Hydrograph is the plot of the stream flow at a particular location as a function of time. Although the flow comprises of the contributions from overland flow, interflow and groundwater flow, it is useful to separate only the groundwater flow (the base flow) for hydrograph analysis.
The hyetograph is the graphical plot of the rainfall plotted against time. Traditionally, the hyetograph is plotted upside down as shown in Figure below, which also shows a typical hydrograph and its components. Splitting up of a complete stream flow hydrograph into its components requires the knowledge of the geology of the area and of the factors like surface slope, etc. Nevertheless, some of the simpler methods to separate base flow are described subsequently.
Hydrology of rivers is required by engineers for estimation of water, design of dams, diversions, and flood control through reservoirs or dykes, etc. Information is gathered through a network of stream gauges. Hydrograph analysis deals with the study of runoff records at a stream gauge. Hydrograph analysis is often combined with rainfall analysis to investigate how a watershed responds to rainfall. In many cases, hydrometric information is not available. This is especially true for small watersheds. In such situations, rainfall information must be combined with rainfall-runoff models
