How Much Water is in the Well?
Well Flow Rate & Water Quantity Explained InspectAPedia® -
How much water is in the well? How long will the water well last? What is the well recovery rate?
Well Flow Rate, Well Yield, & Water Quantity Explained - Problems & Repair Advice for wells
What are well static head, flow rate, and delivery quantity? How is well quantity measured?
How does well flow rate vary over time and what else changes the amount of water in a well?
What happens to well flow when we install a more powerful water pump?
Well diagnosis & repair procedures
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This article series describes how we measure the amount of water available and the water delivery rate ability of various types of drinking water sources like wells, cisterns, dug wells, drilled wells, artesian wells and
well and water pump equipment. The sketch at page top, courtesy of Carson Dunlop, outlines what happens during a well drawdown or well flow test procedure. Details are below.
Readers of this document should also see Water Tank Types and before assuming that a water problem is due to the
well itself, see Water pump and pressure tank repair diagnosis & cost an specific case which offers an example of diagnosis of loss of water pressure, loss of water, and analyzes the actual repair cost.
In a companion article, How to Test Well Water Quantity, we describe both valid and questionable ways people measure well yield, and we offer some simple steps any home owner or home buyer can take to check the adequacy of water pressure and water quantity at a building.
How Much Water is Actually In the Well and How Much Water Can Be Drawn Out of the Well for How Long? Well Flow Rates or Well Recovery Rates Explained
Two Key Questions to Ask About the Well When Buying a Property With a Private Water Well
If you are purchasing a property served by a private well of any sort, here are the critical questions to ask about the well itself:
Where is the well located? Is the well on the property being purchased?
What kind of well is it: hand dug well, driven point well, drilled well, spring, stream, (these water source alternatives have implications for sanitation and water quantity as well as safety)
Three Key Questions to Ask About the Water Provided from a Well
There are three basic questions that must be asked about a private water supply provided from a well. It's helpful to state them since otherwise a property buyer may receive only answers to some of these questions, all of which are critical:
Water Quantity from the Well: how much water, in gallons or liters, can we draw out of the well before we run out, and how fast can we take it out? We discuss these questions in this document after this introduction.
Quality of water from the Well: what's in the water? Is it sanitary? Are there contaminants that are a health threat? That's water potability. But further, are there other water contaminants that are not a health threat but which are aesthetic or even functional concerns such as color, sediment, water hardness, or water odors? You can see that a simple bacteria test to "pass" or "fail" a water well won't address most of these questions. See WATER TESTING GUIDE for a guide to selecting the appropriate tests to perform when purchasing or maintaining a property with a private water well.
Water Supply Functionality: is there a good useable flow rate coming out of the water supply piping at every fixture? The answers to this question usually describe the condition of the piping and well pump, not the condition of the well itself, though in some cases deliberate flow restricters may have been installed at a building which is served by a well that has a very limited water quantity. See WATER PRESSURE LOSS DIAGNOSIS GUIDE for help diagnosing poor water pressure and poor water flow.
This article continues with the links shown just below.
Static Head of Water in the Well explains the role of water stored in the well casing or cavity itself and shows how to determine that volume Well Yield: Well Flow Rate - explains how well yield or well flow rates are measured, provides formulas, and an example of calculations Total Quantity of Water Available - explains the true volume of water that is available from a given well, the role of the static head, the flow rate, the pumping rate, and factors that affect well life and changes in well yield.
Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.
The static head inside a water well tells us how much water is available to the pump after the well has rested, water has risen to its maximum
height inside the well, and the pump is about to turn on.
This sketch, courtesy of Carson Dunlop offers a graphic explanation of well static head. The static head in a well is is not the total amount of water than can be pumped out of the well, it's just where
we start. After all, we will also have to include the rate at which water runs in to the well while we're pumping water out.
Looking at our rough well sketch below and repeated at Components of a Drilled Well with a Submersible Water Pump and just considering the vertical arrows at the left side, we see that we have a total well depth (d), which in more
detail is comprised of the air gap at the top of the well (a), the pump clearance at the bottom of the well (c), and the static head (h) which is
the height of the column of water inside the casing which can be pumped out by the water pump when it operates.
