If the pond has a square shape, multiply two sides
(in metres, or m) or, if it has a rectangular shape,
multiply the length (in m) by the width (in m) to find the surface
area (in square metres or m^{2}).

Examples

If you have a large pond you may want to convert the surface
area from square metres to ares or hectares (ha).

If the pond has an irregular shape but the sides
are generally straight, you can find the surface area by dividing
the pond into smaller areas that can be more easily calculated,
and add these to find the total surface area.

Prepare a plan of the surface area of the pond,
as accurately as possible, on a sheet of paper. Now divide the
plan into squares, rectangles, right (or 90°) triangles.

Note: when dividing the
surface of a large irregular pond, it may help to trace an xy axis
the length of the plan. You can use this axis as reference line
along which you can construct squares, rectangles or triangles.

Calculate the area of each square, rectangle or triangle using
accurate length, width, base and height measurements (in m).

To find the area of a square, multiply two sides;

To find the area of a rectangle, multiply the length by the
width;

To find the area of a right (or 90°) triangle, multiply the
base by the height and divide by 2

After you have calculated all of the smaller areas, add them
to find the total surface area.

If the pond has an irregular shape with a curving side,
you may need to approximate the curved part to find the surface
area. Construct a line across the curved side of the pond so that
the part outside the line is approximately the same as the part
inside. Then calculate area or areas as you did earlier in this
section.

Examples

How to calculate the average water depth of
the pond when it is empty

If the pond is not too large, you can mark the future
water level with strings stretched across the pond and tied to
stakes at AB, CD, and EF. The stakes are placed at the planned
water level. Measure the depth in a number of places along each
string and calculate the average water depth as shown below.

If the pond is large and it will be difficult or
impossible to stretch strings from bank to bank, you may be able
to calculate the average water depth using a combination of strings
where possible, and a square grid, as shown below.

How to calculate the average water depth of the pond when it
is full

If the pond is small, with a regular shape, and has a bottom
with a constant slope from one end to the other, go into the water
and measure the depth at four points, 1, 2, 3 and 4 in the pond.
To find the average depth, calculate the average of these measurements.

If the pond is large, with a regular shape, and has a bottom
with a constant slope from one end to the other, increase the
number of measurements. Go into the water and measure the depth
at nine or more points in the pond.

If the pond is large with an irregular shape and an irregular
bottom, construct a square grid 5 m x 5 m over the pond surface.
Go into the water and measure the depth at each grid intersection.
Average all measurements.

How to calculate the volume of water in the
pond

You have thus calculated the surface area of the pond and the
average water depth of the pond. Now, using the figures you have
found, you can calculate the volume of water in the pond by multiplying
the surface in square metres (m^{2}) by the average water
depth in metres (m) to get the volume of the pond in cubic metres
(m^{3}).

SURFACE AREA x AVERAGE DEPTH = VOLUME

Examples

Surface area (m^{2})

Average water depth (m)

Water volume (m^{3})

235

x

1.0

=

235

450

x

1.2

=

540

2500

x

1.5

=

3750

Note: 1 cubic metre (m^{3}) = 1000 litres
(l). To express water volume (in m^{3}) in litres
(l) multiply by 1000. To express water volume (in l) in cubic metres
(m^{3}) divide by 1000.

2.1
Water losses by seepage

Water that is lost vertically through the bottom
of the pond, horizontally through the dikes by infiltration, and
through the drainage system of the pond is called seepage
water.

If the dikes of your pond are well built and well maintained
and if the drainage system is watertight, the amount of seepage
water lost horizontally will be very small. You will need to calculate
only the vertical seepage.

Water seepage will be greater from a new pond when it is filled
for the first time. The soil structure of the pond will still
be good and water will be lost.

After the pond has been filled with water for some time, the
water tends to break down the soil structure and the soil pores
become sealed by organic matter that collects on the pond bottom.
As a result, the soil permeability and losses by seepage will
decrease.

The amount of vertical water seepage
will depend on the soil texture
and on the soil structure of the
pond bottom. If the composition of the soil is coarse, as in
sandy soils, it will be permeable, and water will be lost by seepage.
Soils with a good structure will allow more seepage than soils with
a bad structure.

How to calculate water losses caused by seepage

The figures below give the rate of seepage
losses in millimetres per day (mm/day) from various soil types
(in the natural state) needed to calculate pond seepage losses over
a period of time.

