Total Well Water Quantity FAQs

How to calculate the amount of water you can get out of a well in 24 hours - GPD or LPD

Hello I am working in an RO [Reverse Osmosis] plant we are taking brackish water from bore well. Daily we are taking flow meter reading of 24hours.

Our pump has a flow rate of 147 gallon per minute.

Could you please explain me about 24 hours quantity of water is pumped in gallons, imperial gallons and also in meter cube. On 2019-02-18 by Jerry

Answer by (mod) - Use GPM to calculate GPD - Gallons Per Day

Sure Jerry:

The 24-hour water quantity from a well starting with a GPM flow rate is

GPM (gallons per minute0 x 60 (minutes per hour) x 24 (hours per day) = GPD or gallons per day.

The calculation is identical for liters if you start with LPM (lites per minute) and will compute LPD or litres per day

The calculation is also identical if you start with Imperial Gallons, ending in Imperial Gallons per day.

Watch out, however: many wells that can deliver some quantity of water in GPM during a short term flow rate test can NOT maintain that flow rate over 24 hours.

For that reason many well drillers will run an adjustable flow rate pump and will explicitly measure the sustainable flow rate of the well over 24 hours.

To understand why a well can't maintain its initial flow rate over many hours consider that the actual aquifer tapped by many water wells is actually a complex of water reservoirs of various sizes at various depths that flow into the well at varying rates whose actual flow rate may slow or even stop after minutes or longer.

For your example

147 GPM x 60 = 8,220 GPH x 24 = 211,680 GPD - though as I warned that's theoretical GPD and may not be true GPD.

You can do the other flow rates using these conversion factors

1 US Gallon = 0.832674 Imperial Gallons

1 US Gallon = 3.78541 litres

Would a 200 ft deep well be vulnerable to damage from nearby roadwork blasting?

Would a well that’s 200 feet deep still be susceptible to the roadwork blasting/silt problem. My well suddenly/ within a few days stopped producing anything over 30 percent of capacity for my cistern. It never recovers beyond that number. Even overnight. On 2018-08-10 by Andy

by (mod) - can nearby blasting drop my well water level? Yes absolutely.

Yes that's possible.

In Poughkeepsie NY when roadwork involved blasting about 100 ft. from our shallow water well the water level dropped and water also became muddy.

How does the term Specific Capacity of a Well fit into the scheme of water quantity

How does the term specific capacity fit into the scheme of water quantity that you present here. specific capacity seems to be used in many states as a measure of well "capacity."

It is the measure of gpm/ft. as I understand it, you measure the drawdown after some period of time of pumping at a certain rate (gpm).

I have been trying to figure out how important or how the time impacts specific capacity. References state the time of measure is important because flows vary over time agreeing with what has been said here.

but I am still having a hard time grasping how this meshes with what you present here on this site. an example is I have a well that pumped 70 gpm for 155 m with 2 ft of drawdown. I am not sure how to get to specific capacity or to your flow rate.

I don't know I am just confused because there are so many ways to measure flow and what you are really measuring seems to be variable depending on the source of the info and who says what. Help?!! An adjacent well is 83 gpm for 155 m at 1.6 ft drawdown. These wells are both 100 feet deep.

The former has a SWL of 32 ft and the latter SWL of 47 ft (below the surface). These number lead people to believe that the water supply is infinite but the houses are going in like wild fire on fairly small lots and they all have wells. I am trying to figure what the risk is for all these wells.

So the numbers I gave are for just 2 of numerous wells being developed. I am trying to see how to figure the capacity out which is how the local jurisdiction requires the driller report when a well is drilled. then I want to move to the more logical? concepts you present to get a better feel to see if there is a risk or not.

This western WA state. thank you for your help since this is clear as mud. :) On 2017-05-23 by stephanie -

by (mod) - what is the specific capacity of a water well?

Thanks for the question, Stephanie, as "specific capacity" is a term used by some well installers and is a simple calculation:

Well Specific Capacity = Water Pumping Rate in GPM / Water Level Drop During Pumping

Watch out: this term is a bit confusing for homeowners because it is dimensionless. For example if a well driller runs a 24-hour flow test or "Specific Capacity" test on a well and the pump used removes water from the well at 100 GPM (Gallons per Minute) and if at the end of that test the water level in the well has fallen by 50 feet, then the specific capacity for that well (ignoring seasonal variation in flow rate) would be

SC = 100 / 50 = 2 - this theoretical well would have a Specific Capacity of "2".

