Infiltration - UniTrentorighetti/lezioni HPP/HPP/Hydrology/Lecture5... · Accumulated Infiltration...
Transcript of Infiltration - UniTrentorighetti/lezioni HPP/HPP/Hydrology/Lecture5... · Accumulated Infiltration...
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Infiltration
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Accum
ula
ted Infiltra
tion (
mm
)
200
300
400
Infiltration depends on the soil type
Accum
ula
ted Infiltra
tion (
mm
)
100
60 120 180
Time (minutes)
Taylor and Ashcroft, 1972]
But also on other factors such as flux (temperature, viscosity,
hydraulic head) and surface conditions (land use and land
cover).
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Rainfall at the soil surface is partitioned into infiltration (I) and surface
runoff(Q).
Experimentally, it is observed that not all the rainfall is able to infiltrate into
the soil... this is due to two main reasons:
1. the rainfall intensity is too high, and the maximum (instantaneous)
Infiltration
1. the rainfall intensity is too high, and the maximum (instantaneous)
infiltration rate is exceeded (Horton mechanism).
2. the cumulated rainfall volume is too high and the soil becomes
completely saturated - there is no room for an increase of the water
storage (Dunne mechanism).
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Horton mechanism (Horton, 1945): the soil ischaracterized (at any time instant t) by a maximuminfiltration capacity, f*(t), which depends on the soilmoisture conditions and decreases during a givenevent. Thus we have:
P
P
qo
f
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Dunne mechanism: water is infiltrated into the soilunless the soil becomes completely saturated (alower impervious surface below the root zone isassumed). After the saturation all the rainfallbecomes surface runoff.
P
P
Pqo
P
f
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• Horton mechanism usually dominates in arid and
semiarid climates, where the storms are concentrated in
short periods and are characterized by large rainfall
heights.
• Hortonian surface runoff is usually widespread in space,
but quite limited and local in time.
Infiltration
• Dunne mechanism usually becomes more important in
humid climates, where rainfall events are characterized
by large annual volumes but lower intensities.
• Dunnian surface runoff could be extended in time (once
that saturation is reached), but is usually limited in
space.
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Subsurface stormflow
PP
P
qs
Surface runoff is not the only component
Perched subsurface stormflow
PP
P
qs
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Flood hydrograph is usually separated into 3 contributions:
• Baseflow
• Slow component (subsurface flow)
• Fast component (surface runoff)
Hydrograph separation
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Hydrograph separation is usually performed in a semi-log plane.
Hydrograph separation
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a constant maximum infiltration capacity Φ is assumed.
Infiltration: Φ index model (Horton)
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an exponential decrease of infiltration capacity f*
through time is assumed.
Infiltration: Horton model
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Infiltration: Horton model
• fc [L/T]: minimum infiltration capacity (proportional to saturated
hydraulic conductivity);
• f0 [L/T]: initial infiltration capacity;
• k [1/T]: coefficient controlling the rate of variation of f through
time.time.
Horton model provides potential infiltration, in other words the
infiltration that is obtained if rainfall is larger than f(t): J(t) > f (t).
This condition is not always satisfied, expecially during the first
part of rainfall events when tipically potential infiltration is high as
a consequence of the low soil water content values.
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Infiltration: Horton model
In order to avoid this inconsistency (potential infiltration larger than
precipitation), the horton curve can be shifted horizontally by a
certain time t* until the following conditions are satisfied:
• First condition imposes that in the time interval [0; t0] real
infiltration (obtained from the shifted curve) equals precipitation.
• Second condition imposes that at time t0 rainfall intensity equals
infiltration.
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Infiltration: SCS model
Hypothesis
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Infiltration: SCS model
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Infiltration: SCS model
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Infiltration: SCS model
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SCS model with spatially variable CN
Catchment subdivided in Np sub-catchments through GIS
analysis. CN and S are constant within the sub-catchment but can
vary betweeen different sub-catchments.
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Calibration of SCS model with spatially
variable CN�Unknowns are S and α ;
�α is usually assumed equal to 0.2 ;
�surface runoff volume obtained through hydrograph
decomposition (vs) is assumed equal to that calculated through
the SCS model,
where:
A: catchmet area;
tp: rainfall duration;
N: number of sub-catchments.
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Calculation of effective precipitation with SCS
We assume that SCS model can be applied in a continuous way
Precipitation is known at discrete time intervals (aggregation of
precipitation occurred during that particular interval), thus J ∆t isprecipitation occurred during that particular interval), thus Jh,i ∆t is
the precipitation in the interval (i-1)∆t - (i)∆t for the sub-catchment
h. It follows that:
if
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Calculation of effective precipitation with SCS
Effective precipitation intensity can be then derived from
cumulative runoff for each sub-catchment h:
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Water infiltrating into the soil during the event in the sub-
catchment h is:
np: number of time step intervals in which the rainfall event has
been subdivided np = tp/∆t.
Calculation of subsurface stormflow with SCS
been subdivided np = tp/∆t.
Total infiltration is then provided by:
and then the subsurface partition coefficient can be computed
as:
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We can then calculate the average intensity of the lost
infiltration:
where n is the number of time steps for which cumulated
Calculation of subsurface stormflow with SCS
where nh,1 is the number of time steps for which cumulated
rainfall is less than the initial loss.
For each time step we can then calulate the following quantity:
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if
otherwise
Effective subsurface infiltration
Calculation of subsurface stormflow with SCS
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2003
AfterBorga,2003