You must first compare your pond water requirements
- the water needed to initially fill your pond and to compensate
for seepage and evaporation losses (see Section 2) - with the water
available from your source (see Section 3).

If you find that your water source provides enough water to fill
the pond in a reasonable period of time, to fill it when you want
to fill it, and to compensate for water losses throughout the year,
you will not need a reservoir.

If you find that your water source does not provide enough water
to fill the pond and to compensate for water losses at certain times
of the year, but there is enough water available over the year,
you may decide to build a reservoir and store the water you will
need.

If you decide to build a reservoir, you will encounter one of two
situations:

Your source provides a supply of water throughout
the year

If your daily water flow is great enough throughout the year to
compensate for water losses by seepage and evaporation, but not
great enough to fill the pond in a reasonable period of time, and
to fill it when you want to fill it, you will only need a
reservoir with a volume equal to the pond volume, or even less,
since the reservoir is being supplied constantly.

Example

If your daily flow is not great enough throughout the year to fill the
pond in a reasonable period of time when you want to fill it, and to compensate
for water losses by seepage and evaporation, the reservoir will
have to be large enough to provide for the total water requirements.

Example

Your water source dries up completely at certain
times of year

If your source does not supply water throughout the year, but dries
up at certain times, you should plan to fill your pond during the
year either from a reservoir, or directly from the source at a time
when there is enough water available.

If you are able initially to fill your pond at a time when water
is available, your reservoir need only contain enough water to compensate
for water losses by seepage and evaporation during the dry season.

If you are unable initially to fill your pond at a time when there
is water available, you may need to build a larger reservoir to
store water over a longer period of time, and that will contain
enough water to fill the pond and to compensate for water losses.

Water losses by seepage and evaporation from a reservoir

A reservoir is subject to water losses by seepage and evaporation
just as a pond is. To offset water losses from a reservoir, plan
the water volume of the reservoir 1.5 times greater than the water
volume you require to satisfy total water requirements.

Example

4.1 Selecting a site for a
reservoir

If you are going to build a reservoir, look for a site that will
allow you to retain the greatest volume of water with the
smallest dam possible. Dams require a great deal of work
to build and to maintain, and the smaller the dam, the better.

Avoid a site in a valley, relatively open and wide
at the downstream end. At such a site you will have to build a large
dam.

The ideal site for a reservoir is in a wide valley
which narrows suddenly, with steep sides at the downstream end.
The narrower the valley end, the smaller the dam will have to
be.

Choose a site with good soil, which will hold water well. Avoid
a sandy site. The place you choose should not have areas of sand
that are too large to seal against water loss. If there are large
sandy areas, it may be better to look for another site.

It is good to clear the site of vegetation
before building a reservoir. If you plan to do this, avoid choosing
a site with too many large trees, which may be difficult to remove.

You should choose a site where you can build a reservoir large
enough to satisfy your total water requirements. It may be to
your advantage to find more than one possible site and choose
among them on the basis of:

Your water requirements (see Section 2.3);

How large a reservoir is possible at each site (see Section
4.2);

The topography, soil and vegetation present at each site,
as you have seen in this section.

Example

Note: if the water flow of a stream increases greatly
during the rainy season, it may be difficult to maintain a dam
during this period. If the water level becomes too high the dam
may be washed away.

Example

4.2 Determining
characteristics of a reservoir

When you have chosen a suitable site for a reservoir and have decided
on the place to build the dam, you will have to determine the height
of the dam you should build to store the volume of water required. To
do this you will need to make one or more approximations to find the
reservoir size that will give you the volume desired.

But first you will have to decide on the type of dam you
will build. A dam may be built either without a spillway
(when the stream water flow is relatively small and constant throughout
the year) or with a spillway (when the stream flow
is relatively large and/or when it greatly varies from one season to
another). If conditions allow it, you should build a dam without a spillway,
since it is easier to build. What is a spillway?

Estimating the volume for a reservoir without
a spillway

This is a relatively simple method that will give you a rough
estimate of reservoir volume. If you plan to build a dam without
a spillway, this rough approximation will be enough.

First approximation

Begin by assuming a maximum water depth at the dam of 1.5 m.

