Runoff: Components and Types

Runoff

• Runoff is the draining or flowing off of precipitation from a catchment area through a surface channel, representing the output from the catchment in a given unit of time.

Components of Runoff

• Surface Runoff: the flow of water that occurs when excess stormwater, meltwater, or other sources flow over the Earth's surface.
• Groundwater Runoff: the part of the total runoff that has infiltrated the ground, entered the saturated zone, and then re-emerges at the surface to contribute to the flow in streams, rivers, lakes, or oceans.
• Interflow/Throughflow: the lateral movement of water in the unsaturated zone of the soil, parallel to the land surface.

Components of Surface Runoff

• Overland Flow: the movement of water over the land surface, usually when the soil has become saturated, or when rainfall exceeds the infiltration capacity of the soil.
• Channel Flow: the movement of water within the confines of a defined channel such as a stream, river, or canal.

Types of Interflow

• Prompt Interflow: the rapid lateral movement of water in the unsaturated zone of the soil, occurring shortly after precipitation events.
• Delayed Interflow: the slower lateral movement of water through the soil, occurring over longer periods after precipitation events.

Two Classifications of Runoff

• Direct Runoff: the part of the runoff which enters the stream immediately after the rainfall.
• Baseflow: the delayed flow that reaches a stream essentially as groundwater flow.

Natural Flow (or Virgin Flow)

• Refers to the flow of a river or stream in its natural state, unaffected by human activities such as dam construction, water withdrawals, land use changes, or other alterations.

Hydrograph

• A graphical representation that shows the variation of a hydrological variable, such as streamflow or groundwater level, over time.
• Typically plots the discharge (flow rate) of a river or stream against time, often in response to precipitation events.

Parts of a Hydrograph

• Rising Limb
• Peak Discharge (Crest Segment)
• Recession Limb (Falling Limb)
• Baseflow
• Lag Time
• Total Runoff Volume

Types of Streams

• Perennial Streams: watercourses that flow continuously throughout the year, regardless of seasonal variations in precipitation or groundwater levels.
• Intermittent Streams: watercourses that flow periodically throughout the year, typically during wet seasons or in response to precipitation events.
• Ephemeral Streams: watercourses that flow only in direct response to precipitation events, such as rainfall or snowmelt.

Flow Duration Curve

• A graphical representation of streamflow data that illustrates the percentage of time that a given flow rate is equaled or exceeded over a specified period.
• X-axis: Probability of Exceedance
• Y-axis: Flow Rates

Probability of Exceedance

• Refers to the likelihood that a particular flow rate or discharge will be equaled or exceeded during a specified period of time.
• Expressed as a percentage and represents the probability that a certain flow will occur or be exceeded.

Flow Mass Curve

• A graphical representation used in hydrology to analyze the relationship between streamflow and the cumulative mass or volume of water over a specified time period.
• Displays the cumulative volume or mass of water flowing past a certain point in a stream over time.

Types of Streams

• Intermittent streams: flow periodically throughout the year, typically during wet seasons or in response to precipitation events, and may dry up for part of the year.
• Ephemeral streams: flow only in direct response to precipitation events, such as rainfall or snowmelt, and do not have a continuous flow.
• Perennial streams: flow continuously throughout the year, regardless of seasonal variations in precipitation or groundwater levels, and typically maintain a consistent flow rate.

Flow Duration Curve

• A graphical representation of streamflow data that illustrates the percentage of time that a given flow rate is equaled or exceeded over a specified period.
• X-axis: Probability of Exceedance
• Y-axis: Flow Rates

Probability of Exceedance

• Refers to the likelihood that a particular flow rate or discharge will be equaled or exceeded during a specified period of time.
• Expressed as a percentage and represents the probability that a certain flow will occur or be exceeded.

Flow-Mass Curve

• A graphical representation used in hydrology to analyze the relationship between streamflow and the cumulative mass or volume of water over a specified time period.
• Displays the cumulative volume or mass of water flowing past a certain point in a stream over time.
• Purpose: analyzing the total amount of water transported by a stream or river over a given period, providing insights into the overall water yield, sediment transport, and other hydrological characteristics.
• Application: assessing total water yield from a watershed, estimating sediment load and transport in rivers, evaluating the effectiveness of reservoir operations, and analyzing the impact of land use changes or climate variability on streamflow.

Two Classifications of Runoff

• Direct Runoff: the part of the runoff that enters the stream immediately after the rainfall.
• Baseflow: the delayed flow that reaches a stream essentially as groundwater flow.

Natural Flow (or Virgin Flow)

• Refers to the flow of a river or stream in its natural state, unaffected by human activities such as dam construction, water withdrawals, land use changes, or other alterations.

Hydrograph

• A graphical representation that shows the variation of a hydrological variable, such as streamflow or groundwater level, over time.
• Typically plots the discharge (flow rate) of a river or stream against time, often in response to precipitation events.

Parts of a Hydrograph

• Rising Limb
• Peak Discharge (Crest Segment)
• Recession Limb (Falling Limb)
• Baseflow
• Lag Time
• Total Runoff Volume

Droughts

• Prolonged periods of abnormally low precipitation and water availability, leading to water shortages and adverse impacts on agriculture, ecosystems, water resources, and society.
• Types of Droughts:
  • Meteorological Drought: a deficit in precipitation compared to the long-term average for a specific region and time period.
  • Agricultural Drought: a lack of soil moisture affecting crop growth and agricultural productivity.
  • Hydrological Drought: reduced streamflow, groundwater levels, and water availability in rivers, lakes, and reservoirs.
  • Socioeconomic Drought: impacts on human activities, such as water scarcity, food shortages, economic losses, and social disruptions.
• Causes of Droughts:
  • Natural Variability: climate fluctuations, such as El Niño/La Niña events and atmospheric circulation patterns.
  • Climate Change: altered precipitation patterns and increased temperatures leading to more frequent and severe droughts.

Learn about the different types of runoff, including surface runoff, and its importance in the water cycle. Understand the concepts of catchment area, precipitation, and flow of water.