The Hydrograph and Flow Regimes

A hydrograph is the record of streamflow over time, and a river's flow regime is the characteristic pattern of its flows; both reveal how a catchment stores and releases water.

Definition

A hydrograph is a graph of discharge against time at a point on a stream; the flow regime is the statistical and seasonal pattern of those flows, characterized by their magnitude, frequency, duration, timing, and rate of change.

Scope

This topic covers the components and interpretation of the storm hydrograph, the separation of quick flow from baseflow, recession behavior, and the longer-term flow regime described by flow-duration curves and seasonal patterns. It interprets streamflow records, complementing the hydraulics and measurement covered elsewhere.

Core questions

• What are the parts of a storm hydrograph, and what do they represent?
• How is baseflow separated from storm runoff?
• What does a recession curve reveal about catchment storage?
• How are long-term flow regimes characterized and why do they matter?

Key concepts

• Storm hydrograph and lag time
• Quick flow and baseflow
• Baseflow separation
• Recession curve
• Flow-duration curve
• Natural flow regime

Key theories

Hydrograph components and baseflow separation

The storm hydrograph is conceptually divided into quick (storm) flow and baseflow sustained by drainage from storage; separating them helps relate runoff to rainfall and characterize catchment response.

The natural flow regime

The natural flow regime concept holds that the full range of flow variability, its magnitude, frequency, duration, timing, and rate of change, structures river ecosystems, providing a framework for environmental flow management.

Clinical relevance

Hydrograph and flow-regime analysis informs flood characterization, low-flow and drought assessment, reservoir and water-supply yield, and the setting of environmental flows needed to sustain river ecosystems below dams and diversions.

History

The hydrograph and its separation into runoff components developed alongside the unit hydrograph in the early-to-mid 20th century. Late-20th-century ecohydrology, notably the natural-flow-regime paradigm of Poff and colleagues, broadened attention from peak flows to the whole spectrum of flow variability and its ecological role.

Key figures

• S. Lawrence Dingman
• N. LeRoy Poff

Seminal works

• dingman2015
• poff1997
• chow1988

Frequently asked questions

What is baseflow?

Baseflow is the portion of streamflow sustained between storms by the slow drainage of water from soil and groundwater storage; it keeps perennial rivers flowing during dry periods, in contrast to the quick storm flow that produces flood peaks.

What is a flow-duration curve?

It is a plot showing the percentage of time that streamflow equals or exceeds given values, summarizing the variability of a river's flow regime and widely used in water-supply, hydropower, and low-flow assessments.