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Surface Runoff
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There are two types of surface runoff that occur during rainfall or snowmelt.
Infiltration excess overland flow occurs with soil that is not saturated. In fact the soil can be quite dry, but soil properties or land cover do not allow for infiltration to keep up with high rainfall or snowmelt rates.
Saturation excess overland flow occurs when the soil becomes saturated and there is no longer any space for water to infiltrate. This can occur even with soil that would typically allow for large amounts of infiltration in sub-saturated conditions.
Infiltration Excess Overland Flow
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Infiltration excess occurs when the rate of rainfall or snowmelt is greater than the infiltration capacity. The water that cannot infiltrate becomes surface runoff.
For example, if the infiltration capacity is 15 millimeters per hour and the rainfall rate is 25 millimeters per hour, then the rainfall rate exceeds the infiltration capacity by 10 millimeters per hour. That 10 millimeters per hour becomes infiltration excess overland flow, even if the underlying soil is dry.
Infiltration excess is most commonly observed with short-duration intense rainfall. It also occurs most often in areas with high clay content or where the surface has been altered by soil compaction, urbanization, or fire.
Infiltration excess overland flow is sometimes called Hortonian flow.
Saturation Excess Overland Flow
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Saturation excess occurs when the soil layers have become saturated and no further water can infiltrate.
It is most common with long-duration, gentle-to-moderate rainfall, or with
the latter of successive precipitation and or snowmelt events.
For example, consider a storm that produces a steady rainfall rate of 10
millimeters per hour for four hours. After three hours of infiltration the
soil becomes saturated. In the fourth hour, the 10 mm/hr rainfall rate cannot
infiltrate the saturated soil and becomes saturation excess overland flow.
Saturation excess overland flow can occur anywhere the soil is wet. It is most common in humid climates with gently sloped or flat basins.
Here we see a farm field where the soil can typically absorb a large amount of water. The standing water in relatively light rainfall suggests that soil saturation is preventing further infiltration of rainwater. Saturation excess overland flow is the result.
Interflow
Interflow, also known as subsurface stormflow, is relatively rapid flow toward the stream channel that occurs below the surface. It occurs more rapidly than baseflow, but typically more slowly than surface runoff. In some cases interflow may be as important as surface runoff for forecasting rapid rises in the stream channel. In fact, in regions with high infiltration rates and steep terrain, interflow may be the dominant process by which streams react quickly to rainfall or snowmelt.
This process is most likely to occur in humid, deep-soil areas. However, significant interflow contribution may occur in thin-soiled regions when there is an impermeable layer such as bedrock beneath the more permeable surface soil layer.
Transmissivity Feedback
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One runoff process that contributes to interflow is transmissivity feedback. This occurs when a network of macropores is activated following rapid infiltration.
Macropores and natural pipes are void spaces in the soil that provide preferential pathways for water to move downslope. Decayed plant roots, burrowing insects and animals, and chemical reactions between water and soil minerals are a few ways that macropores form.
Macropore networks are more likely in deep-soiled areas with considerable organic materials. Thus, humid climates are more likely to have substantial interflow through macropore networks.
Soil-Bedrock Interface
The presence of a soil-bedrock interface enhances interflow. The soil-bedrock interface typically occurs in steep terrain where the soil layer is considerably more permeable than the underlying bedrock. Rainwater or snowmelt infiltrates rapidly to the bedrock interface and then moves rapidly downslope along the interface.
Sometimes a feature called a fragipan exists. A fragipan layer has low permeability, like rock or clay, and may also serve to focus the lateral subsurface flow. Fragipan features can exist at relatively shallow depths and play an important role in enhancing both interflow and even surface runoff after the soil layers above the fragipan are saturated.
Groundwater Ridging
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Groundwater ridging is yet another mechanism that contributes to runoff.
Groundwater ridging is a process that occurs in sloped drainage basins where the water table is much closer to the surface near the stream channel than it is further away from the stream.
Rainwater or snowmelt reaches the groundwater level near the stream channel more quickly than it does further up the hill away from the stream. The water table begins to rise near the stream channel more quickly than it does further away, creating a groundwater ridge close to the stream. The gradient between the groundwater ridge and the stream channel results in more rapid interflow to the stream.
In some cases the groundwater ridge can reach the soil surface and contribute to surface runoff through saturation excess overland flow.
Pre-Event Water
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Some of the interflow that quickly finds its way to the stream is not necessarily from the current rainfall. In these cases there is considerable water already in the soil layers that gets displaced as new water infiltrates. The water that appears in the stream immediately following a rainfall or rapid snowmelt period may be from previous precipitation events, or pre-event water. In humid climates, studies have shown that pre-event water is often the greatest contributor to rapid rises in stream level.
Review Questions:
1. Which type of surface runoff is most likely with an intense 30-minute rainfall?
(Choose the best answer.)
a) Saturation excess overland flow
b) Infiltration excess overland flow
2.
Which type of surface runoff is most likely with 48 hours of gentle steady rainfall in a forest?
(Choose the best answer.)
a) Saturation excess overland flow
b) Infiltration excess overland flow
3. Which type of surface runoff is most likely to occur in a flat, densely vegetated area?
(Choose the best answer.)
a) Saturation excess overland flow
b) Infiltration excess overland flow
4. Which type of surface runoff is most likely to occur in an urban area?
(Choose the best answer.)
a) Saturation excess overland flow
b) Infiltration excess overland flow
5. Water that exists prior to a significant rainfall event may become runoff
as “pre-event water” but never amounts to a major proportion of
the runoff.
(Choose the best answer.)
a) True
b) False
6. Interflow may be more important than surface runoff for generating a
quick rise in streams located in _____.
(Choose all that apply.)
a) urban environments
b) densely vegetated, sloped terrain
c) regions where impermeable bedrock underlies a permeable soil layer
d) fire burned areas
7. Macropores _____ interflow by providing _____ for water
to flow along.
(Choose the best answer.)
a) increase | horizontal surfaces
b) increase | preferential pathways
c) decrease | horizontal surfaces
d) decrease | preferential pathways
8. Groundwater ridging decreases flow to the stream by blocking
the interflow.
(Choose the best answer.)
a) True
b) False
Review Question Feedback
1. Which type of surface runoff is most likely with an intense 30-minute rainfall?
(Choose the best answer.)
The correct answer is
b) Infiltration excess overland flow
2. Which type of surface runoff is most likely with 48 hours of gentle steady rainfall in a forest?
(Choose the best answer.)
The correct answer is
a) Saturation excess overland flow
3. Which type of surface runoff is most likely to occur in a flat, densely vegetated area?
(Choose the best answer.)
The correct answer is a) Saturation excess overland flow
4. Which type of surface runoff is most likely to occur in an urban area?
(Choose the best answer.)
The correct answer is b) Infiltration excess overland flow
5. Water that pre-exists a significant rainfall event may become runoff as “pre-event water” but never amounts to a major proportion of the runoff.
(Choose the best answer.)
The correct answer is
b) False
6. Interflow may be more important than surface runoff for generating a quick rise in streams located in _____.
(Choose all that apply.)
The correct answers are b) and c). Interflow may be more important than surface runoff for streamflow forecasts especially in regions with deep, well-developed soil profiles and/or sloped terrain. In fact, humid climates with hilly terrain often have a significant interflow contribution to runoff.
7. Macropores _____ interflow by providing _____ for water
to flow along.
(Choose the best answer.)
The correct answer is b) increase | preferential pathways
8. Groundwater ridging decreases flow to the stream by blocking
the interflow.
(Choose the best answer.)
The correct answer is b).
End of Section Two: Paths to Runoff
