In rooftop rainwater harvesting, the runoff coefficient relates to the factors such as an evaporation and infiltration but in real case sometimes the rain is falling all the day but it doesn’t give a flush from the roof to drain water to the storage. Besides the runoff coefficient some rainfall must be neglected BECAUSE The RWHS considers the total amount of rainfall.
What is your purpose with a more accurate number?
Depending on the roof you should assume a minimum amount of rain intensity per hour and total rain volume that allows you to collect water. Below that the water collection can be estimated zero to be on the safe side.
Considering the large uncertainty in the hourly, daily, seasonal and yearly precipitation, same in temperature and wind (those influences the evaporation), different roofing materials, shape and exposure of roofs, local climates, this runoff factor does not look significant.
Of course, it depends on your goal, climate, value of water in the study area.
Since the natural variation in the precipitation is larger than 20-50% in different geographical locations, a 0.1-3% variation factor does not look important, just deduct the most realistic number from the yearly precipitation.
Dear Mr. Robert thank you for the nice answer, My calculations is based on the average daily rainfall data, taken from the 6 different rain gauge stations so the average data gives me a minimum intensity mostly non-zero e.g 0.1, 0.002 mm/day etc.
In real case hardly we have few days rainfall and sometimes their is rain but no flush from the roof so how to tackle this issue?
The rainfall amount you have mentioned amounts to be almost nil. However when ever there is slightly a higher intensity of rainfall, if the runoff has to take place, I feel that only solution is to increase the slope angle of the roof.
I Agree with Mr. N.B.Narasimha Prasad . If your roof is maintain some slope the water will drain off very faster rate.
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