Cover Image
close this book Water purification, distribution and sewage disposal for Peace Corps volunteers
close this folder Section 3: Planning the distribution system
View the document Overview:
View the document Design
View the document Existing facilities
View the document Size and nature of the community
View the document System capacity
View the document Water source
View the document Proposed system
View the document Financing the project
View the document Lesson plans

System capacity

The methods for evaluating each type as a potential water system source was covered in Section I. For each source you will have to determine its yield. This is the maximum quantity of water that can be drawn from a source in a given period of time. To calculate the yield for a source of water you:

1. Draw a measured quantity of water from the source;

2. Time how long it takes the source to replenish the drawn quantity)

3. Divide the amount of water drawn by the time taken to refill.

Yield is usually stated in gallons per minute. Below are examples for estimating yield for various types of water sources.

Fig. 24 Cistern Catchment Yield

a. Cistern Catchment Yield

To estimate your catchment area, the minimum yearly rainfall and the amount of water required by the family during one year, must be estimated. Sometimes, the government meteorological section can give you the minimum rainfall expected. If they do not, you can estimate the minimum rainfall at two-thirds of the yearly average. Take the average amount of water needed by the family for one day and multiply it be 365 to learn how much is needed for one year. Then use the chart to find how much roofspace is needed (Fig. 24 )... Suppose you have a rainfall of 60 inches a year and the family needs 20 gallons a day, then...

2/3 x 60 equals a minimum rainfall of 40 inches a year

365 days x 20 gallons a day equals 7300 gallons a year

The chart shows that a catchment area of about 300 square feet is needed to supply the family with enough water for one year.

b. Yield of Small Streams

This is a rough but very rapid method of estimating water flow for small streams. The number of streams that must be used and the flow variations are important factors in determining the necessary facilities for utilizing the water. Here is a way to survey a water supply problem quickly by allowing you to take rapid flow measurements.

The equation for stream flow is - - Q = K x A x V

Q = flow in gallons per minute (8.33 pounds = l gallon)

A = cross section of stream, perpendicular to flow, in square feet.

V = stream velocity, feet per minute.

K = a corrected conversion factor since surface flow is normally slower than average flow. For normal stages use K = 6.4; for flood stages use K = 6.7 to 7.1.

Fig. 25 Determine Stream Yield

Fig. 26 Cross Section of Stream

To find "A"... the stream will probably have different depths along its length so select a place where the depth of the stream is average...take a measuring stick and place it upright in the water about one foot from the bank...note the depth of water...move the stick two feet from the bank in a line directly across the stream...note the depth...move the stick three feet from the bank, note the depth, and continue moving it at one-foot lengths until you cross the stream. Draw a grid, like the one above, and mark the varying depths on it so that a cross-section of the stream is shown. A scale of one inch equals one foot is often used for such grids. By counting the grid squares and fractions of squares, the area of the water can be estimated. For example, the grid shown here has about 15 square feet of water.

To find "V"...put a float in the stream and measure the distance of travel in one minute (or fraction of a minute, if necessary.) The width of the stream should be as constant as possible and free of rapids, when measuring the velocity.


Cross section is 15 square feet.

Velocity of float = 20 feet traveled in 1/2 minute

Stream flow is normal

Q = 6.4 X 15 x 20 feet/.5 minute

3800 gallons a minute