Hydrograph

A hydrograph is a graphical representation of how the discharge of a river or stream varies with time. It typically plots discharge in cubic meters per second (m3/s) or water stage against time, with the time axis ranging from hours to days or longer.

Hydrographs are used to interpret runoff processes and are essential in flood forecasting and water resource planning. Key features include the rising limb, which shows the increase in discharge after rainfall begins; the peak discharge; the falling limb as discharge recedes; and the baseflow, representing sustained flow from groundwater and delayed subsurface sources.

Common types include storm or rainfall-runoff hydrographs, which depict the watershed response to precipitation, and unit hydrographs, which describe the response to a unit amount of rainfall over a specified duration. Hydrographs are constructed from streamflow measurements collected at gauging stations and are related to stage through a stage-discharge rating curve. In some analyses, hydrographs are separated to estimate direct runoff from baseflow.

Applications include estimating flood magnitudes, designing hydraulic structures, calibrating hydrological models, and assessing watershed behavior over time. Limitations include variability in baseflow, storage effects in reservoirs and lakes, groundwater interactions, and land-use or climate changes that alter hydrograph shape. Accurate interpretation requires reliable data, appropriate time resolution, and a solid understanding of watershed characteristics.