Abstract Large diameter wells successfully abstract groundwater from aquifers with low permeabilities and ensures sustainable water supply in many arid and semi-arid regions. Their efficiency can be enhanced by the provision of horizontal bores drilled from near the bottom of the wells; these bores are often unlined. Hydraulic conditions in these bores can limit their effectiveness if non-laminar flows occur. This paper describes conceptual and computational models which represent conditions in an aquifer, in the well, and in the bores, where laminar, transitional, or turbulent flows can develop. When a bore is unlined, the friction factor is unknown; hence the Reynolds number cannot be calculated; consequently, an equivalent conductivity approach is adopted. A large diameter well in Rajasthan, India, is used to explore how conditions in the system change with time. The computational model is verified by replicating data from a pumping test in a large diameter well with horizontal bores; the recovery phase, extending over more than 2?days, is of crucial importance. The operation of the well for longer time periods is investigated, comparing responses without and with bores. When simulating pumping for 30?days, the yield of the system is increased with the bores collecting water from the water table at greater distances from the well.
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