That has meaning to a hydrologist but is in my OPINION it leaves a homeowner or home buyer with not the slightest idea of how much water they'll get from their well. That's why I prefer our simpler definition of WELL FLOW RATE (read at that page please).

Above using an illustration from Carson Dunlop Associates (with my edits) we show that a "Well Drawdown Test" is in essence synonym for a Well Specific Capacity Measurement.

A set of practical definitions that describe the actual or usable volumes of water available and well flow rates can be read at

• WELL FLOW RATE
• WELL DYNAMIC HEAD & STATIC HEAD DEFINITION
• WELL WATER VOLUME CALCULATION
• WELL YIELD DEFINITION
• WELL YIELD, SAFE LIMITS

Put another way, gallons per minute per foot GPM/Ft would, if calculated for the whole well, be absolutely the same as what we call the well flow rate - again is more understandable when we factor in the diameter of the well so that we know that "dropping the water level by one foot" means.

Typically the flow rate is measured over 24 hours in particular to get away from the very problem you so aptly point out that the flow rate into a well from any given point in the open well bore will vary over time.

Typically the flow rate diminishes over time at each measurement depth for which there are rock fissures or other water passages into the well bore as the aquifer outside the immediate area of the well is drawn down.

At those measurement points, the flow rate usually drops to a slower rate, but it can in fact drop to zero and need hours or more to recover. So the 24hour flow rate is a sustainable flow rate from a well while in my opinion, any flow rate that is measured anywhere in the bore for a shorter period is ambiguous unless the flow is measured and reported at regular brief intervals over time to give a flow diminution curve.

In sum, the simple and more-reliable flow rate is for the whole well, from foot valve or submersible pump to the top of the static head starting when the well has been at rest for 24 hours. Keep in mind that further confusion comes from the fact that wells of different depths have different static head volumes (ask if that's not clear). That's another argument for taking the overall well flow rate for a longer period of time. And all of this explains why any short term "flow rate" is not a clear measurement of much.

See our definitions of WELL FLOW RATE
with
additional details on calculation or measurement at WATER FLOW RATE CALCULATE or MEASURE

A description of WELL FLOW TEST for WATER QUANTITY -

And be sure to read our definition of STATIC HEAD in the well

O also read WELL YIELD DEFINITION

Indeed you're spot-on in wanting people to be rather specific when talking about well flow rates and how they are measured.

Years ago I was prompted to start on this article series by frustration over real estate agents and some home inspectors who would put a metering device on a hose spigot and get and report a "well flow rate" based on a few seconds of water flow out of a building outdoor hose bibb - a completely nonsensical measurement.

What do you mean by "SWL" - "static water level" = if so, see my notes on STATIC HEAD as that's the same measurement:

where does the top of water come to rest when a well is fully "recovered" - that is, if left alone no more water flows into the well bore and the level of water in the bore doesn't rise. The Static Water Level is ONLY a measurement of the distance from ground level down to the top of the water.

To make any sense of that number you need to also know the well depth or more usefully, the depth to the bottom of the foot valve or submersible pump water intake.

Once we know the height of the column of water that is available to the pump then

THAT is what I call the STATIC HEAD - the "head" or volume of water that the pump can draw down before it will be dependent on the rate of inflow into the well from the surrounding ground.

When we know the diameter of the casing we convert the static head to gallons.

So you can have a well with a poor flow rate - say 1 gpm, but a huge static head - say (1.5 gal per foot x 400 feet of water = 600 gallons). In that case and given usual water usage at a one family house, even with a poor flow rate the family won't "run out of water" - usually, or until July, or until the well flow rate deteriorates with age.

by stephanie

I have been through all of the sections of the site you have pointed me too.

It is a very well done site by the way. What you present is very logical and I am able to follow. I don't know why the use of specific capacity even came into existence since it really doesn't make much sense.

