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Here we define well yield, safe well yield, the water delivery rate of a well, or well recovery rate, and we explain how to measure or calculate this critical number. 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. Here we explain the following:
    Well Yield: Well Flow Rate
      Define Well Yield, Recovery Rate, Flow Rate
      Calculating the Flow Rate of a Well
      Well Flow Rate in GPM or 24-Hours
      Well's Flow Rate Change Over Time
      Define Safe Yield for a Well

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.

© Copyright 2012 InspectAPedia.com, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use page top links to major topics or use links at the left of each page to navigate within topics and documents at this website. Green links show where you are in a document series or at this website.

Well Yield: Well Water Delivery Rate - the Well Recovery Rate or Well Flow Rate

To determine the well yield we need to know how much water flows into the well from surrounding rock or soil in a given amount of time. In a simple example given here, we start with a dry empty well (it was just pumped dry), we wait a known period of time, and then we measure the depth of water that appears in the well. That's all we need to calculate the Static Head of Water in the Well and then the Well Yield: Well Flow Rate. At Total Quantity of Water Available we translate these numbers into a more realistic picture of how much water you really can expect to get out of a well.

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.

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.

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

The following example of calculating a well flow rate uses well data from a dug well described in detail at Hand Dug Well Procedure.

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. [Note that this is a hand dug well - see Hand Dug Wells for more about that type of water source. But the calculations of well flow rate are the same for any round drilled well or hand dug well.]

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

The formula to calculate the volume of water in a cylinder is

Volume_cylinder = pi x r² x h

where pi = 3.1416,
r is the radius of the circle formed by the cylinder, and
h is the height of the cylinder

Area of a circle = pi x radius squared (radius = 1/2 of the diameter)

For our dug well example under discussion, our diameter is 1.4m so the radius is .7m.

Area of the well in cross-section or bottom = 3.1416 x (.7 x .7)

Volume_cylinder = 3.1416 x .7² x 1.6

Volume_cylinder = 2.46 cubic meters - that's the volume of water found in the well after the overnight waiting period.

Notice that we're being sure to use the same units of measurement for both diameter (or radius) and depth - in this case, we are working in meters.

How to Convert Cubic Meters to Liters or Gallons - Other Common Measures of Water Volume

Convert Cubic Meters of Water in the Well to Liters

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

Convert Liters of Water to Gallons of Water

Gallons: 1 gallon contains 3.7854 Liters

So we can divide the liters, above, by 3.7854 to convert water volume in liters to water volume in gallons.

The example well water volume contains (2460 / 3.7854) = 650 gallons of water.

How to Compute Well Flow Rate in Gallons per Hour

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.

Well Flow Rate = gph

The Well Flow Rate for a water well of any type is normally expressed in gallons per hour or gph that water enters the well from surrounding soils. We calculate a well's flow rate measured in gph by dividing the quantity of water in the well (we calculated that just above, right?) by the number of hours it took for that water to enter the well.

Well Flow Rate = Gallons / Hours - or gph, 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.

Our reader made it easy by telling us that the well was pumped dry at 4PM. He measured the water depth and well diameter and kept track of how much time had passed (17 hours).

From our well volume formulas above we know that starting with zero water, after 17 hours the well contained 650 gallons of water.

For this example, 650gallons / 17hours = 38 gallons per hour - this is the well flow rate for a 17 hour period for this particular well.

Definition of Well Flow Rate in Gallons per Minute & 24 Hour Flow Rate

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. gpm.

In our example above, 38 gallons an hour might look like a huge flow rate, by the way, but it's not when we convert the flow rate in gallons per hour into flow rate in gallons per minute - the standard unit of measure of well yield.

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 temporarily draw water out of a well faster than the gpm flow rate, 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, gpm 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.

Gallons per hour (gph) divided by 60 (the number of minutes in an hour) = well flow rate per minute in gpm.

Well Flow Rate_gpm = Well Flow Rate_gph / 60

For our dug well example, 38 gph / 60 = 0.63 gpm - this is the measured well flow rate in gallons per minute.

In this case that's a weak, marginal well flow rate - just over half a gallon per minute. 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.

Watch out: if you install a pump whose pumping rate exceeds the well yield or flow rate (see Well Yield: Well Flow Rate), the pump may run dry and be damaged. The risk of pump damage is greater in a well that has a small static head (see Static Head of Water in the Well) or in conditions under which the pump is left running for long periods so that the static head is likely to be exhausted. If you have this risk or this problem on a well, see the advice on protecting pumps given at WELL PIPING TAIL PIECE.

Is This Really the 24-hour Well Flow Rate?

Is 38 gph or 0.63 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.

