This “Learning Package on Hydrology” by the National Institute of Hydrology deals with the basic scientific concepts underlying hydrology. In a general sense the package deals with elements of the hydrologic cycle and explains the processes of rainfall, runoff and evapotranspiration and their interaction.  

The topics covered include precipitation, abstraction from precipitation, streamflow measurement, hydrographs, flood routing, flood control, groundwater, runoff and floods. The package deals with the occurrence, circulation and distribution of water of the earth and earth's atmosphere.

Various phases of the hydrological cycle, such as rainfall, runoff, evaporation and transpiration are all non-uniformly distributed both in time and space. Further, practically all hydrologic phenomena are complex and at the present level of knowledge, they can at best be interpreted with the aid of probability concepts.

Hydrological events are treated as random processes and the historical data relating to the event are analysed by statistical methods to obtain information on probabilities of occurrence of various events. The probability analysis of hydrologic data is an important component of present-day hydrological studies and enables the engineer to take suitable design decisions consistent with economic and other criteria to be taken in a given project.

The package deals with the broad subject of inter-disciplinary nature drawing support from allied sciences, such as meteorology, geology, statistics, chemistry, physics and fluid mechanics. The quantities of water going through various individual paths of the hydrological cycle are described by the continuity equation known as water-budget equation or hydrologic equation.

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