It is supposed to be measured until the drawdown stabilizes which like you said would be (more or less) the rate at which the discharge from the well being pumped at that certain rate is equal to the rate at which water is flowing in.

What you would call the well yield (except the period of time being pumped is what ever it takes to stabilize at an unchanging drawdown level; this could be a 1 hour or a 24 hour or more)

The key to the test is finding the pumping rate that allows that drawdown to stabilize (I guess) and then how long it takes to stabilize is not relevant (I guess).

I have the impression that you are actually doing a sort of step-drawdown test.

Anyway I am still confused as to the value of this measure because it is the GPM that you pump at to get that equilibrium in the drawdown.

It is not some round about way to get to the well yield or in flow. It does get at the recovery rate and gives what I call here a "dimensionless" measure of well flow.

It's how much per minute per foot of drawdown to reach that equalized drawdown. What value does this measure have? I am so frustrated with this. grr. But thank you for your efforts to put this site together. I find it very valuable. and understandable.

Also, many of our wells in this area are pulling from aquifers in glacial materials not bedrock and most wells are sealed with bentonite clay slurry not concrete although some wells have been sealed with a cement slurry.

Usually the big production wells for towns and cities. Also, I take it hydrofracing has been in practice for a long time in your area for improving flow into water wells. Seems to give some credence to the concerns people have about hydrofracking for oil and gas. Just a thought. thanks again

by (mod) - how does a well driller measure GPM flow rate? The driller looks at "Maximum Sustainable Flow Rate" for your well; your private homeowner well pump is a different machine pumping at a different rate.

Steph

Re "the GPM that you pump at": a well driller can use an adjustable rate pump to actually find the maximum sustainable flow rate from a well.

That's not the same pump as is installed for the homeowner.

For a helpful review of how a hydrologist might use specific capacity to monitor a well, see

Ballard, Thomas, USING SPECIFIC CAPACITY to MONITOR WELL PERFORMANCE (Website) (2017) Southeast Hydrology, - local backup copy saved as Specific-Capacity-Well-Monitor-Ballard.pdf

Excerpt: Because it usually requires very little effort to collect the data necessary for the calculation of Specific Capacity, it is an extremely useful measurement that can be used to identify declines in well performance, which allows for planning an optimal well rehabilitation schedule. 

The Specific Capacity of a well is the pumping rate (gpm) (Q) divided by the drawdown in feet (s). Specific Capacity can also be used to provide the design pumping rate or maximum yield for the well and to estimate the transmissivity of the surrounding formations penetrated by the well screens.  The following equation is used to calculate Specific Capacity:

SC = Q/s

Where SC = Specific Capacity (gpm/ft);   Q = discharge (gpm);   s = drawdown (ft)

See our Well Drawdown Test illustration I include above.

What would be the optimum measurement for replenishment or rate of recovery? "Gallons per hour".

We want to bring to market a new device and technology to measure key well data.

We want to use industry standard to measure static water and rate of replenishment.

Static water levels will be measured every second and averaged out daily.

What would be the optimum measurement for replenishment or rate of recovery? "Gallons per hour". (July 15, 2014) Andre

Reply:

The minimum well recovery rate you'd want to meet would be that established by some mortgage lenders - say 5 gpm. But that's a *minimum* that would give a sustainable well flow rate of the same number. Be sure to read our articles about well flow rate, well yield, well recovery rate and static head so that you've got a clear understanding of how those measures are used.

Relationship of Pumping Rate to When Your Well Runs Out

Could my well water level drop when using because I am in a shallow supply?

My well pump will only deliver water to my lawn sprinklers for 10 min. then runs dry. After 15 to 20 min. things will work again but again only for 10 min. I have no problems in the house with showers, washers, etc.

Could my well water level drop when using because I am in a shallow supply? (Aug 15, 2014) Jim Park

Reply: Water Use Rate & Total Volume for your lawn sprinklers is greater than your bath shower flow

Jim,

If the sprinkler system uses water at a high GPM rate your lawn watering may be exhausting the well.

If that's the case and if a well protection switch is installed, it may be shutting down the pump for a time - or the well may simply be running out, then recovering on the schedule you describe. Check for those conditions.

Bottom line: your lawn sprinkling water use rate is exhausting your well - taking water out faster than the well can recover.