Effects of Partial Penetration on the Characteristic Well Losses of Naturally Developed Tubewells
Roger A. Luyun, Jr.1 and Wilfredo P. David2
The study was conducted to determine the incremental drawdowns or well head losses due to partial aquifer penetration in naturally developed shallow tubewells (STWs). Three STWs designed for 100, 50 and 25 percent aquifer penetration, were installed, developed and tested.
The transmissivity of the aquifer were found to be constant at 439.6 m2/day. The storage coefficient increases with decreasing distance from the pumped well and with increasing discharge. The incremental drawdowns, characteristic well loses, and effective well radii all increase with decreasing penetration fraction. The incremental drawdowns and well losses, however, were less than expected and are not significant at discharges below 18 L/s.
The computed effective well radii were large and increase with decreasing penetration ratio. Thus, the expected increases in drawdown due to increased flow velocity in the vicinity of partially penetrating STWs were offset by the increased effective well radius of partially penetrating STWs. Hence, the additional cost of drilling and developing fully penetrating STWs that are to be developed naturally should be weighed carefully against slightly higher characteristics well losses or pumping costs.
Keywords: shallow tubewells, transmissivity, drawdown, pumping test, aquifer, well loss, discharge ratios
1Assistant Professor, Land and Water Resources Division, Institute of Agricultural Engineering, College of Engineering and Agro-industrial Technology, University of the Philippines Los Baños, 4031 College, Laguna, Philippines; 2Professor, Institute of Agricultural Engineering, College of Engineering and Agro-industrial Technology, University of the Philippines Los Baños, 4031 College, Laguna, Philippines.