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Rainwater collection is often a good option in areas with regular rainfall and where traditional water sources (i.e. groundwater, surface water) are not available or contaminated. The following ‘how to’ guide comes from my experience implementing Rain Water Harvesting Systems (RWHS) in the Solomon Islands, where many coastal communities don’t have others options for water supply.

The components of Rain Water Harvesting Systems (RWHS)

All RWHS are made of a four elements: a catchment area, a collection system, a storage reservoir and a water collection point.

Your catchment area would normally be a roof. Impermeable ground-level surfaces can also be used to catch rainwater. However, water caught on ground-level surfaces is usually of lower quality than raised surfaces, due to factors including dirty soil, wild defecation, agricultural chemicals and solid and liquid waste. So I’ll focus this guide on raised surfaces, such as roofs.

As you can guess, the bigger your roof, the better your catchment. You want to ensure you have the maximum runoff, by reducing the evaporation and infiltration through the roofing material. The runoff coefficient of a corrugated iron sheet is around 0.8 while for thatch it is 0.2. In Solomon Island, we used public buildings (i.e. schools and churches) for rainwater catchment and few private houses when we didn’t have any other option, making sure that it will be a shared RWHS. We even rehabilitated some of the roofs, using locally available corrugated sheets.

The collection system is made of gutters, placed on one or two sides of the roof, depending on its design. Always keep in mind that you want to get a maximum supply. In Solomon Islands, for gutters we used PVC spouting 150mm width and for downpipes PVC Ø100mm; which are the most common, the easiest to use and allowing a good water collection (limited losses of water and reduced frictions). Then, before the inlet of your reservoir, you need to install a 1st flush pipe – which is simply a vertical closed PVC pipe. The 1st rain will clean your roof and fill this flush pipe with dirty water rather than your reservoir. Quite useful.

The storage reservoir. It could be constructed – using local knowledge and skills, or bought fully equipped, installed outside, inside or even put underground. The local conditions, habits and expectations would guide you. Involve the concerned people during the assessment and design phases. But the reservoir capacity needs to be properly estimated, in order to store enough water for the dry season.

You can place the water collection point wherever you want, directly on the reservoir or further down, or even inside the house. The storage reservoir could also be put underground, with a pump would be needed to lift the water to where it is needed.

Design of the RWHS

Firstly, check if there is enough water to satisfy the demand, comparing the maximum water supply and the maximum demand of water. Then, calculate the required volume of the water storage, to make sure that there will be enough water in the dry season.

Maximum water supply:

Volume of water available [L] = Plan area of roof [m2] x Runoff coefficient [ratio] x Annual rainfall depth [mm]

Maximum water demand:

Volume of water needed [L] = Daily water need [L/day/person] x Nb of person to supply x Nb of days of supply desired

Balance:

If Maximum water supply > Maximum water demand, the RWHS is feasible. If not, you need to change some of the parameters (i.e. roofing material, roofing surface, number of people served, number of days of supply) or to find another source of water.

Calculation of the volume of storage needed for the dry season:

A first estimation would give you an idea of the required size (and the required budget):

Minimum storage volume required [L] = Daily water demand [L] x Nb of days in the longest dry season

Then, you have to do detailed calculations, using monthly rainfall data. For this purpose, draw a table with 7 columns:

  1. Months
  2. Column A = Number of days in month
  3. Column B = Monthly demand [L] = Daily demand x A
  4. Column C = Monthly rainfall [mm]
  5. Column D = Monthly supply = Rainfall captured [L] = Catchment area [m2] x Runoff coefficient x C
  6. Column E = Stored in this month [L] = D - B
  7. Column F = Cumulative amount in storage [L] starting at the end of the dry season For example if the last month of the dry season is August, you'll start to fill the reservoir - and you'll start your calculation in September.

Then, the minimum storage of your reservoir should be the highest cumulative amount in storage, plus an additional volume allowing some mistakes, but also the volume of the first flush, the volume of water below the outlet and the air gap above the overflow.

Maintaining rainwater quality Rainwater is normally considered to be high quality with very low turbidity and zero faecal contamination. However, it can become contaminated after touching any surface, such as:

  • The catchment area (i.e. dirty roofing);
  • The collection system (i.e. dirty gutter);
  • Or inside the storage tank.

So, keep in mind that a regular maintenance of the whole RWHS is crucial to keep water quality safe. The users need to be trained on it. Also, in some highly developed and industrialised urban areas, rainwater might contaminated by atmospheric pollutions. You should plan to check the rainwater quality by bringing samples to a governmental or private lab.

This is it. If you have any questions, do not hesitate to ask.

You can find interesting literature on the subject on line: https://www.ircwash.org/sites/default/files/Thomas-2007-Roofwater.pdf http://espanol.projectwet.org/system/files/materials/rw_harvesting_english.pdf