C3 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 Fiedler COMET Hydromet 00-1 Tuesday, 26 October 1999

C3 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

C3 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

C3 Sacramento Model Components  Tension water  Free water  Surface flow  Lateral drainage  Evapotranspiration  Vertical drainage (percolation)

C3 Soil Tension and Free Water

C3 Sacramento Soil Moisture Accounting Model

C3 Sacramento Model Structure E T Demand Impervious Area E T Precipitation Input Px Pervious Area E T Impervious Area Tension Water UZTW Free Water UZFW Percolation Zperc. Rexp 1-PFREE PFREE Free Water Tension Water P S LZTW LZFP LZFS RSERV Primary Baseflow Direct Runoff Surface Runoff Interflow Supplemental Base flow SideSubsurface Discharge LZSK LZPK Upper Zone Lower Zone EXCESS UZK RIVA PCTIM ADIMP Total Channel Inflow Distribution Function Streamflow Total Baseflow

C3 Model Parameters PXADJPrecipitation adjustment factor PEADJET-demand adjustment factor UZTWMUpper zone tension water capacity (mm) UZFWMUpper zone free water capacity (mm) UZKFractional daily upper zone free water withdrawal rate PCTIMMinimum impervious area (decimal fraction) ADIMPAdditional impervious area (decimal fraction) RIVARiparian vegetation area (decimal fraction) ZPERCMaximum percolation rate coefficient REXPPercolation equation exponent LZTWMLower zone tension water capacity (mm) LZFSMLower zone supplemental free water capacity (mm) LZFPMLower zone primary free water capacity (mm) LZSKFractional daily supplemental withdrawal rate LZPKFractional daily primary withdrawal rate PFREEFraction of percolated water going directly to lower zone free water storage RSERVFraction of lower zone free water not transferable to lower zone tension water SIDERatio of deep recharge to channel baseflow ET DemandDaily ET demand (mm/day) PE AdjustPE adjustment factor for 16th of each month

C3 State Variables ADIMCTension water contents of the ADIMP area (mm) UZTWCUpper zone tension water contents (mm) UZFWCUpper zone free water contents (mm) LZTWCLower zone tension water contents (mm) LZFSCLower zone free supplemental contents (mm) LZFPCLower zone free primary contents (mm)

C3 Percolation Rate Under Saturated Conditions Lower zone  Water balance of lower zone Out In

C3 Percolation Rate Continued...  Water balance of lower zone In Out

C3 Percolation Curve

C3 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.

C3 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

C3 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

C3 Volume Effects

C3 E T Demand Impervious Area E T Precipitation Input Px Pervious Area E T Impervious Area Tension Water UZTW Free Water UZFW Percolation Zperc. Rexp 1-PFREE PFREE Free Water Tension Water P S LZTW LZFP LZFS RSERV Primary Baseflow Direct Runoff Surface Runoff Interflow Supplemental Base flow SideSubsurface Discharge LZSK LZPK Upper Zone Lower Zone EXCESS UZK RIVA PCTIM ADIMP Total Channel Inflow Distribution Function Streamflow Total Baseflow Sacramento Model Structure

C3 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

C3 Hydrograph Produced Mainly by Surface Runoff ( Runoff Breakdown: Surface 64%, Interflow 15%, Supplemental 21%)

C3 Hydrograph Produced by Mixed Runoff ( Runoff Breakdown: Surface 27%, Interflow 33%, Supplemental 40%)

C3 Hydrograph Containing No Surface Runoff (Runoff Breakdown: Interflow 17%, Supplemental 83%)