Rainwater Calculator

Mathematical Principles of the Rainwater Calculator

The mathematical principles of a rainwater calculator are really quite simple. How much rainwater will fall? How long will the rainwater keep falling? Are there any times during the year when no water falls at all? When? For how long? How big and in what condition is the cistern or vault being used? How big is the area for catchment? How much rain can be caught in what period of time? Rainwater calculators are a simple mathematical script that can be easily done by any computer or calculator.

From the ancient times when architects projected megalithic waterways known as Aqueducts, to this very day, the sky has given mysteries to urban planners. The Greek inventor Archimedes knew of the ways in which the abstract world of numbers could be used to manipulate the mechanical nature of the world, and used this to his advantage. A legend is told of the great Athenian inventor Dedalus who invented among many things, a dry treasury under a massive lake, and that whole bodies of water had to be extracted and replaced through great calculation.

Rainwater comes falling down at its own pace. Yet it will fall none the less, it will. What a good sustainable project needs to keep in mind is from when to when does it rain? Is there a dry season? If so how long is it, and how much water is consumed throughout any given month. Knowing the total rainfall in a given period of observation is done by leaving a cylinder out in the rain for the duration of the observation, and recorded every now and a again. But to be wise a person needs to study all the rainwater fall charts and graphs for the last fifty years to make an estimate on how much rain will fall on the average in any given month, and base their abstract numerical predictions on an almost worse case scenario.

The total area of the catchment system will determine how much rain can be caught, during any given rainfall. Catchment systems are the basic area upon which the collection number is based. By multiplying length and width of this two dimensional space (surface of the catchment system), a final area for the rainfall is given a value. This section of space for the water to land on is not perfect, since it’s not a bucket or rain-barrel. It’s usually a flat surface or curved with an inclination to channel the falling water. Obviously some water likes to pick up speed and go flying right off the catchment surface to the ground. This is called efficiency and has its own value in percentage as well. If efficiency is 100%, then calculations run fine, if not, then the end calculation needs to be multiplied by the final efficiency value to determine rainwater calculator loss.

The cistern size will determine how much rainwater can be stored between any given drought or low rainfall season. Rainwater calculators don’t do all the work themselves; the architect must have at least a small notion of his or her objectives. If it only rains for three months in the desert and then there are nine whole months of dry arid conditions, then a precise calculation of how much water is used per month needs to be well known. As the water collected in rainy seasons needs to be more than plenty to sustain its average monthly usage for the later dry months, take into account evaporation and seepage. The last two factors; evaporation and seepage, are directly related to the type of cistern being used. Underground cisterns tend to have a far more controlled evaporation, and if properly built seepage should not be a problem either (contaminants are probably the worse problems a cistern can face, but have little to do with calculations).

A = (catchment area of building)

R = (inches of rain)

G = (total amount of collected rainwater)


As easy to calculate as that is, mathematical principles of rainwater calculators are based around personalized specifications, previous studies about average rainfalls in the area and calculated prediction. How much rain is falling per month, how much is used or wasted in the process? How much can be stored, how much is too much, how much is too little? Are there ever dry months, when, how long? Are there wet months to compensate for those dry months, and if so, how does that effect the potential size of the catchment system, or the cistern for that matter? What kinds of systems are being used? Are they likely to be more or less efficient? When Archimedes was sitting in his bath, passing these subjective philosophical mathematical equations over and over through his meditating mind, he was on the verge of planting the seed for future rainwater calculators 2,500 years later. Now it is we who must meditate and understand rainwater calculators. Meditate to build a better and far more sustainable reality than the one imposed by the Romans.


Posted in Harvesting by Administrator on September 8, 2005.

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