Catchment types
Roof catchments: The rooftop is usually the most common catchment surface and can be flat or sloping.
- Smooth, hard and dense roofs are preferred since they are easier to clean and are less likely to be damaged and release materials/fibres into the water.
- The catchment surface should slope slightly towards the downtake pipes so that water does not stagnate on the roof.
- A catchment that slopes towards a single direction, preferably in the direction of the sump / borewell will reduce piping costs
Ground level catchments: If the storage tank is below the ground level, paved flooring surfaces and open grounds can also serve as catchments.
Figure 3.1: Roof catchments
Runoff coefficient
Rainwater yield and quality varies with the size and texture of the catchment area, climatic conditions, cleanliness of the catchment and the surrounding environment. A smoother, cleaner, and more impervious catchment contributes to better quality and greater rainwater collection. Regardless of catchment material, there will be certain losses due to absorption, evaporation and inefficiencies in the collection process. Run-off coefficient is a factor that accounts for these losses and indicates the fraction of the rainwater that actually leaves the catchment to enter the collection pipes. For example, a runoff coefficient of 1 indicates that there are no losses and all the rainfall that falls on the catchment reaches the downtake pipes and a coefficient of 0.8 indicates that only 80% of the incident rainfall reaches the downtake pipes. The runoff coefficients for different catchment materials are given in the table below
| Type of Catchment | Coefficients |
|---|---|
| Roof catchments | |
| Tiles | 0.8-0.9 |
| Corrugated metal sheets | 0.7-0.9 |
| Ground surface coverings | |
| Concrete | 0.6-0.8 |
| Brick pavement | 0.5-0.6 |
| Untreated ground catchments | |
| Soil on slopes less than 10 % | 0.1-0.3 |
| Rocky natural catchments |
0.2-0.5 |
Source: Rainwater Harvesting Trainers’ Manual published by Karnataka Urban Infrastructure Development and Finance Corporation