Sketchof a drawdown test for a water well (C) Carson Dunlop Associates Total Well Water Quantity Explained
How Much Water is In the Well?

  • WELL QUANTITY TOTAL - CONTENTS: How much water can we take out of a well before we run out? What is the water quantity that a well can deliver, how do we measure or estimate this figure?Well Flow Rate, Well Yield, & Water Quantity Explained - Problems & Repair Advice for wells. Well diagnosis & repair procedures including how to determine the total quantity of water that can be drawn from a well over a given period.
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How to measure or estimate well water quantity or total drawdown volume:

What is the total amount or volume of water that can be taken out of a water well in gallons or litres over some time period?

Here we define and explain the procedure to estimate the total water quantity available from a well.

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Definition of the Total Quantity of Water Available From a Water Well

Well static head sketch (C) Carson Dunlop AssociatesThis 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 well sketches at page top and at left, courtesy of Carson Dunlop Associates.

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 & 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.

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?


Why 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

Definition of Safe Total 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.


Continue reading at WELL YIELD DEFINITION or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

In a companion article, WELL FLOW TEST PROCEDURE, 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.

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WELL QUANTITY TOTAL at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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