Rainfall is the most unpredictable variable in the calculation and hence, to determine the potential rainwater supply for a given catchment, reliable rainfall data are required, preferably for a period of at least 10 years. Also, it would be far better to use rainfall data from the nearest station with comparable conditions.
The number of annual rainy days also influences the need and design for rainwater harvesting. The fewer the annual rainy days or longer the dry period, the more the need for rainwater collection in a region. However, if the dry period was too long, big storage tanks would be needed to store rainwater. Hence in such regions, it is more feasible to use rainwater to recharge groundwater aquifers rather than for storage.
The characteristics of the catchment area determine the storage conditions. All calculations relating to the performance of rainwater catchment systems involve the use of runoff coefficient to account for losses due to spillage, leakage, infiltration, catchment surface wetting and evaporation, which will all, contribute to reducing the amount of runoff. (Runoff coefficient for any catchment is the ratio of the volume of water that runs off a surface to the volume of rainfall that falls on the surface). Water harvesting potential = Rainfall (mm) x Area of catchment x Runoff coefficient Or Water harvesting potential = Rainfall (mm) x Collection efficiency
Runoff is the term applied to the water that flows away from a catchment after falling on its surface in the form of rain. Runoff can be generated from both paved and unpaved catchment areas of buildings. The nature of the catchment determines the quantity of runoff that occurs from the area. For example, about 70 per cent of the rainfall that occurs over the tiled surface of a terrace would flow as runoff while only 10 per cent of the rainfall on a wooded or grassy area would flow, the rest being retained on the surface and getting percolated into the ground. From the point of view of quality
Runoff can be divided into two types: runoff from paved surfaces (e.g., roofs and courtyards) and runoff from unpaved surfaces (e.g., lawns and playgrounds). Quality of runoff from paved surfaces is better since runoff from unpaved surfaces may have bacterial or other contamination. If water is to be stored for drinking purposes, it is advisable that only runoff from paved surfaces is used for the purpose.
Runoff coefficient is the factor which accounts for the fact that all the rainfall falling on a catchment cannot be collected. Some rainfall will be lost from the catchment by evaporation and retention on the surface itself. ( for runoff coefficient). Rainwater yield varies with the size and texture of the catchment area. A smoother, cleaner, and more impervious roofing material contributes to better water quality and greater quantity. While loss is negligible for pitched metal roofs, concrete or asphalt roofs average less than 10 per cent loss, and built up tar and gravel roofs average a maximum of 15 per cent loss. Losses can also occur in the gutters and in storage. Regardless of roofing material, many designers assume loss on annual rainfall up to 25 per cent. These losses are due to several factors: the roofing material texture which slows down the flow; evaporation; and inefficiencies in the collection process.
Water available from annual rainfall & type Roofing with the coefficient runoff as follows.