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Styles of Roofwater harvesting

User regimes

Rainwater harvesting is used in many different ways. In some parts of the world it is used merely to capture enough water during a storm to save a trip or two to the main water source. In this case,only small storage capacity is required, maybe just a few small pots to store enough water for a day or half a day. At the other end of the spectrum we see, in arid areas of the world, systems which have sufficient collection surface area and storage capacity to provide enough water to meet the full needs of the user. Between these two extremes exists a wide variety of different user patterns or regimes. There are many variables that determine these patterns of usage for RWH. Some of these are listed below.

Rainfall quantity (mm/year)

the total amount of water available to the consumer is a product of the total available rainfall and the collection surface area. There is usually a loss coefficient included to allow for evaporation and other losses. Mean annual rainfall data will tell us how much rain falls in an average year.

Rainfall pattern

climatic conditions vary widely throughout the world. The type of rainfall pattern, as well as the total rainfall, which prevails will often determine the feasibility of a RWHS. A climate where rain falls regularly throughout the year will mean that the storage requirement is low and hence the system cost will be correspondingly low and vice versa. More detailed rainfall data is required to ascertain the rainfall pattern. The more detailed the data available, the more accurately the system parameters can be defined.

Collection surface area (m2)

this, where rooftop catchment systems are used, is restricted by the size of the roof of the dwelling. Sometimes other surfaces are used to supplement the rooftop catchment area.

Storage capacity (m3)

the storage tank is usually the most expensive component of the RWHS and so a careful analysis of storage requirement against cost has to be carried out.

Daily consumption rate (litres/capita /day or lpcd)

this varies enormously – from 10 – 15 lpcd a day in some parts of Africa to several hundred lpcd in some industrialised countries. This will have obvious impacts on system specification.

Number of users

again this will greatly influence the requirements.

Cost

a major factor in any scheme.

Alternative water sources

where alternative water sources are available, this can make a significant difference to the usage pattern. If there is a groundwater source within walking distance of the dwelling (say within a kilometre or so), then a RWHS that can provide a reliable supply of water at the homestead for the majority of the year, will have a significant impact to lifestyle of the user. Agreed, the user will still have to cart water for the remainder of the year, but for the months when water is available at the dwelling there is a great saving in time and energy. Another possible scenario is where rainwater is stored and used only for drinking and cooking, the higher quality water demands, and a poorer quality water source, which may be near the dwelling, is used for other activities.

Water management strategy

whatever the conditions, a careful water management strategy is always a prudent measure. In situations where there is a strong reliance on stored rainwater, there is a need to control or manage the amount of water being used so that it does not dry up before expected.

Classification

Ideally, we would like to be able to classify the various common user regimes that are adopted. This can help us to develop a nomenclature for dealing with the systems we will look at later. We can simply classify most systems by the amount of ‘water security’ or ‘reliability’ afforded by the system. There are four types of user regimes listed below.

Occasional

water is collected occasionally with a small storage capacity, which allows the user to store enough water for a maximum of, say, one or two days. During the wet season this means that the user will benefit considerably from having such a system and most, if not all, of the user needs will be met during this time. After a day or two of dry weather the user will have to return to using an alternative water source. This type of system is ideally suited to a climate where there is a uniform, or bimodal, rainfall pattern with very few dry days during the year and where an alternative water sources is close at hand.

Intermittent

this type of pattern is one where the requirements of the user are met for a part of the year. A typical scenario is where there is a single long rainy season and, during this time, most or all of the user needs are met. During the dry season an alternative water source has to be used or, as we see in the Sri Lankan case, water is carted/ bowsered in from a nearby river and stored in the RWH tank. Usually, a small or medium size storage vessel is required to bridge the days when there is no rain.

Partial

this type of pattern provides for partial coverage of the water requirements of the user, during the whole of the year. An example of this type of system would be where a family gather rainwater to meet only the high-quality needs, such as drinking or cooking, while other needs, such as bathing and clothes washing, are met by a water source with a lower quality. This could be achieved either in an area with a uniform rainfall pattern and with a small to medium storage capacity or in an area with a single (or two short) wet season(s) and a larger storage capacity to cover the needs during the dry season.

Full

with this type of system the total water demand of the user is met for the whole of the year by rainwater only. This is sometimes the only option available in areas where other sources are unavailable. Sufficient a/ rainfall, b/ collection area, c/ storage capacity is required to meet the needs of the user and a careful feasibility study must be carried out before hand to ensure that conditions are suitable. In areas where there is a bimodal rainfall pattern (i.e. two rainy seasons) this type of system is far more attractive, as the tank will be recharged during both wet seasons. Where there is a single (unimodal) wet season the storage capacity will normally be very large – and therefore expensive. A strict water management strategy is required when such a system is used to ensure that the water is used carefully and will last until the following wet season.