1. GENERAL BACKGROUND

1.0 Rainfall

The origin of fresh water is rainfall, and rainfall varies with geographic location and climate. Geography and climate influence both the amount of rainfall and the season when the rain falls. As a result of geography and climate, rainfall in a particular place can be concentrated during a few months or it can be spread out over the entire year.		  


     
In subtropical regions, such as African savannah and Asian monsoon areas, rainfall is usually concentrated in one season.   In equatorial regions, such as rain forest areas in Africa, the Far East, Asia and South America, rainfall is usually spread out more evenly over the year, with one or two seasons of particularly intense rain.



1.1 Rainwater infiltration and run-off

When it rains, water falling to the surface of the soil will either infiltrate into the soil or run off along the surface. Part of the water that infiltrates into the soil will be absorbed by the upper soil layers and part will seep down into the deeper soil layers. The part of the water that does not or cannot infiltrate will run off along the surface of the soil.

The degree to which rainwater will infiltrate or run off will depend on the permeability of the soil surface, the presence of vegetation, and local topography.

 


Permeability of the soil surface

The more permeable the soil surface is, the more easily water can infiltrate. In certain situations, the surface may be completely impermeable (for example, when it is rocky), and none of the rain water will infiltrate. It will all run off.

  




Intensity of rainfall

With less intense rainfall, there will be less run-off and more infiltration.

With more intense rainfall, there will be more run-off and less infiltration.

 
     

If the amount of rainfall is less than the soil can absorb, it will all infiltrate; there will be no run-off.

 



Presence of vegetation

The presence of vegetation during intense rainfall can greatly influence water infiltration. Tree limbs, branches and leaves will help to reduce the intensity of the rainfall or slow the water as it falls and allow more to infiltrate into the soil. The more vegetation there is, the less water will run off.

  




Local topography

When water falls on a flat area, where the soil will not allow all of the water to infiltrate immediately, pools of water will form. In time, part of the water in the pools will slowly infiltrate. The water that does not infiltrate will evaporate.

 

When rain falls on a sloping area, and it does not infiltrate immediately, it will run off following the slope to form small streams in the valleys.
     

 



1.2 What happens to water that infiltrates into the soil?

Water that infiltrates into the soil can to a limited extent be absorbed by the soil, depending on its thickness and composition. The more clay contained in the soil, the more water it can hold. But clay does not absorb water quickly.

Remember: water goes into clay soils slowly, but clay soils can hold more water than sandy soils. Water goes into sandy soils quickly, but sandy soils hold less water than clay soils.

     
 

Water that has been absorbed by the soil will be partly used up by plants and partly evaporated at the surface of the soil. Water that has not been absorbed by the soil seeps through to the subsoil. How fast and how deep it goes depends on the composition, structure and layering of the subsoil and of the structure of the rock formations below. Depending on these, thus, the depth at which water is found in the ground will vary from a few centimetres to many metres.

 


When rock formations under the subsoil are porous, water can seep into the rocks, it can seep to great depths and it may be difficult to reach.

When rock formations under the subsoil are impermeable, water cannot seep to great depths. It will accumulate on top of the rock and be easier to reach.

Subsurface water that has not been absorbed by the soil is called groundwater. The level at which groundwater is found in the soil is called the water table. When the water table is high enough to rise above the surface in low areas, a swamp, a lake or a stream is formed (see Section 1.5).

 



1.3 Springs and wells are sources of infiltrated water

When water lying above an impermeable rock layer enters a permeable layer such as sand or gravel, it will begin to infiltrate this layer. Such a water-carrying layer is called an aquifer. If an aquifer comes to the surface of the soil, the water it carries will flow out. This is called a spring.

The period during which water will flow from a spring may vary, because water may move through an aquifer rather quickly or it may take longer, depending on the topography.

 


The amount and interval of flow from a spring will also depend on how much rainfall there is, how it is spread over the year and the size of the area feeding the spring with rainwater.

When an aquifer is close to the surface of the soil but does not break through, it may be possible to reach the water by digging a hole. This is called a well.

  


When an aquifer is far below the surface of the soil, too deep to be reached by digging, it may be possible to reach the water by drilling.

 


When an aquifer becomes trapped between two impermeable layers that, owing to the topography, slope down into the ground, the water it carries will flow down and will be put under pressure. The further trapped water flows down, the more pressure will be developed.

If the top impermeable layer is naturally broken, the water under pressure is forced up. If the pressure is great enough to break the surface the water will be forced out of the ground. This is called an artesian spring. If the pressure is not great enough and the water remains below the surface it may be possible to reach it by digging or drilling a hole. This is called an artesian well.

 



1.4 What happens to water run-off?

When the ground is sloping, water runs off along the surface into adjacent low-lying areas. Excess groundwater in the soil also tends to move toward low areas. If the level of water in the ground is high enough, the run-off will collect above the ground. This is an example of the water table rising above the surface of the ground.

