Rainwater Harvesting: How much rain falls on the earth each year?

“Rainfall in one year is no less (in amount) than that falling in another year.”

After performing extensive studies to estimate the amount of water present on the surface of the earth, it was found to be enormous, as it was estimated to be 1360 million cubic kilometers. Most of this water (97.2%) is in the form of salty water in seas and oceans , while the rest of it (2.8%) forms the fresh water in its three forms (solid , liquid and gas) . 2.15% of the total amount of water on earth is in the form of glacier deposits at the two poles , and on top of their mountains . The rest of the earth’s water, which is about 0.65% of the total amount of water on earth, is mostly stored as groundwater in the lithosphere. Next to it, comes the water of fresh water lakes, then the water stored in the form of humidity in the soil of the earth. The remaining amount is water vapor in the gaseous atmosphere, and lastly the water running in rivers and their headstreams.

The world ocean covers approximately 71 percent of the earth’s surface, estimated to be about 510 million cubic kilometers, which means that the world ocean on the earth is about 361 million cubic kilometers, while the solid area of the earth’s surface is estimated to be 149 million cubic kilometers.

Therefore, the rate of evaporation from surface water is estimated to be around 434,000 cubic kilometers of water per year, while the rate of water evaporating from the soil is estimated to be almost 50,000 cubic kilometers and transpiration from plants 21,000 km3/year. After adding these figures together, we realize that the water cycle between the earth and its atmospheric layer is about 505,000 cubic kilometers per year. Most of this amount of water evaporates from tropical areas, where the average temperature per year reaches 25oC.

When the water evaporates from surface water and soil, it rises up due to its low density, and then air currents drive it to the troposphere; the lowest layer of the earth’s gaseous atmosphere and location of all the earth’s weather. The troposphere is characterized by a marked decrease in temperature as the altitude increases until it reaches -80oC over the equator. In this cold layer the water vapor , rising from the earth , condenses (with the Will of Allah) and then comes back to the earth in the form of rain , water , snow , hail , or drizzle (whether in the form of dew or fog) .

When this water falls back to the earth, it is distributed by the creator , with Perfect Wisdom , as the amount of water precipitated into wet land surface is higher than that evaporating from it (96,000 to 107,000 km3/year precipitate into the earth , compared to 70.000 cubic kilometers which is the total amount of water evaporating from its surface) . On the other hand, the amount of water precipitating into seas and oceans is much less than that evaporating from their surfaces (398,000 km3/year precipitate into them, in comparison to 434,000 km3/year evaporating from them). The difference between the last two figures is exactly the same difference between the amount of rain and the evaporated water on earth, which is almost 36.000 cubic kilometers of water that flows from the solid land to the seas and oceans every year.

Flows (between reservoirs)

Process From/to Reservoir Flow Rate
Precipitation Atmosphere to Ocean/Land 505,000 km3/year
Ocean precipitation Atmosphere to Ocean 398,000 km3/year
Land precipitation (except snow?) Atmosphere to Land/surface 96,000 to 107,000 km3/year
Evapotranspiration Ocean and Land/surface and Plants to Atmosphere 505,000 km3/year
Ocean evaporation Ocean to Atmosphere 434,000 km3/year
Land evaporation Land/surface to Atmosphere 50,000 km3/year
Transpiration Plants to Atmosphere 21,000 km3/year
Uptake by plants Land/surface to Biota 21,000 km3/year
Runoff Land/surface to Ocean 36,000 km3/year
Melting Ice/snow to Land/surface 11,000 km3/year
Snowfall (on land only?) Atmosphere to Ice/Snow 11,000 km3/year
Percolation Underground to and from (??) Land/surface 100 km3/year

The water cycle throughout the earth is a miraculous one, which acts as a testimony to the Absolute Ability of the Creator, and the perfection of His Creation. The amount of water is all in all constant and is accurately calculated to fulfill the needs of life on earth. This hydrologic cycle, which alternates between evaporation and rainfall, also purifies the water of the earth, in which billions of individuals, representing different forms of life on earth, live and die. It also helps keep the temperature balance on the surface of the earth, and reduces the intensity of the heat of the sun in summer. This in turn, reduces the difference between the temperature in summer and that in winter, in order to preserve the life on earth in all its different forms.

Since the total amount of the earth’s water evaporating into its gaseous atmosphere, is fixed every year, and the total amount of water vapor contained in this layer is also constant throughout the year, then the total amount of rain precipitating into the earth also remains constant every year, even though its amount varies from one area to the other, depending on the Will of Allah. The average amount of rain precipitating into the earth’s surface today amounts to 85.7 cubic cm. per year. The amount of rain varies between zero in the dry and arid desert areas, and 11.45 cubic meters of water precipitating into the Hawaiian Islands per year.

“Rainfall in one year is no less (in amount) than that falling in another year.”

With this much water falling on the earth each year I have to ask, why are we paying for our water needs? We already have all the water we could ever need. Yes we need pure water to drink, but if we had the right filtration system(which is not very expensive www.frescohealth.com) there would be no more purchasing of drinking water. But what about the water we are using in other areas? Toilet water, bathing water and laundry water can come directly from nature without having to continuously pay for it. Just for a moment, imagine not having a water bill…..sounds nice doesn’t it. The money saved that was once invested in water use could be invested elsewhere.

Only one percent of the water we use every day is used for personal consumption. The rest is used to clean, flush toilets, shower, water the lawn, irrigate gardens, wash cars, fill  swimming pools, do laundry and complete other household chores. For this reason, rainwater harvesting is practical and offers significant ecological and economic advantages!

We estimate more than half of the water we use each day can be replaced with  rainwater, without any inconvenience. This amount includes flushing toilets, washing  clothes, cleaning, watering the garden and irrigation. This can potentially save  60,000 gallons of water on an annual basis for a family of four using 300 gallons of  water per day!

The history of rainwater harvesting in Asia can be traced back to about the 9th or 10th Century and the small-scale collection of rainwater from roofs and simple brush dam constructions in the rural areas of South and South-east Asia. Rainwater collection from the eaves of roofs or via simple gutters into traditional jars and pots has been traced back almost 2 000 years in Thailand (Prempridi and Chatuthasry, 1982). Rainwater harvesting has long been used in the Loess Plateau regions of China. More recently, however, about 40 000 well storage tanks, in a variety of different forms, were constructed between 1970 and 1974 using a technology which stores rainwater and stormwater runoff in ponds of various sizes. A thin layer of red clay is generally laid on the bottom of the ponds to minimize seepage losses. Trees, planted at the edges of the ponds, help to minimize evaporative losses from the ponds (UNEP, 1982).

The source of figures in this material is Windows to the Universe, at http://www.windows.ucar.edu/ at the University Corporation for Atmospheric Research (UCAR).

Sunnah & Science

By: Dr. Zaghloul El-Naggar

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