Definition of Safe Total Well Yield

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 WELL YIELD IMPROVEMENT.

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.

Watch out: if you install a pump whose pumping rate exceeds the well yield or flow rate , the pump may run dry and be damaged.

See WELL YIELD DEFINITION

The risk of pump damage is greater in a well that has a small static head or in conditions under which the pump is left running for long periods so that the static head is likely to be exhausted.

See STATIC HEAD of WATER in the WELL

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?

At WELL YIELD, SAFE LIMITS 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 the Total Quantity of Water Available From a Water Well

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 well sketches at page top and at left, courtesy of Carson Dunlop Associates, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

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.

This article is a companion to WELL YIELD DEFINITION where we define well recovery rate, water well yield, and where we make sure you know the difference between well pumping rate (how fast you can take water out of the well) and well recovery rate (how fast water flows from the surrounding soils back into the well).

Watch out: Measurements like the well depth , 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?

See details at WELL DEPTH, HOW TO MEASURE to get the depth of the well and the size of the water column or static head in the well.

See details at

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 WELL FLOW RATE to understand the amount of water the well can deliver per minute, hour, or day.

And see WELL FLOW TEST PROCEDURE for how to do an accurate well flow test.

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.

See WELL YIELD IMPROVEMENT.

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.