Calculating the gallons of water per foot of well casing
We have about 1.5 gallons of water per foot of depth of a well when we're using a standard residential 6" well casing. The height of water column inside the well and available to the pump is less than the total well depth. Except in artesian walls the water column does not extend from the well bottom to the top of the ground.
Static head location in a well
In this sketch, distance (h) is the "static head" which is the total volume of water available to the pump. The static head in a drilled well extends from the very bottom of the pump (since water can't jump up to the pump) upwards to the highest point that water reaches inside the well casing when the well has rested and reached its normal maximum height.
Well water quantity calculation at pump startup
(a) air in top of the well casing: water rises only to a height somewhere below the very top of the well.
(d) total well depth: measured from the top of the ground to the bottom of the well
Static head water quantity (h) = Total well depth (d) - Air (a) - Clearance at bottom (c)
Calculating the volume of water or gallons of water stored in water piping
In some circumstances such as deciding how much water to flush out of a pipe for certain water tests, it is useful to know the volume of water required to fill well piping or water piping.
For long runs of well piping there may be a significant volume of water in the piping itself. Using 600' of plastic well piping as an example, we need simply to calculate the volume of a cylinder (the inside of a water pipe) into cubic inches per foot.
The volume of a cylinder V = pi x r2 x h
where pi = 3.1416,
r = cylinder radius (1/2 the diameter) and
h = the cylinder height or length of pipe in our case and
G = the volume of water in gallons = 0.004329 gallons per cubic inch
There is more water in long piping runs than one would have guessed.
To translate cubic inches of water inside of a pipe, 1 cu. in. is about 0.004329 gallons
1/2" internal diameter piping contains 2.35 cubic inches or (2.35 x 0.004329) = 0.01 gallons per linear foot:
.5 / 2 = r = .25", and h = 12" per foot,
V = (.25)2 x 3.1416 x 12 = 2.36
cu. in. per foot. Multiplying this by 0.004329 we obtain
G = 0.01 per linear foot of pipe
3/4" internal diameter piping contains (.75)2 x 3.1416 x 12 x 0.004329 = 0.02 gallons per linear foot
1" internal diameter piping contains (1)2 c 3.1416 x 12 x 0.004329 = 0.04 gallons per linear foot
Well Yield: Well Water Delivery Rate - the Well Recovery Rate or Well Flow Rate
Well Recovery Rate is the rate at which water runs into the well from the rock fissures and openings into the lower portion
of the well below the steel casing, while we're pumping water out of the well. Some other terms for well recovery rate include well yield, well flow rate, and well water quantity. Since the "recovery rate" of a well describes the rate at which water runs into the well, a well recovery rate also defines the rate at which water can be pumped out of a well without pumping the well down so far that the pump "runs dry".
Typical numbers for well recovery rates (if measured honestly
over a 24-hour period) run from a fraction of a gallon per minute (a terribly poor well recovery or flow rate) to 3 gallons a minute of water flow (not great but useable) to 5 gallons per minute (just fine for
residential use) to more than 10 gpm (a great well recovery rate for residential use).
Difference Between Well Recovery Rate or Well Flow Rate & Well Pumping Rate
The well flow rate or recovery rate is not equal to the well pumping rate: that is, most water pumps can pump water out of a well faster than
water runs in unless the well has a great recovery rate. For wells with modest recovery rates of say 2-3 gpm, some well installers or plumbers design the
pump so that it cannot pump faster than this rate, thus avoiding pumping the well dry and possibly damaging the water pump itself.
The well pumping rate is limited by the horsepower of the well pump, pump type, pump location, and other factors. The maximum well pumping rate set by the pump is normally a number stamped on the data tag attached to the well pump itself. The well pumping rate defines how fast in gallons per minute (GPM) the pump can deliver water if it has an infinite quantity available.
The well flow rate, as we discuss in this article, is the rate that water flows into the well itself from the surrounding soils. The well flow rate is the true limit on a well's ability to deliver a sustained water flow to its users.
