Impact of unplanned withdrawal of graoundwater on shallow-tubewell irrigation in Bangladesh

Abstract

Review of available literature, discussion with agriculturists and a questionnaire survey carried out among farmers in 6 districts revealed that the failure of STW due to lowering of groundwater level during the peak dry season (March-April) is a major problem being faced by increasing numbers of farmers in different regions of the country. The problem became more pronounced since 1992 when the tubewell sitting rule was suspended and huge numbers of STW were sunk indiscriminately, which on one hand helped increase food production, while on the other hand aggravated the groundwater lowering problem. Among the respondents of the questionnaire survey, about 60% suffered from tubewell failure due to lowering of groundwater level; over 80% of these farmers reported reduced crop yield and financial loss due to lowering of groundwater level. No groundwater zoning map has so far been developed, which could be utilized by the engineers and agriculturists working in the field for advising farmers regarding proper irrigation equipment. Since April is the critical period (panicle initiation and flowering stage) for bora rice cultivation and during this month groundwater level usually reaches its lowest level, in this study groundwater zoning map was prepared for the entire country using data from 3819 groundwater level monitoring wells for the month of April 2004 (the latest year for which data were available). Areas where groundwater table lies within 7.6 m during the peak dry season are suitable for STW irrigation. According to the zoning maps, such areas, designated as "perfect STW zone" cover about 70,564 sq. km. Areas where water table lies below 11.3 m during April, deep tubewells are generally used to irrigate crops and such areas, designated as "deep tubewell zone" covers about 6,664 sq. km of the country. In areas where water table remains within 7.6 to 9.8 m during the peak dry season of April, farmers have to use Deep Set Shallow Tubewell (DSTW); areas where water table remains within 9.8 to 11.3 m are suitable for Very Deep Set Shallow Tubewells (VDSTW). Such areas have been designated as the "critical STW zone" and cover about 23,389 sq. km, which is about 23.3 and 15.9 percent of the gross irrigated area and total area of the country, respectively. Areas under critical zone could be found mostly in the north-western, central and south-central regions of the country. The critical STW zone covers some areas of 31 districts. In 9 districts (e.g., Jessore, Satkhira, Sunamganj, Dinajpur), the critical zone covers';; 10 percent of the total area of the respective districts, while in 6 districts (e.g., Pabna, Comilia, Kushtia), the critical zone covers greater than 50 percent of the area of the districts. An estimated 0.3 million STWs operate in the critical STW zone during the dry season. But these STWs cannot irrigate the crops properly due to lowering of groundwater during the peak irrigation period, resulting in reduced crop (mainly bora rice) yield. The zoning map developed in this study could be used to clearly identify the "critical STW zone" and the Govt. can take necessary measures to advice and encourage farmers in these areas to use force mode pumps, instead of suction mode pumps (i.e., STW, DSTW and VDSTW) to ensure proper irrigation and food production. Besides, in these critical areas farmers may be advised to cultivate low water requiring crops like wheat, potato, maize, etc. Critical STW zone covers large areas of the Rajshahi, Dhaka and Chittagong divisions. In Rajshahi division, the areas of the greater Rangpur and Dinajpur districts are still safe for STW irrigation, but almost the entire Barind tract area belongs to the critical zone. Significant parts of Nawabganj, Rajshahi, Nagaon, Bogra and Pabna districts fall under the critical zone. The Chittagong division includes the three hill districts of Khagrachari, Rangamati and Bandarban for which no groundwater level data were available. This division also includes some of the coastal districts, e.g., Cox's Bazar, Chittagong, Noakhali and Laksmipur, where surface water is mainly used for irrigation due to high salinity in the groundwater (especially at shallower depths). The critical STW zone covers about 8225, 6516, 5838 sq. km area in Rajshahi, Dhaka and Chittagong divisions, respectively. In Khulna division, no groundwater level data were available for parts of Satkhira district, almost the entire Khulna district, and the entire Bagerhat district. However, groundwater is not used in these areas for irrigation due to its saline nature. The groundwater level is very close to the ground surface in most areas of the Sylhet division due to the presence of a large number of hoors, beels and jheels that are constantly recharging the groundwater aquifer. Most of the area of this division falls under the perfect STW zone. In Khulna and Sylhet divisions, the critical zone covers 1,803 and 573 sq. km, respectively. Only limited data are available for assessment of the trend of change of groundwater level in the country. Evaluation of the available data of 6 monitoring wells located in Dhaka, Comilla and Naogaon districts for the period 1973 to 2003 suggests that the lowest groundwater level did not change significantly during the 70's. A sharp lowering of groundwater level has been observed during the early 1980's. Rapid increase in bora rice cultivation using groundwater irrigation during the early eighties could be responsible for this observed phenomenon.