Well water quantity: all about well flow rate, well yield, and water quantity: this article series describes how we measure the amount of well water available and the well flow rate - 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.
We warn that the depth of a well does not by itself tell us how much water it can deliver, and in some cases a very deep well may indicate trouble. We explain the difference between total water quantity available and the actual well flow rate, and we describe both an actual well flow test and a rather bogus "in the building with a bucket" flow test.
The sketch at page top, courtesy of Carson Dunlop, outlines what happens during a well drawdown or well flow test procedure. Details are below.
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How Much Water is Actually In the Well and How Much Water Can Be Drawn Out of the Well for How Long?
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:
What we really need to know is the total quantity of water that can be drawn from the well and the quality of that water: is it potable, hard (mineral laden), smelly, dirty, requiring treatment for any aesthetic or health-concern contaminant?
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:
Properly speaking, the flow rate of a well is usually defined as
But an actualor true well flow rate is
since that inflow rate ultimlately sets the maximum rate at which water can be taken out of the well once any water reservoir in the well bore (the static head plus contents of any water storage tanks) has been consumed.
Typically for real estate transactions or for evaluating a newly drilled well, the flow rate is measured over a 24 hour period and is referred to as the well yield.
See WELL FLOW TEST PROCEDURE if you need to have an accurate well capacity test performed on your water well
See WELL YIELD DEFINITION for a complete, detailed explanation of the factors that go into a true measurement of the capacity of a well to deliver water.
Watch out: If you are given a well flow rate that was measured over some shorter interval or worse, over some un-specified interval, you cannot be sure how the well will perform in actual use. For example someone may measure a pseudo-well-flow rate by just measuring the well output at the pump for a few minutes, or at a bathtub spigot or an outdoor hose bib.
Because well flow rates for many water wells are not constant but rather may diminish from an initial maximum in gallons per minute to a lower but sustainable flow rate, these short well flow tests can be misleading.
Brief water flow tests may actually just be measuring the rate that the well pump draws water out of the well bore - pumping out of the water reservoir in the well bore itself. This static head pumpout is not the well's sustainable water delivery capacity.
The well quantity is the total amount that can be drawn out of a water well before running out - see WELL QUANTITY TOTAL.
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 [click to enlarge] 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
For complete details about the definition, role & importance of the static head in a well see STATIC HEAD in the WELL
How to calculate 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. Below we show how the volume of a well casing is calculated. If your well casing or dug well or other round well is larger than 6-inches in diameter just adjust the radius or r-figure to equal 1/2 of your well's diameter in inches.
Based on simple geometry & the formula for the volume of a cylinder: we calculate the area of a cross section, or top, or bottom of the cylinder, then multiply that area by the cylinder's height.
Watch out when estimating how much water is in the well. The depth of the well from bottom to top of the ground is usually not the height of actual water in the well. 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
Static head water quantity (h) = Total well depth (d) - Air (a) - Clearance at bottom (c)
Details for 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. Below we are repeating the well casing volume calculation, just changing the diameter or radius number to the inside diameter of the piping, and for h or height we use the length of piping.
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
Details of this topic are at WELL YIELD DEFINITION
Excerpts are below
Definition of Well Recovery Rate - 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.
Watch out: 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
We offer a more detailed (and more confusing) equation used to calculate the details of a well recovery rate in our discussion
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.
WELL FLOW TEST PROCEDURE 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
Area of a circle = pi x radius squared (radius = 1/2 of the diameter)
Area = 3.1416 x .49 .... or
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
WATER FLOW RATE MEASUREMENT describes measuring water flow rates at or in a building - which is not the same as a well flow test.
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.
How to State 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 Empty) 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 pressure 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.
You can indeed measure water flow rate in a building by running one (or more) fixtures into a bucket, knowing the volume of the bucket and just watching how long it takes to fill the bucket. But this approach is usually wrong, as we explain at
WATER FLOW RATE MEASUREMENT - how much water is delivered at a plumbing fixture
For the most complete discussion of this topic please see WELL QUANTITY TOTAL.
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.
Watch out for errors or deliberate misrepresentation about well flow rate and water quantity - 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.
Adding a More Powerful Well Pump Can Improve Pressure, Increase Flow Rate at Faucets ... and can Lead to Trouble
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.
Watch out: 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 & REPAIR 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
Watch out: Measurements like the
are all useful, but taken by themselves some of these numbers can give a false reading about the basic question of how much water is in the well?
Before assuming that a water pressure, flow, or quantity problem is due to the
well itself, see WATER PUMP REPAIR GUIDE an specific case which offers an example of diagnosis of loss of water pressure, loss of water,
Reader Question: does the rate at which water flows into a well vary depending on whether the well is full or empty?
7/10/2014 William said:
Does the rate at which water flows into a well vary depending on whether the well is 'full' or 'empty'. In other words, will lowering the pump help increase the recovery rate, or simply add to the static quantity?
This question was originally posted at WELL YIELD IMPROVEMENT
Yes the well flow rate into a well does vary in relation to the level of water that is already inside the well bore (or dug well).
In sum, water flow into at least deeper wells and even some shallow wells from a variety of water passages, typically rock fissures, that occur at various depths in relation to the well bore. The total well flow rate is the sum of all of these smaller individual flows.
Even more complex, not all of the individual flows into a well flow at the same rate, nor do they necessarily flow continuously at a fixed rate over time. For example
Fissue A may flow at 7 gpm for 20 minutes then diminsh to 2 gpm for a longer or even indefinite interval, while fissure B may flow at 3 gpm for 12 minutes, then fall to 0.5 gpm for a longer interval.
When the well bore is "full" - that is up to its normal static head top level, water stops flowing into the well bore because the pressure of the water in the bore is sufficient to stop inflow at the various water entry points along its height (I'm using the term "fissures" or water flow passages as usually there is more than one).
An exception are artesian wells whose aquifer feeds into the well bore at sufficient pressure to actually push water out at the well top and even to higher levels.
Continue reading at STATIC HEAD, WELL DEFINITION or select a topic from the More Reading links shown below. or select a topic from the More Reading links shown below.
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