Determine and mark the contour line at this assumed
maximum water level using topographical measuring devices. The
contour, when marked, will outline the extent of the planned reservoir.

Now measure this contour line (in
m) and calculate the approximate volume of water (in m^{3})
that can be held in a reservoir of this size and maximum depth
(1.5 m). To do this:

Square the length of the contour line;

Divide the result by 37.5 (conversion factor);

Multiply by the assumed maximum water depth (in this case,
1.5 m) to find the approximate reservoir volume.

Example

If this approximate water volume is between
10 and 20 percent greater than the water storage needed,
you can build the dam and the reservoir based on the assumed depth
of 1.5 m.

Example

Additional approximation

If your first approximation is less than 10 percent or more
than 20 percent of the water storage requirement, you will have
to adjust the assumed maximum water depth and make
another approximation.

If the approximate reservoir volume is much greater than
the water storage requirement, reduce the assumed
maximum water depth by 30 cm to 1.2 m and repeat the calculations,
using this new assumed depth. If the calculated reservoir volume
is still too great, reduce the assumed maximum water depth by
another 30 cm to 0.90 m and repeat the process.

If the calculated reservoir volume is much smaller than
the water storage requirement, increase the assumed
maximum depth by 30 cm to 1.8 m and repeat the
calculations, using this new assumed maximum depth.

Estimating the volume
for a reservoir with a spillway

If you decide to build a dam with a spillway, it will require
more work and time than a dam without a spillway. Therefore,
it is advisable to estimate the reservoir volume more accurately
before you begin to build. This is a method that will allow
you to calculate the volume of a planned
reservoir with greater accuracy.

To employ this method, you will need
various topographical measuring devices for levelling - such as
an A- frame, a straight edge and mason's
level or other sighting device (that can also measure 90� angles)
such as a cross staff, or a more sophisticated
surveyor's level.

First calculation

Begin by assuming a maximum water depth of 1.5 m in the reservoir,
when the water level reaches the crest of the spillway.

Determine and mark the contour line
at this assumed maximum water level, using a suitable levelling
method. The contour, when marked, will outline the extent of the
planned reservoir.

Using a suitable leveling method, mark on the poles,
at the corners of each square, the level that the water would
reach if the maximum water depth of the reservoir were 1.5 m.

Now you are ready to calculate the water volume of each square.
The sum of the volumes of all the squares will equal the total
water storage volume of the reservoir.

Calculate the volume of
each full square

Measure the water depth (in m) marked on the pole at each
corner of the square;

Add the four water depth figures and divide by 4 to find the
average water depth in this square;

Multiply the average water depth by 400 m^{2}, which
is the area of the 20 m x 20 m square, to find the total water
volume of the square (in m^{3}).

Example

Calculate the volume of each partial square

Measure the water depth (in m) marked on the pole at the
two corners of the square that are inside the planned reservoir
area (the depth of the other two corners of the partial square
is 0);

Add the two water depth figures and divide by 4 to find
the average water depth in this partial square;

Measure the length of the two partial sides of the square
(in m);

Add these two length measurements and divide by 2 to find
the average side length;

Multiply this average side length by 20 m to find the area
of the partial square (in m^{2});

Multiply this area of the partial square by the average
water depth to find the total water volume corresponding to
the partial square (in m^{3}).

Example

After you have calculated the water volume for each full square
and each partial square, add all these volumes to find the total
water storage volume. This is the capacity of the planned reservoir
if it has a maximum water depth of 1.5 m at the crest of the
spillway.

Additional calculation

If your first reservoir capacity is much greater or much smaller than
the water storage requirement, you will have to adjust the
assumed water depth and make another series of calculations.

If the calculated reservoir capacity is much greater than
the water storage requirement, decrease the assumed maximum
water depth of 1.5 m by 0.2 m to 1.3 m. and calculate
each square again. Repeat this process until the calculated reservoir
capacity is equal or nearly equal to the water storage requirement.

Example

If the calculated reservoir capacity is much smaller than
the water storage requirement, increase the assumed maximum
water depth of 1.5 m by 0.2 m to 1.7 m. and calculate each square again. Repeat this process until the
calculated reservoir capacity is equal or nearly equal to the water
storage requirement.