A hydrograph illustrates a type of activity of water during a specific time frame. Salinity and acidity are sometimes measured, but the most common types are stage and discharge hydrographs. These graphs show how surface water flow responds to fluxes in precipitation.
“Stage hydrographs” show fluctuations in the height of water in reference to a certain point, while “discharge hydrographs” depict the volume of the discharge itself. Discharge hydrographs are the most commonly known. They demonstrate the rate of flow – usually measured in cubic feet per second – during a period of time.
Steepness of a peak indicates how quickly water is transported through a stream, which may be impacted by physical barriers or limitations. Hydrographs with sharp jumps are sometimes referred to as “flashy.”
Types of (Discharge) Hydrographs
Natural Hydrograph: Recorded at the stream gaging site, directly depicting the stream’s discharge.
Synthetic Hydrograph: Simulated from known strength of precipitation and watershed characteristics that would impact water’s flow, including soils, vegetation and slope.
Unit Hydrograph: Made from either natural or synthetic hydrographs, showing results from exactly one unit (inch, cm, etc.) of constant rainfall. To predict results from more precipitation, the volume of discharge in this unit hydrograph (the area under the curve) is simply multiplied by the number of units of interest.
Dimensionless Unit Hydrograph: Made from several natural hydrographs, depicting the ratio of the actual discharge rate to the peak discharge rate.