Runoff and Runoff Volume
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Understanding Runoff
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Today, we're starting with an important concept in hydrology: runoff. Can anyone tell me what runoff means?
Is it the water that flows after it rains?
Exactly! Runoff is the part of the precipitation that flows over the land surface into streams and other water bodies. It's crucial to understand the different types of runoff, such as surface runoff, subsurface runoff, and base flow.
Whatβs the difference between those types?
Great question! Surface runoff refers to water that flows directly to streams, while subsurface runoff involves water that seeps into the soil and then emerges into streams later. Finally, base flow is the groundwater contribution during dry periods.
So base flow is like the 'slow and steady' water that keeps streams flowing?
That's a perfect way to think about it! Remember, understanding these differences will help us learn how to manage water resources effectively. Let's review: Surface runoff goes directly to streams, subsurface runoff seeps in first, and base flow happens from the groundwater.
Exploring Runoff Volume
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Now, let's talk about runoff volume. It represents the total runoff from a watershed after a storm. How do you think we can express this volume?
Maybe in terms of mΒ³ or mm?
Yes! It can be expressed as total volume in cubic meters or as depth in millimeters over the basin area. This helps us understand how much water is being generated in a watershed.
What about losses? How do they factor in?
That's important! Losses include infiltration, evaporation, and detention. These need to be factored in when calculating the final runoff volume.
Could you give us an example of how this is done?
Certainly! If we have 100 mm of precipitation and we account for 20 mm of losses, the runoff volume would be 80 mm. This calculation helps us predict flooding and manage water resources effectively.
Introduction to the SCS-CN Method
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Letβs dive into the SCS Curve Number method. Who can tell me what it is used for?
Is it something that estimates runoff from rainfall?
Correct! It estimates direct runoff by considering factors like land use and soil type. It uses an equation involving precipitation and the curve number for specific conditions.
What does the curve number represent?
The curve number indicates how much infiltration occurs; it ranges from 30 to 100, with higher numbers meaning less infiltration and more runoff. It depends on soil group, land use, and moisture conditions.
So we can predict how various land types affect runoff?
Exactly! Understanding this helps in urban planning and agriculture. Always keep in mind: Curve Number = lower infiltration leads to higher runoff!
Understanding Hydrographs
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Now weβll discuss hydrographs. Can someone explain what a hydrograph shows?
Isn't it a graph that shows streamflow over time?
That's right! A hydrograph plots streamflow against time, showing how discharge changes. It includes parts like the rising limb, peak discharge, and falling limb.
What do the different parts mean?
The rising limb indicates the steep rise in flow after a storm, the peak is the maximum flow, and the falling limb shows the decline back to base flow. This can help in flood forecasting!
How do all the factors we talked about affect the shape of a hydrograph?
Great connection! Factors like rainfall intensity, soil type, and vegetation can influence how quickly the runoff occurs, altering the hydrograph's shape. Remember, steeper curves suggest higher variability.
Introduction & Overview
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Quick Overview
Standard
The section explains runoff, detailing surface and subsurface runoff, and base flow. It introduces the SCS Curve Number method for estimating runoff volume based on precipitation, land use, and soil conditions. The section also discusses flow-duration curves, flow-mass curves, hydrographs, and the factors affecting runoff.
Detailed
Runoff and Runoff Volume
Runoff is defined as the portion of precipitation that flows over the land surface into water bodies, occurring when infiltration, evaporation, and other losses are balanced. There are different types of runoff:
- Surface Runoff: This is the water that flows directly over the land to streams.
- Subsurface Runoff: Water that infiltrates the soil and later re-emerges into streams.
- Base Flow: This is the groundwater contribution to streamflow, especially during dry periods.
The concept of Runoff Volume (Q) is crucial, as it represents the total quantity of runoff from a watershed after a storm. This can be calculated with various losses considered, including infiltration and evaporation. The volume is typically expressed in terms of depth over the area of the basin (mm) or as total volume (mΒ³).
To estimate runoff more effectively, particularly in varying land uses and soil types, the SCS Curve Number (SCS-CN) Method is used. This method formulates a direct runoff estimate from rainfall, integrating factors such as land use, soil type, and antecedent moisture conditions.
Other important concepts covered are the Flow-Duration Curve (FDC) and Flow-Mass Curve, which help analyze flow reliability and storage requirements, respectively. A Hydrograph is introduced as a crucial tool for visualizing streamflow over time, with distinct components such as rising limb and base flow.
