Selection of Groundwater Development Zones and Upazilas for Study: According to a BWDB- UNDP study (UNDP, 1982), Bangladesh has been divided into 15 groundwater development zones based on aquifer, aquifer properties and water quality. The development zones demarcated by BWDB- UNDP, have been considered as basis for selecting study area for well spacing. Considering insufficiency of data and physical constraints for groundwater development, two zones namely Zone M, M? (Hilly areas in Sylhet & Chittagong) and Zone N (Coastal area) comprising of 131 Upzilas have been excluded from the analysis. Well spacing depends mainly on physiographic factors, aquifer properties and irrigation water requirements. Since these factors widely vary from Upazila to Upazila, even it may vary in the same Upazila, as such it would not be realistic to estimate well spacing on a zone basis, rather it would be wise to carry out the analysis at least on Upazila basis
Description of Different Methods for Estimating Tubewell Spacing: In general, the following four methods are used to calculate well spacing.
- Crop Water Requirement Method
- Groundwater Recharge Method
- Aquifer Hydraulic Properties Method
- Command Area Method
Brief description of the methods is given below:
1: Crop Water Requirement Method:The spacing of Tubewells must consider the peak irrigation requirement to irrigate all the areas under the command of irrigation well. Irrigation water requirement depends on rainfall, evaporation, soil percolation rate and meteorological conditions such as temperature, sunshine hour, wind speed and humidity. Besides the maximum water requirement, spacing is also controlled by well discharge and period of pumping. The required spacing can be calculated from the following equation (UNDP, 1982) modified from the relationship given by (Chowdhury, 1979).
2: Groundwater Recharge Method: Spacing of tubewells in an area may be controlled by the amount of recharge to the groundwater reservoir. Total extractions should not exceed the long term recharge on an annual basis.
3: Aquifer Hydraulic Properties Method: Well spacing is usually determined by the hydraulic characteristics of the groundwater system. Aquifer test data are used to calculate hydraulic characteristics of aquifer. By plotting distance and drawdown of detail aquifer test data in a semi log paper, distance-drawdown curve can be found. This curve, when extended to zero drawdown, provides radius of cone of influence (re). The spacing of tubewell is more than 1.5 times of ‘re’ in case of water table aquifer and equals to ‘re’ in case of semi-confined aquifer. This method is the direct method of calculation of well spacing. It needs detail aquifer test data which is not available for each Upazila. So, indirect method developed by Cooper and Jacob (Bouwer, 1978) can be used for the present purpose.
4: Command Area Method: In Bangladesh, deep tubewell and shallow tubewell are being used for irrigating HYV Boro and Wheat during dry season and T. Aman in late monsoon since late sixties. In this method, average distance of tubewells can be calculated from the number of deep tubewells and shallow tubewells and area irrigated by them.