2290 East Prospect Road, Suite 1

1
National Weather ServiceRiver Forecast
SystemSacramento Soil Moisture Accounting Model
COMETHydromet 00-27 March 2000

• 2290 East Prospect Road, Suite 1
• Fort Collins, Colorado 80525

2
Sacramento Soil Moisture Accounting Model

• Physically-based
• Conceptual
• Lumped (versus distributed)
• Continuous (versus event-based)

3
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

4
Sacramento Model Components

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

5
Soil Tension and Free Water
6
Sacramento Soil Moisture Accounting Model
7
Sacramento Model Structure
E T Demand
Precipitation Input Px
Impervious Area E T
Direct Runoff
PCTIM ADIMP
Pervious Area
Impervious Area
Upper Zone
Surface Runoff
EXCESS
Tension Water UZTW
Free Water

UZFW
E T
UZK
Interflow
E T
Percolation Zperc. Rexp
Total Channel Inflow
Distribution Function
E T
RIVA
Streamflow
1-PFREE
PFREE
Lower Zone
Free Water Tension Water
P S LZTW
LZFP LZFS
RSERV
Supplemental Base flow
LZSK
E T
LZPK
Total Baseflow
Primary Baseflow
Side
Subsurface Discharge
8
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)

9
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)

10
Percolation Rate Under Saturated Conditions

• Water balance of lower zone

In
Lower zone
Out
11
Maximum Percolation Rate

• Water balance of lower zone

In
Out
12
Percolation Curve
13
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.

14
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

15
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

16
Volume Effects
17
Hydrograph Produced Mainly by Surface Runoff
(Runoff Breakdown Surface 64, Interflow 15,
Supplemental 21)
18
Hydrograph Produced by Mixed Runoff ( Runoff
Breakdown Surface 27, Interflow 33,
Supplemental 40)
19
Hydrograph Containing No Surface Runoff(Runoff
Breakdown Interflow 17, Supplemental 83)