Natural soil type

Seepage losses (mm/day)

Sand

25.00 - 250

Sandy loam

13.00 - 76

Loam

8.00 - 20

Clayey loam

2.50 - 15

Loamy clay

0.25 - 5

Clay

1.25 - 10

Example

Reducing seepage water losses
by puddling

One way to reduce seepage water losses is to break the soil structure
of the pond bottom before it is filled with water. This is a common
practice in irrigated rice fields, and is called puddling.

The soil of the pond bottom is first saturated with water. The
amount of water you will need initially to saturate the bottom
(200-300 mm) will vary slightly with the type of soil. Assume
a standard requirement of 300 mm, or 0.3 m.

When the water has soaked into the soil of the pond bottom enough
to permit working, you are ready to puddle. This is done by hoeing,
ploughing or working the soil by any other suitable means.

How to calculate water needed for puddling and water losses by
seepage after puddling

To calculate the amount of water needed for puddling multiply the
pond area (in m^{2}) by 0.3 m.

Example

The figures in the chart give the rate of seepage losses from various
soil types (after puddling) needed to calculate pond seepage losses
over a period of time.

Puddled soil
type

Seepage losses (mm/day)

Sandy loam

3-6

Loam

2-3

Clayey loam

1-2

Loamy clay

about 1

Clay

about 1

Example

To calculate the total water required both for puddling and to
compensate for seepage losses for 6 months thereafter, add the two
values.

Example

2.2 Water
losses by evaporation

The water that is lost to the air from the surface of the pond
is called evaporation. The amount of water lost by
evaporation depends largely on local climate conditions.

High air temperatures, low humidity, strong winds and sunshine
will increase evaporation.

Low air temperatures, high humidity, rainfall and cloud cover
will decrease evaporation.

Evaporation will also depend on the water
surface area. The larger the pond, the more water will evaporate
from its surface.

Evaporation rates

You will need to know your local evaporation rate
in order to calculate the amount of water lost from the surface
of a pond by evaporation. Evaporation rates, which are provided
by meteorological stations, are found by measuring and recording
water losses by evaporation over many years.

Evaporation rates are usually expressed as the water
depth lost in millimetres over a period of time, e.g., 2 mm/day, 14
mm/week or 60 mm/month.

Evaporation rates by Class A Pan

One of the most common methods to find the evaporation rate is
accurately to measure daily water losses from a standard-size
container called a Class A Pan. Evaporation rates
by Class A Pan can be obtained from many meteorological stations
throughout the world.

In choosing a meteorological station for evaporation rates, be
careful to select one where climatic conditions such as sun, wind
and rainfall are similar to conditions in your locality. If you
are not sure ask the advice of a technician from the meteorological
station.

Class A Pan evaporation rates may be expressed as either mm/
day, mm/week or mm/month, over a period of years. Usually you
will be able to obtain the average monthly evaporation rates,
which are based on measurements made during several years. If
you can get average monthly evaporation rates, this will be the
most convenient for calculating water losses by evaporation.

Note: water evaporates faster from a Class A Pan
than from a large water surface such as a pond. When using Class
A Pan evaporation rates you must multiply by a correction
factor of 0.75 to better approximate evaporation losses.

Example

How to calculate water losses by evaporation
using Class A Pan evaporation rates

To calculate evaporation losses, multiply the water surface area
(in m^{2} ) by the corrected evaporation rate (in m) for
the length of time your pond will be in use.

Obtain Class A Pan average evaporation rates (in mm) for each
month your pond will be full from an appropriate meteorological
station;

Class A Pan average monthly evaporation rates needed for this
example are as shown below:

Month

Evaporation rate (mm)

April

56

May

63

June

68

July

75

August

84

September

79

Add the rates (in mm) for each month and multiply this sum
by 0.75 (correction factor for Class A Pan rates) to find the
total corrected evaporation (in mm) for all the months;

Divide this total corrected evaporation (in mm) by 1 000 to
express the evaporation in metres;

Multiply this value (in m) by the water surface area (in m^{2})
to find the total water losses by evaporation (in m^{3})
for the months your pond will be in use.

Example

Evaporation rates by the Penman Formula

Some meteorological stations may not record evaporation rates using
a Class A Pan. If this is the case, you may be able to get evaporation
rates calculated by the Penman Formula. The Penman
Formula is based on data of atmospheric pressure, radiation, sunshine,
humidity, air temperature and wind speed.

Note: under some conditions, such as when there are
high winds, and especially in arid climates, the Penman Formula
may provide evaporation rates that are too low. If this is the case
in your locality, ask the advice of a technician from the meteorological
station.

