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General Concepts Physically-based conceptual model
– based on physical concepts that describe water movement trough a watershed
Lumped versus distributed models
Continuous versus event-based models
Two-layer soil model
Presented by Dr. Fritz FiedlerCOMET Hydromet 00-1Tuesday, 26 October 1999
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Design Considerations Conceptual approach provides means to assess changes
in watershed morphology
Detailed modeling of the many actual watershed processes was too complex for operational application
Detailed modeling required more data than available
System developed that integrates primary physical processes without excessive data and/or computational needs
Essentially based on basic water balance equation:
Runoff = Rainfall - Evapotranspiration - Soil Moisture Changes
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Sacramento Soil Moisture Accounting Model
Represents soil moisture characteristics such that:
– Applied moisture is distributed in a physically realistic manner within the various zones 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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Sacramento Model Structure
E T Demand
Impervious Area
E T
E T
E T
E T
Precipitation Input
Px
Pervious Area
E T
Impervious Area
Tension Water
UZTW Free Water
UZFW
PercolationZperc. Rexp
1-PFREE PFREE
Free WaterTension Water P S
LZTW LZFP LZFS
RSERV
Primary Baseflow
Direct Runoff
Surface Runoff
Interflow
Supplemental Base flow
Side Subsurface Discharge
LZSK
LZPK
Upper Zone
Lower Zone
EXCESS
UZK
RIVA
PCTIM
ADIMP
Total Channel Inflow
Distribution Function Streamflow
Total Baseflow
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Model ParametersPXADJ Precipitation adjustment factorPEADJ ET-demand adjustment factorUZTWM Upper zone tension water capacity (mm)UZFWM Upper zone free water capacity (mm)UZK Fractional daily upper zone free water withdrawal ratePCTIM Minimum impervious area (decimal fraction)ADIMP Additional impervious area (decimal fraction)RIVA Riparian vegetation area (decimal fraction)ZPERC Maximum percolation rate coefficientREXP Percolation equation exponentLZTWM 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 rateLZPK Fractional daily primary withdrawal ratePFREE Fraction of percolated water going directly to lower zone free water storageRSERV Fraction of lower zone free water not transferable to lower zone tension waterSIDE Ratio of deep recharge to channel baseflowET Demand Daily ET demand (mm/day)PE Adjust PE adjustment factor for 16th of each month
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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 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
0
200
400
600
800
1000
12/29 2/17 4/8 5/28 7/17
Me
an
Da
ily F
low
(C
FS
)
Small Tension Water Zones Large Tension Water Zones
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E T Demand
Impervious Area
E T
E T
E T
E T
Precipitation Input
Px
Pervious Area
E T
Impervious Area
Tension Water
UZTW Free Water
UZFW
PercolationZperc. Rexp
1-PFREE PFREE
Free WaterTension Water P S
LZTW LZFP LZFS
RSERV
Primary Baseflow
Direct Runoff
Surface Runoff
Interflow
Supplemental Base flow
Side Subsurface Discharge
LZSK
LZPK
Upper Zone
Lower Zone
EXCESS
UZK
RIVA
PCTIM
ADIMP
Total Channel Inflow
Distribution Function Streamflow
Total Baseflow
Sacramento Model Structure
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Evapotranspiration ET Demand: Evapotranspiration from land surface when
soil moisture is not limiting (tension water at capacity)
Potential Evaporation: Evaporation from free water surface (lakes, wet grass)
PE Adjustment Curve: Seasonal curve reflecting type and activity of vegetation
ET Demand = PE * PE Adjustment
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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%)