Water is a transparent and nearly colourless chemical substance that is the main constituent of Earth's streams, lakes, and oceans, and the fluids of most living organisms. It moves continually through the water cycle of evaporation and transpiration (evapotranspiration), condensation, precipitation, and runoff, usually reaching the sea. The table below shows an approximation of where the water is being held and is only an approximation as the table is changed by drought and flood but also by usage of ground water. This source of water has taken millions of years, in some cases, to reach its present storage cavity and is used as if the source the a permanent one, in fact it is not and takes a long time to replace. 

Functions of Water 

​We have an affinity with water hence the 'sea change'. Recreation is often centered around water.
 
Water is essential for all life of earth.  Plants and animals contain mostly water and all cellular chemistry occurs in water.
 
Water has unique chemical properties of H2O make this possible (hydrogen bonding). Hydrogen sulphide H2S is very similar in molecular weight but has very different properties (boils at negative 60C and is toxic).
 
Water is a universal solvent capable of dissolving almost anything given time.
 
Water has energy storage properties. The calorie is defined as the amount of energy to raise the temperature of 1millilitre of water by 1 degree centigrade. One food Calorie is defined as 1000 real calories. The energy storage capacity of water moderates our climate with warm ocean currents.
 
Water flowing and falling in our landscape has enormous potential energy which has been imparted to it by solar evaporation. We have harnessed this potential energy in hydroelectric systems. This energy can also be a source of great destruction.

Water Distribution

Water on Earth is a finite source. Protecting the water means protecting all forms of the Groundwater can be found in aquifers. water found on Earth. Water at the surface, under the ground, in vapor form, and as precipitation. Pollution from using fossil fuels can impact all forms of water (from crude oil leaks to acid rain generated from coal burning). Acid rain falls onto the land and flows into the surface water, back into the ground, and back into the air. It can be an endless cycle. As contamination infiltrates the water cycle, more water will be impacted. Most of the water on Earth is saline. Fresh water is and will be in demand and become a very valuable resource. Care must be taken to prevent overuse of potable water sources. Care must also be taken to protect the Earth's waters from contamination. Water is indeed a valuable resource.
http://www.ngwa.org/Fundamentals/teachers/Pages/information-on-earth-water.aspx
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Trapping & Storing Water on your Land

The first thing to do is to improve the structure of your soils by making it more penetrable and also its water holding capacity.

How

• Try to let no water escape as run-off.
• Use water as many times as possible.
• Slow down its flow.
• Tackle excess water problems as close as possible to where they start e.g top of a path or your block.

Trapping Water in the Soil

• Trap your water as high as possible on you block.
• Build swales (ditches) to halt waters downward journey and encourages absorption. 

1. This can be site specific i.e. the steeper the site the closer the swales need to be. 
2. The weaker the soil structure the further apart the swales. 

Holding Water in the Soil

Water is held by mulch.
They:-

• reduce water evaporation.
• increase water infiltration.
• minimise erosion.
• regulate soil temperature.
• suppress weeds.
• supple nutrients & organic matter.
• use garden surplus.

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Storing Water 

Tanks

Earth Storages

Vegetation

Water Stored In Soil

• Galvanised, aquaplate, plastic membrane, concrete, fibreglass, poly
• Add calcium to reduce uptake of heavy metals
• Calculating water yield from tables
• Use lower quartile when estimating yield
• As a rule the lower the rainfall the less the reliability

• Ground water
• Dams, (cost about 100 times cheaper than tanks)
• Ponds/wetlands
• Soil (up to 40% by volume) (greater than 10 times cheaper than dams)

 The closer your system mimics nature the cheaper it becomes
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• 75 mm healthy soil can store 25 mm of water
• Ideal Soil Structure
• Effect of compaction