The evaporation rates calculated by the Penman Formula are more
accurate than the rates recorded using a Class A Pan. To calculate
evaporation losses by Penman rates, you can use the
method shown above but, since these rates are more accurate,
omit the step where you multiply the total evaporation by the correction
factor of 0.75.

2.3 Total water requirements

The total water requirements for a pond are:

The amount of water needed to fill the pond in a reasonable
length of time;

The amount of water needed to compensate for seepage and evaporation
losses over the planned fish-growing period.

Pond size and water flow required

To begin growing fish as soon as possible, you should have enough
water available to fill your pond in a reasonable length of time.
For ponds smaller than 1 500 m^{3} , eight days is reasonable.

Before you begin to build a pond it will be helpful to compare
the number of days needed to fill ponds of various sizes and the
rate of water flow required. Table 1 will give
you a quick idea of some of the combinations possible.

Approximate filling time (days)

Pond volume (m^{3})

Required water flow (l/s)

8

400

0.5

1000

1.5

2500

3.5

10000

14.0

4

400

1.0

1000

3.0

2500

7.0

5000

14.0

10000

28.0

2

400

2.0

1000

6.0

2500

14.0

10000

56.0

Examples

If you measure the available water flow (see Section 3) before
you begin to build your pond, you will be able to estimate more
precisely the number of days needed to fill a pond. Table
2 gives the water volume per day (in m^{3}) provided
by various rates of water flow. To calculate the number of days
to fill a pond, divide the planned pond water volume by this daily
water flow.

TABLE 2

Amount of water provided per day by various
rates of water flow

l/s

l/min

l/h

l/day

m^{3}/day

1

60

3600

86400

86.4

2

120

7200

172800

172.8

3

180

10800

259200

259.2

4

240

14400

345600

345.6

5

300

18000

432000

432.0

6

360

21600

518400

518.4

7

420

25200

604800

604.8

8

480

28800

691200

691.2

9

540

32400

777600

777.6

10

600

36000

864000

864.0

14

840

50400

1209600

1209.6

15

900

54000

1296000

1296.0

20

1200

72000

1728000

1728.0

^{1}Z

Zx60

Zx3600

Zx86400

Zx86.4

^{1} The bottom line of this table shows how to convert
water flow values (Z) in l/s into l/min, l/h, l/day and m^{3}
/day.

Example

Using Table 2, you find that a water flow of 3 l/s will provide
259.2 m^{3} of water per day.

The time required to fill your pond is 1000 m^{3} ÷ 259.2
m^{3} /day = 3.86 days, say 4 days.

As a check, compare this result with Table 1 and you will confirm
this by reading across from 4 days that you need 3 l/s to fill a
pond of 1000 m^{3}.

Pond volume and the number of ponds possible will depend on the
water flow available

The size and number of ponds you will be able to build will depend
on the water flow available at the time you plan to fill them. The
paragraphs above together with Tables 1 and 2 give you several ways
to estimate the pond volume possible with various rates of water
flow.

You have measured the water flow and have found that you have 14
l/s available:

Using Table 1 you find that with 14 l/s you can fill one pond
of 2500 m^{3} in 2 days;

Or, with 14 l/s, you can fill one pond of 5000 m^{3}
in 4 days;

Using the values in Table 1 you can also calculate that with
14 l/s you can fill one pond of 10 000 m^{3} in 8 days.

Number of ponds to be built

With the same water flow of 14 l/s, you may decide to build more
and smaller ponds than shown above:

For example, with 14 l/s, you can fill two ponds of 2500 m^{3}
(= 5000 m^{3}) in 4 days;

Or, with 14 l/s, you can fill five ponds of 500 m^{3}
(= 2500 m^{3} ) in 2 days.

Planning for future expansion

You may also decide to build only one pond this year and another
next year:

With 14 l/s you can build one pond of 2 500 m^{3} this
year and fill it in 2 days and expand your operation next year
to two ponds of 2 500 m^{3} that can both be filled in
4 days with the water flow available.

Note: when you have several ponds, they need not be
filled at the same time. First fill one and then another as your
water supply permits.

Losses by seepage and evaporation

In addition to the water needed initially to fill a pond, you will
need to add water regularly, over the length of the growing season,
to compensate for seepage and evaporation losses.

Before you begin to build a pond you should estimate how much water
you will need to compensate for seepage and evaporation
losses, per hectare of pond area, so that your available water
supply will be sufficient during the driest season. On this basis,
you can then calculate the pond area that can be maintained with
this minimum water flow only.

Remember:
1 ha = 10 000 m^{2}
1 m^{3} = 1 000 l
1 day = 86 400 s