RatingKurve

RatingKurve, commonly known in English as a rating curve, is the empirical relationship between water level (stage) and discharge at a specific gauging station on a river or stream. It enables hydrologists to estimate streamflow from continuous stage measurements, particularly when direct discharge measurements are impractical or infrequent.

Derivation and form: The curve is developed from paired measurements of stage and discharge collected during various flow events. Discharge is measured directly with instruments such as current meters or acoustic Doppler devices while a concurrent stage reading is recorded. The resulting data are plotted as Q (discharge) versus H (stage), and a mathematical function is fitted to describe the relationship. The resulting form is typically monotone and nonlinear, and may be piecewise or expressed as a power-law, polynomial, or tabulated series.

Characteristics and limitations: Rating curves assume stable channel geometry and hydraulic conditions at the measurement site. In practice, channel changes from sedimentation, erosion, vegetation, backwater effects from upstream reservoirs, or dam operations can alter the relationship, necessitating re-rating. Hysteresis can occur, leading to different curves for rising and falling stages. Extrapolation beyond observed data carries additional uncertainty, especially during extreme floods.

Applications and maintenance: Rating curves are used to convert stage data into discharge records, generate hydrographs, support flood forecasting, and calibrate hydrological models. They require periodic updating with new measurements to maintain accuracy and are a fundamental tool in hydrometry and water resources management.