So you could pump water out of a well very fast pumping rate, say at 10 or even 15 gpm. But if the well recovery rate is less than the well pumping rate, you're going to run out of water. How soon you run out of water depends on how much water was in the well casing when you started pumping (the static head), and ultimately on the well recovery rate. We explain this in more detail at Definition of the Total Quantity of Water Available From a Water Well.
We offer a more detailed (and more confusing) equation used to calculate the details of a well recovery rate in our discussion at Drilled Wells - steel casings. But it's easier to simply pull water out of a well at a given rate and see how long we can do so. That's about what a well driller does to determine the effective well flow rate when a new well is drilled. Pulling water out of the well (using a variable-rate pump running at a rate set by the well test professional) integrates all of the different rock fissure flow rates into a single quantity of water.
Example of Calculating the Flow Rate of a Well
Question: I'm digging a well, not yet in the driest part of our dry season. I'm at about 10 meters depth, well diameter about 1.4 meters. At 4 pm when the digger stops for the day (by hand hammering through rock with a mallet and chisel), he drains the water. At 9 am the next day the well has 1.6 meters of water in it. I intend to complete digging further into the dry season. However, based on the above data, how many liters of water can the well produce in a 24 hour period? -- A. Starkman, Oaxaca, Mexico.
Answer: We can calculate the well flow rate from the reader's example above, using the formula for the volume of a cylinder and a constant to convert between volume of well water in cubic meters and liters or gallons.
This well water flow rate calculation case provides exactly what we need to calculate the quantity of water in a well from direct measurements of the well diameter, depth, and water depth, presuming that the well, a dug well in this case, is round. We just need the depth of water and the diameter of the cylinder formed by the well.
Then we use the formula for volume of a cylinder - which in turn means we calculate the area of the circle formed by the bottom of the well (or the well's cross-sectional area) and we just multiply that area by the height (or depth) of the water. How to Test Well Water Quantity describes how we test well flow rate and quantity when the well is already built, is covered or sealed, and we can't conveniently make well diameter and water depth measurements.
So for this real-life example of a dug well for which we want to calculate the well water volume and the well flow rate:
Well Diameter D = 1.4 Meters
well Radius r = 1/2 of diameter or .7 meters
Depth of water in the well (reported after a specific time interval discussed below) = 1.6 Meters
Area of a circle = pi x radius squared (radius = 1/2 of the diameter)
Area of the well in cross-section or bottom = 3.1416 x (.7 x .7)
Area = 3.1416 x .49 .... or
Area = 1.54 meters
Volume of a cylinder (in this case a round, hand dug water well) = Area x depth
Volume the well = 1.54 m x 1.6m .... or
Volume of water observed inside this well = 2.46 cubic meters
Converting Cubic Meters to Other Common Measures of Water Volume
Liters: one cubic meter contains 1000 liters.
So for our example well, the well cylinder of water contains (2.46 x 1000) = 2460 liters of water
Converting Liters of Water to Gallons of Water
1 gallon = 3.7854 Liters so we can divide the liters, above, by 3.7854 to convert to gallons.
The example well water volume contains (2460 / 3.7854) = 650 gallons of water.
Well Flow Rate in Gallons per Hour for a Given Time Interval
Now we can also obtain the well flow rate - the rate at which water is flowing in to the well - though this will change seasonally as well as change if the well is dug further or other steps are taken that affect well yield.
At the time of our reader's observations, from 4PM on a given day to 9AM the next day (that's a total of 17 hours on the clock) the new well collected 650 gallons of water.
Gallons / hours = gallons per hour or water flow rate into the well, provided that no one is taking water out of the well during this same interval.
Well Flow Rate Per Hour = WFh is normally expressed in gallons per hour or gph.
WFh = (Total Gallons of Water in The Well Starting from Empth) divided by Number of Hours of Elapsed Time between empty well and the observed water volume in the well.
WFh = Gallons / Hours = gph or gallons per hour
For this example, 650gallons / 17hours = 38 gallons per hour - this is the well flow rate for a 17 hour period. This is a huge flow rate, by the way.