 





1.5 Swamps, lakes and streams are sources of water run-off

When water collects above the surface in low areas with little or no natural drainage and little or no natural slope to the land, a swamp or a lake is formed.

  When water collects above the surface in low areas with good natural drainage and sloping land, a stream is formed.
     

 


The effect of groundwater infiltration on bodies of surface water is gradual and hard to see.

 

The effect of surface water run-off on bodies of surface water is more noticeable. Immediately after a rain storm, for example, the water level in swamps, lakes and streams may increase rapidly as a result of run-off.

     

 



1.6 Catchment basins and stream networks

Streams are fed by groundwater infiltration and surface water runoff from adjacent areas sloping toward the stream channel. The total land area feeding a stream is called the catchment basin of that stream.

The limit of a catchment basin is defined by the ridges of the highest hills between stream valleys. Rain falling on one side of a ridge will run off into one valley and rain falling on the other side of the ridge will run off into the next valley.

As a stream continues to flow, it may join with other streams from other valleys and form a stream network. Streams grow in size as a stream network develops, and may become very large streams called rivers.

The water available at any given point in a stream network is the total water provided by all catchment basins upstream from that point. When choosing a point to collect water, for example by building a barrage dam, a very short distance up or down the stream network can make a great difference in the amount of water available.

Example

 


1.7 Water requirements for a fish pond

For a fish pond, you will need an initial amount of water to fill the pond. This amount must equal the required pond volume.

   

 

     

When the pond has been filled, there will be some loss of water through infiltration into the soil, through the pond bottom and, banks, and water loss through the drainage system. Water lost this way is called seepage.

 

     

When the pond has been filled there will be some loss of water through evaporation. Water evaporates from the surface of the pond into the air as water vapour. The amount of evaporation will increase with rising air temperatures, with higher winds and with greater air dryness.

The total amount of water needed for fish culture can be determined by adding:

  • The pond volume at the start of the fish-growing period;
  • The seepage losses over the entire growing period;
  • The evaporation losses over the entire growing period.
  



1.8 Water supply for a fish pond

You will need a supply of water sufficient to fill the size of pond you want to build, to fill it in a reasonable length of time, to compensate for seepage and evaporation losses over the length of the fish growing period, and to be able to operate your fish pond continuously throughout the year.

   


The water table as a water supply

It is best to avoid using the natural level of water in the ground to fill your pond. Depending on your location, the water table tends to fluctuate greatly from season to season. This will make it difficult to control the level of water in your pond, to manage the pond and to harvest the fish.

   
     

In the dry season, when the water table is low, it may sink below the pond bottom and you will have no water to fill the pond and to replace seepage and evaporation losses.

 

In the wet season, when the water table is high, you will not be able to drain the water out of the pond, for example, when you are harvesting the fish.

     

 



Other sources of water supply, springs, streams and rivers

It is best to build your pond well above the seasonal fluctuations of the water table (where possible) and to choose a source of water for your pond other than the water table.

If you build your pond above the water table you will have to bring water from a source higher than the pond. Depending on the sources of water available, you may decide to use a spring, a stream or a river.

After you have chosen a source of water you will need to estimate how much water is available from that source at various times throughout the year. You will have to measure the water flow.

 



Measuring the water flow

Using the method in Section 3.1, the quick rough method, approximate the water flow at various times of the year. If you find that the amount of water available appears to be greater than the water requirements of the pond you want to build, no further water measurements are necessary.

If you find that the amount of water available appears to be very near the water requirements of the pond you want to build, it is better to make more accurate measurements. Using a more precise method (see Sections 3.2 to 3.6), measure the water flow again at various times of the year. You must be sure that there is enough water to fill your pond and to operate it continuously throughout the year.


If the variations of water flow from your source are so great throughout the year that it is difficult to determine accurately the water supply available, you may choose to make an annual record.

Using one of the more precise methods, measure the water flow at least once a month for a period of one year. Make the measurements at the same time each month and each time at the same place in the spring, stream or river. List all of your measurements carefully. With this series of measurements you will be able to plan your fish farming operations better.

 



When to build a reservoir

If, after making accurate water flow measurements, you find that at certain times of the year there is too little water available to satisfy your water requirements, but that over the year the total water flow is enough to supply your pond, you may choose to build a reservoir.

With a reservoir you will be able to store water when it is available and to use it as needed during dry periods. For information on storing water in a reservoir, selecting a site for a reservoir and estimating water volume of a reservoir, see Part 4 of this Manual.

  

     
Note: you may find that the existing source will not provide enough water to operate a pond throughout the year. During the season when your water supply does not permit fish culture, you may choose to use the pond bottom to raise a plant crop instead.