Drought mitigation using rainwater harvesting in the dry zone of Sri Lanka

Abstract

The government of Sri Lanka proposed to provide safe drinking water to all by 2025. However, this proposal is becoming a challenging task due to depletion and pollution of water resources combined with a growing demand and conflict among users. On the other hand, pipe borne water to all households is becoming costly due to the high level of investment required and increased operation & maintenance cost. Due to this situation, a large percentage of the both urban and rural population would be deprived of any form of safe drinking water. Therefore it becomes nationally important to look for low cost options which could be managed and afforded by the communities themselves. A rainwater based water supply system is an alternative option to solve this problem./ The objective of this research work is to investigate the more suitable storage volume required for rooftop rainwater harvesting system and determine the probability of success of practicing rainwater harvesting for domestic use in the dry zone of Sri Lanka./ The analysis uses thirty years of daily rainfall data (1968-1998) from Puttalam, Hambantota and Anuradhapura districts in the dry zone of Sri Lanka. The results show that the probability of success of a rainwater harvesting system shows a bimodal pattern during the year./ It has been found that the more suitable tank capacity required is 5 m3 corresponding to a roof area between 75m2 to 100 m2. Such a system can provide 120 liters per capita per day to a household of five persons and give an overall probability of success of approximately 30%. At 300 liters per day per household (ie. 50% of demand), the probability of success improves significantly to 50%.Thus rainwater harvesting can be a valuable tool of drought mitigation within the dry zone. The probability of success of a system varies throughout the year due to the seasonal variation of rainfall. In particular, during the period February to March and June to September a rainwater harvesting system alone will not be able to meet the required demand. The percentage saving of monthly water bills for urban schemes can vary from 25% - 33% considering the subsidy given by NWS & DB, and for consumers in rural schemes the amounts are 25% - 35%. These rainwater harvesting systems utilize 5 m3- 7.5 m3 tank capacities with 75 m2 - 100 m2 roof areas.