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DRWH Technology: Catchments

The catchment determines the quantity and to some extent the quality of water that enters the tank. Most losses are through infiltration, although some water will also bounce off the edge of the surface in heavy downpours and usually some water will be list in wetting the surface. The loss is usually represented by a "run-off coefficient" CR which is a number between zero and one: (1- CR) expresses the loss fraction averaged over a year. A good impermeable roof such as corrugated iron will deliver to the guttering system almost all of the water that lands on it. Ground catchments tend to have a lower runoff coefficient as rainwater infiltrates into the ground and flows away as groundwater.

Runoff quality also varies by catchment type. Ground catchments are prone to contamination from many sources including human and animal faecal matter, rotting vegetation and the soil itself. Higher quality water for drinking must be caught from a surface that is less easily contaminated. This usually comes in the form of the roof of the building but can be an separate structure. GI sheet roofs fare best due to their relative smoothness (Fujioka, 1993) and the sterilising effect of the metal roof heating under the sun (Vasudevan, Tandon, Krishnan, & Thomas, 2001).

Types of catchment

Roofs

Roofs are the most popular catchment for water for domestic purposes. An impermeable roof will yield a high runoff of good quality water that can be used fore all household purposes. Roof types are detailed in Table 1.

Table 1: Characteristics of roof types

Type

Runoff coefficient

Notes

GI Sheets

>0.9

  • Excellent quality water. Surface is smooth and high temperatures help to sterilise bacteria

Tile (glazed)

0.6 - 0.9

  • Good quality water from glazed tiles.
  • Unglazed can harbour mould
  • Contamination can exist in tile joins

Asbestos Sheets

0.8 - 0.9

  • New sheets give good quality water
  • No evidence of carcinogenic effects by ingestion (Campbell, 1993)
  • Slightly porous so reduced runoff coefficient and older roofs harbour moulds and even moss

Organic (Thatch, Cadjan)

0.2

  • Poor quality water (>200 FC/100ml)
  • Little first flush effect
  • High turbidity due to dissolved organic material which does not settle

The poor performance of organic roofs would seem to preclude them from use for rainwater harvesting systems, however organic roofs have been employed with varying levels of success. The water is usually used for secondary purposes but can is sometimes used as drinking water in particularly desperate circumstances (Pacey & Cullis, 1996). Various treatments for thatch roofs have been tried such as polythene sheeting, however the sheeting tends to degrade in the sunlight quickly and can only be used for a single season. A novel solution has been tried in Ethiopia where a foldaway roof was constructed (Hune: personal correspondence). Problems remain, however as it relies on user intervention to fold and unfold the roof at height and when it is raining. A more practical solution may be the use of auxiliary catchments such as those used in the ALDP project in Botswana (Gould, 1993). These can also be simple polythene structures (Ariyabandu, 2001). The problem of UV degradation vs. user intervention remains but the latter is now at a more sensible height.

Ground catchments

A ground catchment has a much lower runoff coefficient than a hard roof (in the region of 0.1 - 0.3), however they are usually much larger and so can yield a high overall runoff. Other ground level catchment surfaces yield a higher runoff with paved surfaces having a coefficient on the area of 0.6 - 0.7 and so courtyard runoff is often collected (Li & Liang, 1995) and stabilised threshing floors are also employed (Pacey & Cullis, 1996). The water from these catchments is not usually of high quality, however and so can only be used for secondary purposes, watering livestock or gardening