hydrographs

A hydrograph is a graphical representation of how the flow of a river or stream changes over time. It is typically plotted with discharge (often in cubic meters per second) or water stage on the vertical axis and time on the horizontal axis. Hydrographs are used to describe the hydrologic response of a watershed to precipitation, snowmelt, dam releases, or other hydrological events.

For a rainfall event, a hydrograph usually shows a rising limb, a peak discharge, and a falling limb. The response is influenced by factors such as catchment size, land cover, soil moisture, and drainage networks. Important features include baseflow (the background contribution from groundwater), lag time (the delay between rainfall and peak discharge), time to peak, and the magnitude and duration of the peak. Hydrographs can be recorded for individual events or produced as continuous records over long periods.

Data for hydrographs come from gauging stations that measure water level and, through rating curves, convert it to discharge. They can also be derived from stage sensors or remote sensing in some cases. Types of hydrographs include direct runoff or rainfall–runoff hydrographs, unit hydrographs used in hydrological design, and synthetic hydrographs produced by rainfall–runoff models to forecast flood behavior or plan infrastructure.

Applications include flood forecasting and risk assessment, design of hydraulic structures, water resources management, and hydrological research. Limitations arise from data quality, assumptions in baseflow separation, spatial variability within a watershed, and changes in land use or climate that alter hydrological response.