How to Calculate ‘Total Dynamic Head’
Total Dynamic Head (TDH) = (Static Water Depth + Drawdown + Additional Lift) + Frictional Losses in Pipe
Some creative ways to approximate Static Water Depth:
– A recent well survey will have this information (sometimes called ‘Static Water Level’ or just ‘Static’) or a call into the well driller as they should have that on file or in the well log they submitted to the county.
– If a neighboring property knows theirs, we can sometimes estimate until it’s measured (lots of other variables here but a reasonable proxy)
– Measuring a fishing line with a ½ full water bottle dropped in the well (tied tight!)
– Using a 500ft measuring tape spool with a small eyebolt and flat washer (to make a splash when you hit water) lower it down until you get water and read the tape!
– Dropping a small stone in and timing it exactly (this needs to be precise)
Estimating Drawdown for your well:
If your well drillers were especially thorough they may have logged some water level readings at different GPM in your well as they pumped it. Generally you’ll just have a single GPM measurement from the very bottom of the well. In our experience wells that have less than 10 GPM will need to consider drawdown, the less GPM, the more potential drawdown. 10+ GPM wells may not draw down much, if at all when pumping at 5 GPM with a solar pump.
Estimating Frictional Losses over distance of pipe:
See the table below. Generally the higher GPM and the narrower the pipe, the more frictional losses. That means more head on the pump and lesser flow at the outlet. Size your pipe up if the head increase (also called pressure loss) becomes significant.
Calculating Additional Lift:
If you don’t know the elevation of your well head, sometimes a phone app can help. If you can get an elevation at your well head and then one at the top of the tank you are going into, you’ll have the additional lift the pump will need to fight gravity to get water into the tank.
Pumping into Pressure Systems:
When pumping into pressure, you’ll have to account for increased head on the pump. The formula is based on gravity. 1 psi = 2.31 feet of head. So a 40 psi pressure tank is 92.4 feet of head and significant with some pumps so good to remember to take into account when sizing your pump!
