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Well static head sketch (C) Carson Dunlop AssociatesDefinition of the Static Head in a Water Well
Table of Well Water Volumes & Volume Calculations

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Definition of well water static head:

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.

The static head is basically a reservoir of water in the well bore or casing. It might be significant or trivial, but depending on your well bore recovery rate, the size of the static head can make the difference between a usable well with a low flow rate and running out of water. Also if you don't understand well static head you can be fooled by a "well flow test" result.



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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.

We will discuss the following: 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.

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.

Article Contents

Definition of static head in a water well

The static head is the actual volume of water that is inside a well bore, cylinder or dug well when the well is at rest (not in use) and after it has fully recovered from prior or recent use.

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. As we will explain below, since water does not rise all the way to the top of the well bore or dug well opening, the static head is a volume that is less than the total volume of the interior of the well bore.

[Click to enlarge any image]

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 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

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.

[Click to enlarge any image]

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

This Simple Table Gives The Volume of Water in a Well

Table of Volume of Water in a Well
(Volume of a Cylinder of Various Diameters per Foot or Per Meter)

Well Casing or Well Bore Diameter 
Volume of Water in the Well
Well Diameter
in Inches or cm

Volume of Water Per Foot or per Meter
in Gallons or in Liters

3 inch diameter well casing 84.8 cu. in. of water or about 0.37 gal per Foot of Depth2
8 cm (0.08 m) 5026 cc or cm3 or about 5 Liters per Meter of Depth3
4 inches 150 cu. in. or about 0.6 gal per Foot of Depth
10 cm (0.1 m) 7,854 cc or cm3 or about 7.9 Liters per Meter of depth
5 inches 235 cu .in. or about 1 gal per Foot of Depth
13 cm (0.13 m) 13,273 cc or 13.3 L of water per Meter of Depth
6 inch diameter well casing 339 cu. in. or about 1.5 gal per Foot of Depth
15 cm (0.15 m) diameter well casing 17,671 cc = about 17.7 Liters per Meter of Depth 1
8 inches 653 cu.in. or about 2.8 gal per Foot of Depth
20 cm (0.20 m) 31,416 cc or about 31.4 Liters per Meter of Depth
48 inches (Four Foot Diameter Dug Well) 21,714 cu. in. or about 94 gal per Foot of Depth
120 cm (1.2 m) (1.2 m Diameter Dug Well) 1,130,976 cc or about 1,131 L of water per Meter of Depth

Notes:

The Static Head or Actual Water Volume in the Well Bore is Less Than Total Well Bore Volume

The table above gives the volume of a cylinder (or well bore) of various diameters per foot of height or per meter of height.

Watch out: The volume of water inside of a well bore or well casing is normally less than the total volume inside the well bore cylinder.

The actual water volume is just for the portion of the casing that actually contains water when the well is at rest - don't count the air. The portion of the well casing that is filled with water, or slightly more narrowly, the cylinder of water that extends from the pickup bottom point of the foot valve or submersible pump (the bottom of the usable water column) up to the top of the water column when the well is fully rested and recovered, is the well's static head.

The static head will generally be less than the volume of the whole well bore since water in the well does not rise to the top of the well bore.

Exception: the level of water in artesian wells will, if not deliberately blocked from doing so, rise to the top of the well bore.

1. Special thanks to readers Steven (October 17 2017) and Valerie (5 August 2017) for correcting our arithmetic errors.

2. 1 U.S. gallon contains 231 cubic inches

3. 1 Litre or liter contains 1000 ccs or cm3

4. 1 cubic foot contains 1728 cubic inches

The formulas for volume of a cylinder and thus of water in a well casing are shown and a examples are calculated below.

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

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.

Calculate the volume of water or gallons of water stored in water piping or in a well bore

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)

h = the cylinder height or length of pipe in our case and

G = the volume of water in U.S. gallons = 0.004329 gallons per cubic inch or more conveniently 1 U.S. gallon contains 231 cubic inches of liquid.

For U.K. readers,

GU.K. = the volume of an imperial gallon is 277.419 cubic inches.

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

Divide the cubic inches by 231 (cu. in. per gallon)

G = 0.01 gallons per linear foot of pipe

How to Calculate the Volume of Water Inside a Cylinder

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 = 339.29 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 gallons

1 U.S. gallon contains 231 cubic inches

so for our 6-inch well casing that contains 339 cubic inches per foot,

339 / 231 = 1.4675 gallons per foot or about 1.5 gallons.

Alternatively:

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.

Metric Calculation of Well Casing Volume

Using the formula for the volume of a cylinder

For a well casing that is 15cm in diameter and 1 meter in height:

Radius r = 1/2 the diameter or 7.5 cm

Vcyl = pi x r2 x h

Vcin cc = pi x (7.5cm)2 x (100cm)

Vcin cc = 3.1416 x 56.25cm [the bore radius in cm, squared] x 100cm [the bore height in cm) = 17,671 cc or cubic centimetres

1 cc = 0.001

1 m of a 15cm diameter well bore will contain 17.671 L - that would be the correct entry in our table

Water volume calculation for other metric well diameters:

1m of a 13cm well bore will contain 3.1416 x (13/2)2 x 100 ccs [per L] = 1.1416 x 42.25 x 100 = 13,273 ccs or 13.2L of water per meter

1 m (or 100 cm) of a 20 cm diameter well bore will contain 3.1416 x (20/2)2 x 100 ccs = 3.1416 x 100 x 100 = 31,416 ccs or 31.4L of water per meter

1 m of a 120cm diameter dug well will contain 3.1416 x (120/2)2 x 100 ccs = 3.1416 x 3600 x 100 = 130,976 ccs or 130L of water per meter

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:

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

Or see WELL CASING DIAMETER CHOICE - the effects of larger diameter well casings on the static head in a well

Or see WATER PRESSURE MEASUREMENT - calculations of water volume, weight, and pressure

Or see WELL CHLORINATION SHOCKING PROCEDURE if you need to disinfect a well

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

INDEX to RELATED ARTICLES: ARTICLE INDEX to WATER SUPPLY, PUMPS TANKS WELLS

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