; 58; 59; 60]. The thickness of aquifers differs depending on the type of aquifer and location. For example, the thickness of shallow aquifers ranges from a few meters in the northwest to 60-75 m in the south, and the deeper aquifers range from 190-960 m in the northwest to 250-1500 m in the south [60]. The Pleistocene aquifers are observed in terrace areas, including the north-central (i.e., greater Dhaka) region and the Barind Tract in the northwest part of Bangladesh [59; 60]. According to [PERSON] et al. [61], the recharging of aquifers in Bangladesh occurs by surface water during the dry season (November-March) and by rainfall during March-June. Typically, 40% of the yearly recharge is contributed by surface water and 60% by rainfall. The groundwater is rarely recharged during a flood due to the absence of a hydraulic gradient. The extraction of groundwater has increased vastly in the last three decades. According to [PERSON] and [PERSON] [60], between 1985 and 2004 throughout the country, the number of shallow tube wells increased from 133,800 to 925,152, and the number of deep tube wells increased from 15,300 to 24,718. The lowering of the water table occurs when the extraction is higher than the recharge over a period. It is particularly evident in vast agricultural areas that rely on groundwater for irrigation. One such agricultural area is the Barind tract, a major rice-producing area situated in the northwest part of the country. Starting in 1986, an estimated 62% of the 767,900 ha in the Barind tract are irrigated with groundwater [60]. The groundwater is extracted using shallow and deep tube wells. [PERSON] et al. [33] reported a falling trend in the water table after analyzing the water table trend in 350 monitoring wells between 1985 and 2016; in about 60% of these wells, the water table was below the suction limit for three to six months of the year. A similar lowering trend in the water table was also observed by other researchers [62; 63; 64; 65; 33]. According to [PERSON] et al. [59] the groundwater level in this area in the pre-monsoon months shows a declining trend (due to continuous withdrawal of water for irrigation), but, in post-monsoon months, the level of the water table regains some extent. The regain of the water table depends on the recharge and is affected by the variability in the rainfall and, thus, climate change [59]. Apart from climate change, other significant reasons for less groundwater recharge are due to changes in the courses of major rivers (i.e., Ganges, Teesta, and Kosi), unsustainable irrigation practices (by the overexploitation of groundwater), and drainage characteristics [64]. The construction of the Farakka barrage upstream of the Ganges River in 1975 by India, which restricts water flowing downstream during the dry season, is also responsible for water scarcity in this area [66]. The lowering of the water table is also observed in metropolitan cities, such as Dhaka, the capital city of Bangladesh and one of the largest of 22 growing megacities in the world [67]. Unplanned groundwater extraction, rapid urbanization, land transformation, and growth of the urban population are some of the reasons for the water table lowering in Dhaka [60; 68]. In four decades (1970-2013), groundwater extraction in Dhaka city and its surroundings has increased from 0.5 million m\({}^{3}\)/day to 5.9 million m\({}^{3}\)/day to meet the domestic, commercial, industrial, and irrigation demand [67].