National Weather ServiceRiver Forecast System

Cooperative Program for Operational Meteorology, Education and Training

Larry BrazilSacramento Soil Moisture

Accounting Model

2290 East Prospect Road, Suite 1Fort 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

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 Rate Under Saturated Conditions

Lower zone

Water balance of lower zone

Out

In

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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

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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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%)