Hydrology deals with all forms of water on earth, its occurrence, distribution, circulation, physical properties, chemical properties, its effect on the environment and on life in all forms. It is an interdisciplinary science. The study of the occurrence, distribution, and movement of water below the surface is known as groundwater hydrology.

Approximately 97% of the total water occurs as salt water in the oceans and seas. Only 3% is fresh water, of which more than half is trapped in ice caps and glaciers. The rest is groundwater.

Rock formations that contain water are known as aquifers. Clay and shale are known as aquicludes, which contain sufficient amounts of water but only a small part of it can be transmitted.

Unsaturated limestone and unfractured granite or basalt fall into the category of aquifers. They are waterproof and therefore do not contain water or transmit it. The aquitard is a semi-permeable formation and can transmit water at a very slow rate. Clay lenses interspersed with sand are a case in point.

The concept of hydrology was first correctly postulated by Sir Vitruvias.

The interdependence and continuous movement of all phases of water, liquid, solid and gas, form the basis of the hydrological cycle. It is the central concept of hydrology.

Condensation, precipitation, interception, evaporation, etc. are the various processes involved in the cyclical movement of water.

Inflow = outflow plus or minus variations in storage. This is known as the water balance equation.

The porosity of saturated materials determines the amount of groundwater stored. The ability to transmit fluids is known as permeability.

The porosity of rock formations is a measure of voids. It is calculated using an equation, v/V, where ‘v’ is the volume of voids and ‘V’ is the total volume of the rock.

According to Sir Darcy, the velocity of flow in any porous medium is proportional to the hydraulic gradient.

Aquifers serve as reservoirs for groundwater storage and pipelines for groundwater movement.

Water plays a predominant role in the transport of chemicals. It has certain unique characteristics. Pure water is colorless, tasteless and odorless. The unique nature is due to its molecular structure. Its unique nature facilitates the solubility of many materials in it.

There is an urgent need to monitor and control the increasing human impact on water chemistry through various forms of pollution. The release of industrial waste into water is a major cause for concern.