Impact assessment of rain water harvesting in an urban environment

Abstract

In recent decades, many countries are facing serious issues of water quantity and quality. Rainfall is the main source of water to the earth that is stored and used in the form of surface and groundwater. Groundwater is the largest reservoir of fresh water and is intensively exploited in developing countries. Natural replenishment of groundwater is a slow process and is often unable to keep pace with the excessive and continued drawl in urban areas. This results in declining groundwater levels, leading to several vexing problems. In order to over-come these serious environmental implications, it is necessary to artificially recharge the depleted groundwater aquifers. In recent times, artificial recharge techniques using rain water known as Rain Water Harvesting are practiced in many tropical countries. The concept of Rain Water Harvesting (RWH) lies in tapping the rain water where it falls. There are a variety of techniques adopted in rural and urban areas throughout the world. Widespread implementation of RWH warrants huge outlay of funds requiring an assessment of their impacts. But, there is a need to develop a comprehensive methodology to assess the impact of RWH on groundwater potential. Consequently, the specific objectives of the study are (i) to evaluate the design of implemented RWH structures: (ii) to prepare recharge response zonation mapping using cross-correlation technique: (iii) to study the impact of RWH on improvement in groundwater potential; and (iv) to model the groundwater dynamics by simulating the effect of RWH and to predict the future scenario. The study is taken up in Chennai, the coastal urban area in India, where major RWH implementation has taken place during the year 2003. The specific area chosen is that of 12 sub-watersheds between rivers Adyar and Cooum. For the purpose of comparing the relative performance of RWH in different areas, three distinct watersheds influenced by urban developments viz. i) Coastal urban watershed (ii) Dense urban watershed and (iii) Less dense sub-urba