Supply of water in any region can only be ensured if there is sufficient available water to meet the demands as per the requirement. The water availability of a region depends primarily on its climate and then on topography and geology. The climate factor connected to the rainfall and its variability of occurrence in space and time, the humidity, the temperature and wind, all of which affects the rate of evaporation and evapotranspiration. The topography is important since it controls the way rainfall is discharged both in terms of quantity as well as rate of development of lakes, marshlands and provision of surface water to infiltrate down to the aquifers. Geology affects the topography and controls the availability of suitable underlying rocks that forms aquifers to which water can infilters and be available for exploration and supply.
The most alarming and exponential increase in water scarcity cannot be linked exclusively to abnormalities in rainfall because there is no long term change in rainfall though there has been some annual variation in rainfall. Further water scarcity is not only restricted to the arid regions with scanty rainfall. High rainfall areas like Meghalaya and most of the north-east regions, Goa, Kerala, Jharkhand etc are experiencing acute water shortage. The common correlation between failure or variation in rainfall and scarcity is unrealistic and inappropriate. From the hydrogeological point of view, the hard rocks are those lithological units which lack primary porosity. All the igneous and metamorphic rocks namely granites, gneisses, schist, khondalites, dolerites, gabbro, basalt etc. can be grouped in this category.