National Weather Service River Forecast System Cooperative Program for Operational Meteorology, Educ

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National Weather ServiceRiver Forecast
SystemCooperative Program for Operational
Meteorology, Education and Training

• Larry Brazil
• Sacramento Soil Moisture
• Accounting Model
• 2290 East Prospect Road, Suite 1
• Fort Collins, Colorado 80525

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General Concepts

• Physically-based conceptual model
• Lumped versus distributed models
• Continuous versus event-based models
• Two-layer soil model

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Sacramento Soil Moisture Accounting Model

• Represents soil moisture characteristics such
  that
• Applied moisture is distributed properly in
  various depths and energy states in soil
• Rational percolation characteristics are
  maintained
• Streamflow is simulated effectively

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Sacramento Model Components

• Tension water
• Free water
• Surface flow
• Lateral drainage
• Evapotranspiration
• Vertical drainage (percolation)

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Soil Tension and Free Water
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Sacramento Soil Moisture Accounting Model
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Sacramento Model Structure
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Model Parameters

• PXADJ Precipitation adjustment factor
• PEADJ ET-demand adjustment factor
• UZTWM Upper zone tension water capacity (mm)
• UZFWM Upper zone free water capacity (mm)
• UZK Fractional daily upper zone free water
  withdrawal rate
• PCTIM Minimum impervious area (decimal fraction)
• ADIMP Additional impervious area (decimal
  fraction)
• RIVA Riparian vegetation area (decimal fraction)
• ZPERC Maximum percolation rate coefficient
• REXP Percolation equation exponent
• LZTWM Lower zone tension water capacity (mm)
• LZFSM Lower zone supplemental free water capacity
  (mm)
• LZFPM Lower zone primary free water capacity (mm)
• LZSK Fractional daily supplemental withdrawal
  rate
• LZPK Fractional daily primary withdrawal rate
• PFREE Fraction of percolated water going directly
  to lower zone free water storage
• RSERV Fraction of lower zone free water not
  transferable to lower zone tension water
• SIDE Ratio of deep recharge to channel baseflow
• ET Demand Daily ET demand (mm/day)

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State Variables

• ADIMC Tension water contents of the ADIMP area
  (mm)
• UZTWC Upper zone tension water contents (mm)
• UZFWC Upper zone free water contents (mm)
• LZTWC Lower zone tension water contents (mm)
• LZFSC Lower zone free supplemental contents (mm)
• LZFPC Lower zone free primary contents (mm)

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Percolation Rate Under Saturated Conditions

• Water balance of lower zone

In
Lower zone
Out
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Maximum Percolation Rate

• Water balance of lower zone

In
Out
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Percolation Curve
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Percolation Characteristics

• PBASE
• The continued percolation rate under saturated
  conditions
• A function of the lower zone capacities and the
  lower zone withdrawal rates
• PBASE LZFSM LZSK LZFPM LZPK
• ZPERC
• The number of PBASE units that must be added to
  the continuing saturated percolation rate to
  define the maximum percolation rate
• REXP
• The exponent which defines the curvature in the
  percolation curve with changes in the lower zone
  soil moisture deficiency.

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Effect of Soil-Moisture Parameters on Model
Response

• Volume
• Altering these parameters changes volume, but
  not the relative breakdown of runoff among
  various non-impervious components
• UZTWM, LZTWM, ET (Demand curve), PE (Adjustment
  curve)
• Timing
• Altering these parameters changes the relative
  breakdown of runoff between various
  non-impervious components always causes timing
  changes and (in some cases) can cause significant
  overall volume changes
• UZFWM, LZFPM, LZFSM, UZK, LZPK, LZSK, ZPERC, REXP

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Effect of Soil-Moisture Parameters on Model
Response (continued)

• Impervious runoff
• Altering these parameters determines how much of
  the rain melt goes directly to runoff both
  have a volume and timing effect, though PCTIM
  mainly affects volume, and ADIMP primarily
  affects timing
• PCTIM, ADIMP
• Baseflow volume
• Altering these parameters primarily changes the
  amount of baseflow volume while having little or
  no effect on other runoff components
• SIDE, RIVA, PFREE
• Minor effect
• Generally has little effect on model response
• RSERV

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Volume Effects
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Hydrograph Produced Mainly by Surface Runoff
(Runoff Breakdown Surface 64, Interflow 15,
Supplemental 21)
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Hydrograph Produced by Mixed Runoff ( Runoff
Breakdown Surface 27, Interflow 33,
Supplemental 40)
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Hydrograph Containing No Surface Runoff(Runoff
Breakdown Interflow 17, Supplemental 83)