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The water cycle and rain information
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Introduction
Only 3% of all water on Earth is fresh water and 68% of the water is frozen ice on the North and South Poles. All humans and most animals as well as nearly all vegetation can only survive on fresh water free of salt and minerals. Water for irrigation must also be fresh water and applied sparingly by flood, furrow or drip irrigation, otherwise the irrigated soil will turn saline and unproductive for many years to come.
The other 97% of the water on planet Earth is saline seawater in which whales, fish, corals and plankton flourish. Slightly salty water may be used for watering livestock and other animals, although fresh water is healthier.
Nearly half of all deep boreholes are dry or contain saline (brackish) water with minerals harmful to humans, animals and irrigation. Where fresh water is pumped up from deep boreholes it is called mining fossil water beacuse the drwan water cannot be replaced by rains.
Therefore, there is only two sustainable solutions to the world's increasing demand for water, namely rainwater harvesting and water conservation.
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| Don't waste rainwater falling on the roof |
| © E. Nissen-Petersen, Kenya |
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| Don't waste rainwater falling on the road |
| © E. Nissen-Petersen, Kenya |
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| Deep gullies in the road |
| © E. Nissen-Petersen, Kenya |
The Water cycle
- Evaporation is an almost invisible vapour rising from water surfaces.
- Transpiration is evaporation from vegetation and soil surfaces.
- Evaporation and transpiration create clouds.
- Precipitation from clouds falls as fogs, mists, rains, hails and snow.
- Rainwater run-off is rainwater running off all kind of surfaces.
- Drainage is the ability to drain excess water away from catchments.
- Infiltration is the movement of water into the soil from the surface.
- Percolation is the movement of water through the soil to the underground layers.
- Permeability is the rate at which water penetrates through soils down into the underground water. table.
- In situ storage is storage of water in the voids between particles of soil and sand.
- Subsurface flow is a flow of water in the voids of soil and sand particles.
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| This illustration of the water cycle shows the way of the water from the ocean, through the atmosphere, brought back to the earth by rain, evaporating again or flowing back to the see, either as surface water or in groundwater flows (Vukasin et al. 1995). |
| © Natural Farming Network 1995 |
Water can be harvested at several stages in the water cycle, such as:
- Fog screens placed on hills, mountains and near the sea for domestic water.
- Gutters attached to roofs for domestic water.
- Garlands of stone gutters on rock outcrops for domestic water.
- Soil bunds and trenches on farmland for crops and animals.
- Diversion trenches from roads, rocks and hillsides for seasonal flood irrigation.
- Hand dug wells in shallow ground water to supply water for all uses.
- Subsurface dams, weirs and sand dams to increase the yield of hand dug wells situated along seasonal water courses.
- Boreholes drilled into deep ground water where none of the above options are replicable, although expensive and up to 40% may not supply fresh water.
The quality of water from all these sources can be tested by either a portable testing kit or by a laboratory if the samples can be delivered before deteriorating due to heat over long distances.
Contaminated water may be treated by several methods, such as SODIS (Solar disinfection), boiling, water filters, crushed seeds from the Moringa tree, ultra-violet rays (UV) or artificial chemicals.
For more information on water treatment click here
Rainfall
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All fresh water comes from rains, including water in deep boreholes which originates from rains infiltrated into the underground thousands or millions of years ago. Fresh water can only be obtained from four main sources; rainwater harvesting, shallow ground water, deep ground water and desalination of which the two latter options are too expensive to be discussed here.
Farmers are often saying: "It has not rained at all for the last 6 months". Such statements can be correct because the South-East monsoon that brings cold and dry winds from the Antarctic to East Africa may be delayed for up to 8 months in a year during which there will be no rainfall.
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The North-East monsoon coming from India brings rains to East Africa from October to December every year. The South-East monsoon brings rains from March to June.
Rain falls where and when the two monsoons meet. The area of convergence is called the Inter-Tropical Convergence Zone. This zone follows the apparent movement of the sun, north and south, bringing rain in its wake. But the pattern is influenced by mountain ranges, Lake Victoria and periodic westerly winds from the Atlantic.
The two monsoons bring an annual average of about 600 mm rainfall to the semi-arid eastern and northern parts of Kenya, while the highland zone at Mount Kenya has a mean average of 1,000 mm and the Lake Victoria zone has a mean average of 1,800 mm. The rainfall pattern of East Africa is also presented as a map with different colours for the various average annual rainfalls under "More images".
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Information Source Links
- The Web of Rain: www.gdrc.org
- Water for Arid Land: www.waterforaridland.com
