hydrograms

Hydrograms, in hydrology, are graphical representations of how a hydrological variable such as stream discharge or river stage changes over time at a specific location. They are commonly used to visualize responses of rivers to rainfall, snowmelt, or dam releases and to analyze the timing and magnitude of runoff.

Discharge hydrographs plot discharge (Q) versus time and are often derived from stream gauges. They may show unit hydrographs, which scale a standard response to rainfall of unit depth in a given area, or rainfall–runoff hydrographs that relate rainfall input to resulting discharge.

Constructing a hydrograph involves collecting time series data from gauging stations, rainfall measurements, and sometimes radar rainfall estimates; baseflow separation methods may be used to distinguish groundwater contributions from direct runoff.

Hydrographs are interpreted to determine peak discharge, lag time between rainfall and response, duration of high flows, and the rate of recession. They support flood forecasting, water-resource planning, reservoir operations, and hydraulic design.

Limitations include measurement errors, spatial variability within a catchment, and the influence of antecedent conditions; as such, hydrographs are often complemented by hydrological models.

See also: hydrograph, unit hydrograph, rainfall–runoff model.