Recommended Operating Range - Deep Well Pump

It's normal to know the lift the pump has to overcome, and the flowrate you want from the well. So, there's something I don't quite understand here, Guvnor. If the pump is operating at maximum head, then it isn't producing any flow, so it isn't doing any work, so its efficiency will be zero; you might as well turn it off. Can you explain a bit further, please, 'coz I don't quite understand what you're asking?

Not enough information to go on.

Contact some pump suppliers and talk to them. They will want much more information than you have presented here. Efficiencies are directly related to specific pump/motor combinations, and anything else is pure speculation.

Talk to your pump supplier, everything else is wasted effort.

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Your mention of line shaft pumps implies that you are talking about fairly large pumps, but we need more information:

Well depth, casing size, casing depth, well design flow rate. minimum flow required, maximum flow required, static water level, pumping water level, discharge pressure/head required at surface, power available voltage/phase/hz, experience of other wells nearby, pumps which have been proposed, etc.

I don't understand your emphasis on minimum efficiency - sure, we'd all like 100% but that isn't going to happen. If you wanted maximum efficiency, you'd have a storage tank that you pumped into at maximum efficiency until it filled, then turn off the pump until the tank needed filling again. But if the pumps are as large as I think, that's not advisable. I think your concern is the pump's minimum allowble flow, and that should come from the manufacturer.

When you say drawdown, are you concerned with fluctuations in the pumping level caused by daily/hourly fluctuations in flow, or are you talking about a long-term lowering of the water table?

I think you've covered the ways to increase head. If you think you'll need more head in the future, blank stages may be installed to allow adding stages in the future without buying a new shaft. If I recall, trimming impellers on vertical turbines causes a big hit on efficiency (plus, then you have to buy a whole new set of impellers). If you're thinking about variable speed, I don't know that you'll find a VFD rated submersible motor. I think I'd prefer a line-shaft anyway.

First install a temporary pump and measure flow(rate/quantity) and drop in water level ,time taken etc after your required quantity of water has been pumped. Then measure the water level after about 8hrs and you will know the capacity of the well,based on which write to manufacturers to quote for a suitable pump. Also send a water analysis report to manufacturer. Either employ a consultant or ask manufacturer to quote for both submersible and vertical turbine pumps including supply,installation and maintenance for a period of say 15years,based on which you can decide which pump is most suitable.