1st International Conference on Rain Water Cistern Systems
Honolulu, Hawaii, USA - June 1982

Section 3: Design, Cost, And Policy

Page 151

Design And Calculation Of Rainwater Collection Systems

C.L.P.M. Pompe
West Java Rural Water Supply, Indonesia

Introduction

Dry seasons occur even in monsoon areas. If there are no nearby wells or rivers, people will walk long distances to obtain water. A solution to this problem may be the catchment of rain water-an ancient custom still practiced in many countries in the world where rainfall is collected during the wet sea-. son to use in the dry season. The questions are (1) what should the volume be of such a rainwater collection system and (2) how should such a collector be designed?

This paper deals mainly with the first question: the calculation of the volume of the rainwater collector.

H.M.C. Satijn (1979), who was a former project participant, analyzed the problem and designed a computer simulation model of the rainwater collection system. Because computers are not widely available, I was asked to review Satijn's work and to develop a slide rule-calculation method that could be used by regional technicians. Thus, a step-by-step method was designed to enable technicians to calculate the volume of rainwater collectors by using available monthly rainfall data.

The results of the calculation are not sacrosanct because, first of all, the variables of themselves vary widely, such as consumption or the water demand. Thus, if 5 litres/person/day is the basis for calculating drinking water needs, there is no way of knowing the actual use and the variation in conservation and utilization. What must be borne in mind in the statistical calculation of rainwater catchment systems is that its accuracy will depend on correct input. It is also important to be realistic in using the statistical calculation method, rather than concentrating on complicated statistical computations.

The vital question is "Who pays?" If the farmer (user) pays, he will construct a 15-m3 collector and in the dry season will be conservative in using his water while praying to Allah that rain will soon fall again. When the government or an international organization pays, the designer might think of future users and design a 20- or even 25-m3 collector.

The basic principle of this: method is the importance of calculating storage to provide enough water of the period of the year when there is no rainfall or when it is insufficient to meet water needs. In the following sections the step-by-step method and the theoretical background of the method are presented, and the last section includes some design criteria for the rainwater collection system.

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