Factors like rainfall intensity, soil type, land use, watershed shape, topography, antecedent moisture, and vegetation can significantly affect runoff and the shape of hydrographs. Furthermore, effective rainfall is defined, which is the portion contributing to direct runoff minus initial losses. Finally, the Unit Hydrograph (UH) concept allows the prediction of flood hydrographs based on effective rainfall, emphasizing the linear response of watersheds to rainfall events.
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Definition of Runoff
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Chapter Content
Runoff is the portion of precipitation that flows over the land surface into water bodies when infiltration, evaporation, and other losses are satisfied.
Detailed Explanation
Runoff occurs when precipitation, such as rain or snow, falls to the ground. Some of this water is absorbed by the soil (infiltration) or evaporates back into the air. The remaining water that flows over the surface into rivers, lakes, and oceans is known as runoff. This happens only after the ground has absorbed as much water as it can, and the excess flows across the land.
Examples & Analogies
Think of a sponge that is placed under a faucet. When you first turn on the water, the sponge absorbs some of it, but once it is full, the extra water drips off and flows down to the counter. In this analogy, the overflowing water represents runoff.
Types of Runoff
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Chapter Content
Types of Runoff:
- Surface runoff: Water that flows directly over land to streams.
- Subsurface runoff: Water that infiltrates then emerges into streams.
- Base flow: Groundwater contribution to streamflow during dry periods.
Detailed Explanation
There are three main types of runoff:
1. Surface runoff is the water that travels on the ground surface directly into water bodies, such as rivers or lakes.
2. Subsurface runoff refers to water that seeps into the ground but then flows back to the surface into streams.
3. Base flow is the steady flow of groundwater into streams, especially important during times of low rainfall.
Examples & Analogies
Imagine a sponge in a wet environment. Initially, when it rains heavily, excess water will drip off the surface β thatβs surface runoff. If the sponge is saturated, some water inside can seep out and make its way to the bottom of a container β thatβs subsurface runoff. Over time, as the sponge gradually gives off moisture, it represents base flow.
Calculating Runoff Volume
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Chapter Content
The total quantity of runoff from a watershed after a storm is:
Where:
$ P $ = Precipitation
Losses include infiltration, evaporation, and detention (e.g., interception, depression storage).
Volume is often expressed as depth over the basin area (mm) or as total volume (mΒ³).
Detailed Explanation
To calculate the runoff volume from a storm, we start with the total precipitation (P). From this amount, we subtract the losses, which are the amounts of water that were absorbed into the soil (infiltration), turned into vapor (evaporation), or held temporarily in other forms (detention). The final result tells us how much water is left that contributes to runoff. This volume can be represented as a depth (in millimeters) over the area of the watershed or as a total volume (in cubic meters).
Examples & Analogies
Consider filling a bucket with water from a hose. If your bucket has some holes (representing infiltration and evaporation), not all the water from the hose will remain in the bucket. The amount that stays in the bucket after accounting for the water lost through holes is analogous to runoff volume.
Key Concepts
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Runoff: The flow of water from precipitation.
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Surface vs. Subsurface Runoff: Distinction between runoff types.
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Base Flow: Groundwater component of streamflow.
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SCS Curve Number Method: Estimation of runoff based on land characteristics.
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Hydrograph: Visual representation of streamflow over time.
Examples & Applications
Finding the runoff volume after a rainfall event with specified losses.
Using the SCS-CN method to estimate runoff based on land use types.
Memory Aids
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Rhymes
When rain pours down and ground gets wet, surface runoff drifts, oh don't forget!
Stories
Imagine a thirsty plant waiting for rain. When the drops fall, some water stays and others rush off to streams, while the roots sip slowly through the ground - that's runoff!
Memory Tools
RUB: R for runoff, U for underground flow (base flow), B for built-up areas (urban runoff).
Acronyms
SCS-CN
for Soil type
for Land Cover
for Saturation level
and CN for Curve Number.
Flash Cards
Glossary
- Runoff
The portion of precipitation that flows over land and into water bodies.
- Surface Runoff
Water that flows directly over the ground to streams.
- Subsurface Runoff
Water that infiltrates and then emerges into streams.
- Base Flow
Groundwater contribution to streamflow during dry conditions.
- SCS Curve Number (SCSCN) Method
A method to estimate direct runoff based on land use, soil type, and moisture condition.
- FlowDuration Curve (FDC)
A plot showing the percentage of time a certain flow rate is equaled or exceeded.
- Hydrograph
A plot of streamflow (discharge) versus time for a specific point in a river.
- Effective Rainfall
The portion of rainfall that generates direct runoff after initial losses.
- Unit Hydrograph (UH)
The hydrograph resulting from 1 unit depth of effective rainfall uniformly distributed over a catchment.
- Environmental Flows (EFlows)
Water flows required to sustain fresh and estuarine ecosystems.
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