Definition of Well Flow Rate in Gallons per Minute
The most common measure of a well's ability to deliver water, that is the answer to "how much water can we get out of a well" is the measurement or calculation of the well flow rate per minute - the water flow rate into the well expressed in gallons of inflow per minute. WFm.
The well flow rate in gpm defines the maximum rate at which water can be drawn out of the well over a sustained period. Actually we can draw water out of a well faster than WFm, because the well pump has available to it the reservoir of water already in the well when it starts pumping - the well's "static head". But once that static head of water has been exhausted, WFm is the absolute limit of further water delivery rate possible.
For our well flow rate calculation example above, we found that this well had a water in-flow rate of 38 gph or 38 gallons per hour.
Just divide this number by 60, the number of minutes in an hour to obtain the well flow rate per minute.
Well Flow Rate per Minute = WFm = gpm or gallons per minute
For this example, 38 gph / 60 = 0.6 gpm - this is the measured well flow rate in gallons per minute.
In this case that's a weak, marginal well flow rate. In the U.S. most building or health departments who must approve a private well water supply when issuing a final certificate of occupancy for new construction want to see 3 to 5 gallons per minute or 3-5 gpm.
Is This Really the 24-hour Well Flow Rate?
Is 38 gph or 0.6 gpm really the true well flow rate? Maybe. Maybe not.
The property owner's observation was that from "an empty well" at 4 PM on a given day, the well water level rises to 1.6 meters of depth by 9AM the following day.
So what was observed was a flow rate of 38 gallons per hour over a 17 hour period. Not a 24-hour period. Will the well water level continue to rise past the 17 hour period. Maybe, maybe not.
While a hand dug (or drilled) water well fills as water flows into it, the well water in-flow rate will slow down and eventually stop. This is true except for artesian wells. That's because eventually the pressure exerted on the well sides by water in the well equals the pressure of water in rock fissures or passages from which water is trying to enter the well.
When the water pressure exerted on the well sides and bottom by water inside the well itself equals the water perssure exerted by water trying to enter the well, at that point water flow into the well will stop. The well water level won't change much until someone draws water out of the well, thus lowering its in-well water level back down and allowing more water to flow in.
Well flow rates will vary by season, weather conditions, and other factors such as well age and history of usage. The well flow rate may also be affected by the chemistry of the water itself - if water is high in minerals, over time the rock fissures through which water flows into the well become mineral clogged and the well flow rate may diminish.
How to Determine the True 24-hour Well Flow Rate
So the owner will want to either measure the well depth again after 24 hours, repeating our calculation from above with the well depth measured at the end of 24 hours, with water only flowing into the well, that is, no one draws any water out of the well during that period.
We prefer to simply measure the water in the well at the end of 24 hours and calculate the 24-hour flow rate. When the well is a drilled well rather than a hand-dug well, the well driller may measure the well flow rate by use of a well pump whose output is adjustable.
The well driller measures the well draw down rate in the well opening while the well pump is running, and compares that to the rate at which the pump is removing water from the well. But a true well flow rate, whether obtained by simple observation or by use of a calibrated pump, should be measured over a 24 hour period, not a shorter interval.
Determine the Well's Static Head
Alternatively the owner might want to watch the well water level increase until the water level has stopped rising in the well. It might take longer than 24 hours for the water in flow to stop.
When the water level has stopped rising on its own in the well, the depth of water in the well is measured and is referred to as the static head - the amount of water in the well when the well is fully recovered and at rest.
Definition of the Total Quantity of Water Available From a Water Well
People sometimes confuse things by describing what we call the well 'flow rate" as the "water quantity" available from a well. They're different. You could have a great well water flow rate - say 20 gallons per minute - but if it the water will only run at that rate for five minutes before you run out, the well has a very poor water quantity (5 minutes x 20 gpm = 100 gallons of water) and it's not a satisfactory well.
So watch out for errors or deliberate misrepresentation about well capacity when buying a property. A true well flow rate is not what we can measure in the building over five minutes, it's the ability of a well to deliver a sustained water flow rate over a longer period, usually measured over 24-hours. When a local health department or building department approve the flow rate of a water well, that rate should have been measured by a plumber or well driller and should represent something more than a five minute test. The standard period over which a well flow rate must be measured varies among communities. Find out what the standard is for your area.
