Just remember if you put the plant down the hill, that the voltage will need to be high enough that your losses wont overcome your gain of potential power.
You can run a partial vacuum in the tailstock and get the advantage of some of the drop after the turbine without actually moving the turbine down the hill and the power back up. 20 feet is pushing it a bit but might be doable.
You'll need to do a few things to get away with this:
- Good seals on the turbine to keep it airtight or very close to it.
- Discharge the water from the tailstock into a container that keeps the output pipe covered at all times, to prevent air from being sucked back into the tailstock.
- Run your lines so that there are no high spots to trap air bubbles, and any air in the line will come back through the turbine to where you can get rid of it.
- Provide some means to exhaust (bleed) trapped air. This will require cutting off or restricting the tailstock flow to bring the pressure above atmospheric at the bleedout point. (You can make it automatic by using a local standpipe to a height above the spring's height.
With enough flow and narrow enough pipes at a low slant, trapped air will tend to be sucked out the tailstock (though narrow pipes also cause friction, lowering the effective head).
Also note that running a vacuum means you'll depressurize the line for feeding water to your house - potentially risking backflow conatminating your water supply. You can avoid this by running an additional line from the spring for the turbine - tapping the spring box at a slightly higher point to automatically cut off the turbine water if there isn't enough to supply the house. Or you can make some arrangement of transfer valving (perhaps an automatic one using a valve like that used for a sink hand-sprayer), and depend on check valves and anti-siphon valves to defend your house plumbing from backflow contamination.
Note that depressurization of your house plumbing can also occur from running the turbine even if you DON'T run a below-atmospheric-pressure tailstock. So you might consider that second headstock in any case.