Well static head sketch (C) Carson Dunlop AssociatesDefinition of the Static Head in a Water Well
     

  • STATIC HEAD, WELL DEFINITION - CONTENTS: Definition of a well's "static head" in water well terminology. 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 static head vary over time? What happens to well flow when we install a more powerful water pump? We exhaust the well static head more quickly.
  • POST a QUESTION or READ FAQs about the well static head and how much water is contained in a well casing - water volume reserve
  • REFERENCES

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Here we define the static head in a well and we explain how the well's static head can compensate for a well with a poor flow rate.

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Static Head: Well Water Starting Quantity - What is the Static Head of Water in the 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 sketch at page top, courtesy of Carson Dunlop, shows how the static head of water in a well is located and estimated. Details are below.

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.

[Click to enlarge any image]

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.
Sketch of a drilled well installation

How to calculate the gallons of water per foot of a 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.

With the exception of artesian wells, the well water column height does not extend from the well bottom to the top of the ground. Rather the top of the water in a conventional drilled or dug well at rest will be somewhere between the well bottom and well top, depending on the seasonal water table and other factors.

In an artesian well natural water pressures in the aquifer would force well water out at the top of the well casing.

For artesian wells the well is usually constructed with a seal inside the well casing to prevent water from rising above the point at which a water supply pipe exits the well casing. In an artesian well the static head water height will normally be from the well bottom to the point at which the casing seal has been installed.

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 water intake at the pump (since water can't jump up to the pump intake) 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.
  • (h) static head: explained above
  • (c) well bottom clearance: our well pump or foot valve (if the pump is not in the well) was placed 5' off of the well bottom © in the sketch, a distance to avoid drawing mud into the pump
  • (d) total well depth: measured from the top of the ground to the bottom of the well. More about measuring the depth of a well is at DEPTH of a WELL, HOW TO MEASURE.

The Formula to Calculate the Static Head of Water in a Well

To find the amount of water in the static head of a well we find (h), the depth of the column of water in the well when the well is at rest, and then based on the well diameter we calculate the volume of (h) in cubic meters, feet, or inches. Last we convert that volume into common liquid measures such as liters or gallons.

Using the symbols and definitions given just above, the formula to express the size of the static head of water in a well first in feet of height is simply:

(h) = (d) - [(a) + (c)] - we subtract the well top air air space and pump to bottom clearance distances from total well depth

The actual water quantity in (h) is calculated based on the volume of the well cylinder interior.

In a standard 6" steel casing well, the water volume is about 1.5 gallons per foot of height of the static head

But remember that's just for the portion of the casing that actually contains water when the well is at rest - don't count the air. The formulas for volume of a cylinder and thus of water in a well casing are shown and an example are calculated just below.

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

Vcyl = pi x r2 x h

where pi = 3.1416,
r = the radius of the circle formed by the cylinder (the well shaft or casing), and is simply 1/2 of the well diameter
h = the height of the cylinder of water (the static head height that we measured above).

Watch out! be sure to write the radius and height in the same units of measure - here we're going to use inches.

Vcylinches = 3.1416 x r2inches x hinches

So for a 12-inch (one foot) height of 6" diameter steel well casing,
r = the radius = 1/2 of the diameter of the pipe, or 3" and
h = the height is 12"

Now we can calculate the static head water volume in cubic inches:

Vcylinches= 3.1416 x 32inches x 12inches

Vcylinches = 339 cubic inches (in this example, for a one foot high, 6" diameter cylinder of water in a well casing)

How to convert cubic inches to cubic feet

Since there are 1728 cubic inches in a cubic foot (12 x 12 x 12) we divide:

Vcfeet = 339 / 1728

Vcfeet = 0.196 cubic feet

How to Convert Cubic Feet to Gallons

since 1 ft³ = 7.48051 gal(US Liq),

Vcgallons = 0.196 x 7.4 = 1.46 gallons

That's why we use an easy to remember "rule of thumb" of 1.5 gallons per foot of static head of water found inside of a 6" drilled well casing.

Formula for Static Head in Gallons of Water in a Typical 6" Steel Well Casing

Static Head (h)gallons = (1.5 gallons per foot) x (h) measured in feet

Here's a simple example to calculate the volume of water in the static head of a particular 100 foot deep well. Remember that for your well you'll need to plug in the actual measurements.

(d) = total well depth = 100 ft.
(a) = air in top of well casing = 45 ft.|
(c) = well bottom clearance between pump intake and well bottom = 5 ft.

We want to calculate (h), the static head, in gallons of water - we just need to calculate the height of the column of water (in feet) inside the 6" diameter well casing and multiply it by 1.5 (gallons per foot)

Static head water quantity (h)gallons = (Total well depth (d) - Air (a) - Clearance at bottom (c) ) x 1.5

Or if you prefer

(h)gallons = (h)feet x 1.5

For this example, using the (d), (a), and (c) measurements from above, we calculate (h)feet and multiply it by 1.5 to find the static head in gallons - (h)gallons

(h)gallons = [(100 - 45 - 5) feet of height of static head ] x [1.5 gallons per foot]

(h)gallons = (50) x 1.5

(h)gallons = 75 gallons of water - that's how much water is in the static head of the example well.

 

Static Head Measurements for Other Well Types

Note that the static head description and calculations given in this article apply to round drilled wells and round dug wells.

If your dug well is a different shape, say a rectangle, the principles are the same but you'll need to use the formula for volume of a rectangular shape V= length x width x height rather than a cylindrical shape given above and again just below.

The static head of a driven point well is practically zero - just the volume of water inside the lower section of the driven well point (a pipe) below ground. For a driven point well, if you still want to know its static head, you might try the calculation of volume of water stored in water piping, just below.

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. But let's be clear - the volume of water resting in well piping does not increase the volume of water available at a property. That is, the water stored in well piping does not increase (nor decrease) the well's static head as we defined it above.

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

Does the Static Head Quantity in a Well Change?

Absolutely. The static head, the amount of water in a well when the well is "at rest" - that is, no one has pumped water out of the well for some time and the well has filled back up as much as it's going to - changes:

  • The well's static head will usually be at its lowest during the dry season of the year
  • The water well's static head will usually be at its highest during the wettest season of the year, or after a period of heavy rainfall or heavy snow melt
  • A well's static head may drop precipitously in response to events that disturb the soil or water bearing rock nearby, such as road or foundation blasting
  • A well's static head may drop precipitously or even disappear entirely - leaving the well "dry" in response to changes in the underlying rock strata or on occasion if someone drills another water well nearby, tapping the same aquifer.

 

Continue reading at WELL QUANTITY TOTAL or select a topic from the More Reading links shown below.

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STATIC HEAD, WELL DEFINITION at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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