The amount of water that can be pumped out of a well at any given time is limited by the size of the static head and the
well recovery or well flow rate, and of course by the pump rate the gallons per minute that the pump itself can or is set to
deliver.
Well pumps are usually intended to pump water out of a well slowly enough that the pump and well don't run dry. Some pump systems have fittings that recycle the very last water in the well through the pump, ceasing delivery of it to the building, to protect the pump from overheating.
For these reasons, we've occasionally found clients dissatisfied with their well after they install a new, more powerful water pump. The owners install a more powerful pump to increase water pressure in the home, but the effect may be also to draw water out of the well faster than ever before, thereby disclosing a marginal well flow rate that they had not understood.
For this reason it's a dangerous simplification to simply assert "we can put on a bigger pump" when water flow rate is poor in a building. See WATER PRESSURE LOSS DIAGNOSIS GUIDE for more diagnosis of bad water pressure.
Remember that water quantity(how much water we can obtain) is not the same thing as water pressure (how fast water comes out of the tap). Water quantity comes from what the well can deliver. Water pressure is the amount of force with which the water pump can push water into the building piping and fixtures. Higher water pressure does give us more gallons per minute flow but that's describing a condition at the plumbing fixture. It's not measuring how much water the well can deliver.
A Poor Flow Rate Well Might Seem to Work Acceptably
If our well has a huge static head, say 300 gallons of water, and considering that at most buildings, certainly at residential properties, most water usage occurs in two big surges, in the morning and in the evening (giving the well time to recover between), the well could have a terrible recovery rate, say 1/2 gallon a minute or less, but we might never notice it in the building. We're always running off of the "reserve" or static head.
But over time, as minerals and debris clog those rock fissures that feed water into our well, and if we started with just a small recovery rate of less than a gallon, our well may not continue to deliver the water quantity we need.
A Good Flow Rate or Good Well Recovery Rate is Best
A well with a good recovery rate, flowing at say 5 gpm or more, is more likely to continue to give good service over time, and we might get by with a small static head if the flow rate is good enough.
These are the parameters that a well driller is considering when they decide how deep to go in drilling and how much well flow rate is going to be acceptable.
Well Flow Rates are Not a Simple Number, and Need to Be Measured Over Time
Because ground water typically flows into a drilled well through multiple rock fissures or other underground passages, and because these passages are at different depths, the actual total flow rate into a well is made up of flow from multiple individual openings. Each of these may have its own characteristic flow rate and also flow duration. For example a fissure may flow at a high rate for 20 minutes and then drop to a slow rate or even stop entirely.
This is why the flow rate at a new well is typically measured over a long period, say 24 hours. If you measure the flow rate at a well for just a few minutes, you can have no idea of the well's actual ability to deliver water over any sustained time of usage.
Well Flow Rates Can Change
Someone drilling a new well nearby can tap into the same water table causing it to drop
Local blasting can change the level of a local water table. Local blasting can also change water quality. The well at one of our properties gave seemingly limitless quantity from a shallow 29' deep drilled well from about 1920 up to about 1999. Then the town was improving the nearby roadway and had to do some blasting. The well quantity did not change but suddenly wells along this section of roadway had red silt in their water - it has remained a problem for some home owners in the area.
Seasonal water level variations: Local ground water table levels are often lower at different seasons (less in dry weather)
Permanent water level shifts & Global Warming: Local ground water tables may drop permanently. In some areas of Florida so much water has been pumped from below ground that salt water has begun to intrude into the aquifer. Changing sea levels due to global warming can be expected to affect coastal drinking water wells by raising the level of salty water.
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Additional technical contributors & reference sources for this article are listed below.
Carson Dunlop, Associates, 120 Carlton Street Suite 407, Toronto ON M5A 4K2Toronto. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.
Thanks to reader Alan Starkman, Oaxaca, Mexico, for discussing well flow rate calculations 2/17/2010.
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