Runoff/streamflow major focus in hydrology due to relation to:

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Runoff and Streamflow

• Runoff/streamflow major focus in hydrology due to
  relation to
• Floods high runoff events --gt flooding (need
  flood protection)
• Water supply runoff can be captured in storage
  reservoirs for water supply
• Runoff in streams is the integration of several
  upstream processes in a watershed (can be thought
  of as integrated response to storm events)
• Recall Watershed (or river basin) is defined
  based on topography, e.g.

P
Q
Based on a chosen outlet point, can trace out all
points that would ultimately flow to the outlet.
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Stream Network

• Similarly, the stream network (within a basin) is
  defined as those points where flow accumulation
  area is large, where accumulated flow area (Ac)
  is the upstream area flowing to a certain point.
• Together, the watershed consists of a
  well-defined area with interconnected hillslopes
  and a channel network.

Note Topographic ridges have low Ac (blue) and
main stream channel has high Ac (red/yellow)
Map of log(Ac)
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Runoff and Streamflow

• The collection of hillslopes and channels
  generate runoff from a given pixel and ultimately
  contribute to streamflow
• Questions
• What are mechanisms that generate runoff?
• When/why/where do they occur?
• How do they contribute to the overall flow in
  stream?
• Two main classes of runoff
• surface (overland) runoff (two mechanisms)
• subsurface runoff (two mechanisms)

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Surface (overland) Runoff

• Mechanism 1 Infiltration Excess Runoff
  (Hortonian runoff)
• Occurs when precipitation rate (P) exceeds
  infiltration rate (f) of soil
• Occurs only during storm often in localized low
  conductivity soils (PgtK) where ponding at surface
  occurs

hillslope
P gt f
Saw this when discussing infiltration models
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• Mechanism 2 Saturation excess overland flow
  (Dunne runoff)
• Occurs when groundwater table saturates soil
  from below (rising water table) precipitation
  falls on saturated surface
• Dynamics of water table leads to
  expanding/contracting variable contributing
  areas
• Runoff occurs only during storm in lowland
  areas near streams (i.e. where shallow GW exists)
  

Water table moves up/down in response to
storm/inter-storm periods
During/after big storm
1 week after storm
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Subsurface Runoff

• Mechanism 1 Interflow (perched stormflow)
• Lateral movement of water through unsaturated
  zone often resulting from temporary perched
  water table on low conductivity soil lens
• Generally small component of total runoff due
  to unsaturated flow velocities water may reach
  storm after storm ends
• Mechanism 2 Baseflow (GW flow)
• Flux of water into streams from
  unconfined/confined aquifers baseflow
  responsible for perennial streams where flow
  exists during interstorm periods
• Recharge and aquifer flow time scales such that
  flow reaching stream occurs during interstorm
  period

interflow
baseflow
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• Snowmelt
• Note Snowmelt is an important contributor to
  runoff in many regions (including ours). For
  snow, the melt output acts the same as a
  precipitation input to the surface.
• Hence snowmelt can ultimately lead directly to
  runoff via the following mechanisms
• Infiltration excess runoff (if soil beneath
  snowpack has low conductivity)
• Saturation excess runoff (if soil beneath
  snowpack is saturated)
• The subsurface mechanisms may also ultimately
  contribute snowmelt to streamflow
• The key point is that runoff generation may occur
  for long periods in the absence of precipitation
  when snowmelt is involved.

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Hydrographs

• Streamflow at basin outlet is often measured
  distribution vs. time is called a hydrograph,
  e.g.
• How much each mechanism contributes to overall
  streamflow depends on
• Individual storm (intensity/location/etc.)
• basin characteristics (topography, soil types,
  vegetation types, etc.)
• For flood forecasting/estimating design flows,
  generally most interested in stormflow
  immediate runoff response to storm (baseflow
  generally does not contribute much to stormflow
• Goal Build models for representing runoff
  processes in basin (i.e., given measured P ?
  predict Q).
• Note The subject of hydraulics of flow once
  water is in channel is one of the main topics of
  CEE 151.

Note Time delay between precip. onset and
streamflow peak allows for potential prediction
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Figure 8.1.3a (p. 249)(a) Separation of sources
of streamflow on an idealized hydrograph (from
Mosley and McKerchan (1993)).
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Basin-scale Rainfall-Runoff Modeling

• Modeling approaches
• Empirical/Conceptual Models for Estimating Design
  Floods/Forecasting
• usually treat basin as lumped unit
• use historical data to develop predictive tool
• systems (black-box) response approach
• make simplifying assumptions
• computationally efficient
• Examples SCS method, unit hydrograph method,
  etc.
• Physically-based Distributed Hydrologic Modeling
• account for physical processes distributed
  throughout basin
• explicitly model states/fluxes as function of
  space/time

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Figure 8.2.1 (p. 252)Concept of rainfall
excess. The difference between the total rainfall
hyetograph on the left and the total rainfall
excess hyetograph on the right is the abstraction
(infiltration).
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Figure 8.2.3 (p. 253)Storm runoff hydrographs.
(a) Rainfall-runoff modeling (b) Steps to define
storm runoff.
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Note By construction the response time of the
hydrograph (e.g. 42 hours in this case) remains
unchanged for a D-hr event
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Note As more complex responses are built-up
from multiple UHs, the response time may change
(e.g. in this case 48 hours for a 2D-hr storm)
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Effective Rainfall

• Being able to apply the UH method requires a
  mechanism for estimating the effective rainfall
  (Peff).
• This can be done using a physical model (i.e.
  attempting to predict the infiltration/runoff
  partitioning).
• Alternatively, it is often done empirically. One
  example is using the Soil Conservation Service
  (SCS) method
• where Peff in this equation is given in units of
  inches (as is the measured rainfall P) and Vmax
  is the watershed storage capacity (also in
  inches) which is estimated via
• where CN is the so-called SCS curve number and
  can be determined from tabulated values for
  different soil or land-use types. The single
  curve number for a basin is often determined from
  a weighted average of the curve numbers for
  different soil/land-use types in the basin based
  on their relative areas.
• Note This type of method is highly empirical and
  will generally introduce some error in the
  estimate of effective rainfall.

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Figure 8.6.1 (p. 262)Variables in the SCS
method of rainfall abstractions Ia initial
abstraction, Pe rainfall excess, Fa
continuing abstraction, and P total rainfall.
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Figure 8.6.2 (p. 263)Solution of the SCS runoff
equations (from U.S. Department of Agriculture
Soil Conservation Service (1972)).
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Table 8.7.3a (pp. 265-267)Runoff Curve Numbers