Measure Well Flow Rate After You 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.

Does A Well's Flow Rate Change Over Time?

The flow rate of a drilled well, driven point well, or dug well can certainly change dramatically over time. But other water wells continue to produce water at the same rate for decades. A 27 foot deep well at our office has delivered the same flow rate without change from 1920 to 2010.

When the flow rate of a well varies the underlying causes might include the following:

• Seasonal water table variations - during the dry season the water table may drop surrounding the well. The result is that some of the rock fissures or soil passages that previously fed water into the well may not be producing. Other (typically lower) rock fissures or soil passages may still be delivering water into the well. The effect is that the total well inflow-rate is reduced.
• Permanent water table variations - pumping from a large number of wells in a given area can permanently lower the water table, often dramatically, particularly when the aquifer that has been tapped was one that built-up over eons or during weather conditions that no longer are common. We anticipate that some areas will be affected by global warming to have dramatically lowered water tables while others may actually see their water table increased - it may depend on local weather trends.
• Drilling of other wells nearby can lower the water table and thus reduce the well recovery rate for an existing water well.
• Age, usage of the well, combined with water high in mineral content can reduce the well flow rate as minerals in the water flowing into the well precipitate out as solids that clog rock fissures that were feeding the well.
• The well pump itself may be damaged, worn, or well piping may have leaks that appear to lower the well flow rate when the problem is with the equipment. See WATER PRESSURE LOSS DIAGNOSIS & REPAIR.

Definition of Safe Yield for a Well

We define safe well yield as the combination of total water quantity that can be drawn out of a well without dropping the water level in the well low enough to introduce air into the system or damage the well pump.

A well with a large static head and/or a well with a very good flow rate may have a high safe yield while a well with a small static head and the bad luck to also have developed a low well flow rate will have a very small safe yield - in some cases less than 50 gallons of water.

If a poor safe yield is likely to be permanent, solutions include increased water storage capacity in the building, steps to increase the well yield, or installation of a drawdown cutoff device that prevents the pump from dropping water in the well to a level that risks air entry into the piping or damage to the pump. See How to Get More Water From a Well.

Watch out: if you install a pump whose pumping rate exceeds the well yield or flow rate (see Well Yield: Well Flow Rate), the pump may run dry and be damaged. The risk of pump damage is greater in a well that has a small static head (see Static Head of Water in the Well) or in conditions under which the pump is left running for long periods so that the static head is likely to be exhausted. If you have this risk or this problem on a well, see the advice on protecting pumps given at WELL PIPING TAIL PIECE.

Watch out: Measurements like the well depth (WELL DEPTH, HOW TO MEASURE), well flow rate (WELL FLOW RATE), well recovery rate 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? 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? See How Much Water is In the Well? and see How to Test Well Water Quantity.

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.

WELLS CISTERNS & SPRINGS
  AIR DISCHARGE at FAUCETS, FIXTURES
  Basement Wells
  CHECK VALVES
  CISTERNS
  DEPTH of a WELL, HOW TO MEASURE
  Drilled Wells - steel casings
    Drilled Well with Submersible Pump
    Shallow Well with One Line Jet Pump
    OLD WELL - RETURN TO SERVICE
    Well with Two Line Jet Pump
  Driven Point Wells
  How Much Water is In the Well?
    Static Head of Water in the Well
    Well Yield: Well Flow Rate
      Define Well Yield, Recovery Rate, Flow Rate
      Calculating the Flow Rate of a Well
      Well Flow Rate in GPM or 24-Hours
      Well's Flow Rate Change Over Time
      Define Safe Yield for a Well
    Total Quantity of Water Available
  How to Test Well Water Quantity
  How to Get More Water From a Well
  Hand Dug Wells
  Hand Dug Well Procedure
  Jetted Wells
  OLD WELL - RETURN TO SERVICE
  Springs as Water Supply
  Wash Wells
  Well Pits
  WATER HAMMER NOISE DIAGNOSE & CURE
  WATER PRESSURE LOSS
  WATER PUMP SAFETY
  WATER TANK SAFETY
  WATER TANK TYPES
  WELL CHLORINATION SHOCKING PROCEDURE
  WELL CLEARANCE DISTANCES
  WELL DEPTH, HOW TO MEASURE
  WELL FLOW RATE
  WELL LIFE EXPECTANCY
  WELL PIPING CHECK VALVES
  WELL PIPING FOOT VALVES
  WELL PIPING LEAK DIAGNOSIS
  WELL PIPING TAIL PIECE
  WELL PUMP PRIMING PROCEDURE
  WELL PUMP TYPES & LIFE EXPECTANCY
WELL CHLORINATION SHOCKING PROCEDURE