Water Supply System Planning by Artificial Groundwater Recharge from Rooftop Rainwater Harvesting

Abstract

Rainwater harvesting could be an effective means of supplementing the growing demand of potable water in urban areas in India, especially in high rainfall areas such as the north-eastern part of the country. Although rainwater harvesting has been practiced in India since a long time, but it has never been seriously thought of as a component for integrated management of water resources for urban water supply. Among the various rainwater harvesting techniques, artificial recharge of groundwater from rooftop rainwater harvesting is the most effective means of collecting a sizeable quantity of the rainwater for future use. As per the estimation of the Central Ground Water Board (2000), about 0.008 m3 of rainwater can be obtained from the rooftops of buildings per square metre area for 1 cm of rainfall. A mathematical model for simulation of the three-dimensional transient groundwater flow process is developed for a confined aquifer using finite difference method. The model results are validated by comparing with the results obtained from MODFLOW simulation. This model is then applied in a hypothetical urban water supply system to provide drinking water to a population of about 75,000 considering pumping from a set of discharge wells along with a set of recharge wells for rainwater harvesting. The recharge to the aquifer takes place from eight injection wells within the study area; each collecting rainwater from the roof tops of a network of buildings located within 200 m radius. The recharge to the aquifer is considered to be lumped for each well in quarterly time steps. The recharge wells are also used for pumping water to the supply system. The operation policy of the wells are designed in such a way that the recharge wells shall be used for pumping water to the supply system only for 6 months during the period from October to March. This will permit the recharge water from monsoon storm to settle down in the aquifer and will lead to substantial water quality improvement. However, the discharge wells shall be operated throughout the year. An optimization problem is formulated with the objective of minimizing the squared difference of supply and demand. This optimization model is externally linked to the groundwater flow simulation model to observe the tradeoff between withdrawal from aquifer and recharge from rainwater. The constraints to the optimization model are related to groundwater depletion and temporal and spatial operation of the two sets of wells. The optimization problem turns out to be nonlinear due to incorporation of the simulation constraints. A simple genetic algorithm is used to solve the problem and the results are obtained in the form of head distribution and drawdown after every successive time steps.