View source for Model Formulation:Processes Simulated

GSSHA is a process-based model.  Hydrologic processes that can be simulated and the methods used to approximate the processes with the GSSHA model are listed in Table 1.  With the exception of channel routing, all processes and approximations in the original CASC2D model are also contained in the GSSHA model.  The Preissmann channel routing routine (Cunge et al., 1980) was excluded because of known stability problems with the scheme when simulating trans-critical flows (Mesehle and Holly, 1997).  Also, the upwind explicit channel routing method was replaced with a similar up-gradient explicit method.

{| class="thin"
|-
! Process	
! Approximation
|-
| Precipitation distribution
| 
: Thiessen polygons (nearest neighbor), 
: Inverse distance-squared weighting
|-
| Snowfall accumulation and melting
| 
:Energy balance
|-
| Precipitation interception
| 
: Empirical 2 parameter with seasonal variance
|-
| Overland water retention
| 
: Specified depth
|-
| Infiltration
| 
: Green and Ampt (GA),
: Multi-layered GA, 
: Green and Ampt with Redistribution (GAR),
: Richard’s equation (RE)
|-
| Overland flow routing
| 
:2-D diffusive wave 
:* Explicit,
:* Alternating Direction Explicit (ADE),
:* ADE Predictor-Corrector (ADEPC)
|-
| Channel routing
| 
: 1-D diffusive wave – up-gradient explicit
|-
| Reservoir simulation
| 
: Inflow from overland
: Inflow from streams
: Rainfall input
: ET - Dingman (1995)
: Outlet strucuture control
: Variable area/volume
|-
| Evapo-transpiration
| 
: Deardorff, 
: Penman-Monteith with seasonal canopy resistance
|-
| Soil moisture in the Vadose zone
| 
:Two layer model,
:RE
|-
| Lateral groundwater flow
| 
: 2-D vertically averaged
|-
| Stream/groundwater interaction
| 
: Darcy’s law
|-
| Exfiltration
| 
: Darcy’s law
|-
| Overland Erosion
| 
: Rainfall Impact
: Rill and Gully
:*Kilinc Richardson
:*Engelund Hansen
:*Shear Stress
|-
| Overland Sediment Deposition
| 
: Shield's law
|-
| Overland Sediment Routing
| 
: Transport Capacity
: 2-D Advection
|-
| Channel Routing of Fine Sediments
| 
: 1-D Advection-Dispersion
|-
| Channel Routing of Sand
| 
: Bedload according to Yang's method
|-
| Reservoir Sources of Sediment
| 
: Overland lateral flow
: Stream flow
|-
| Reservoir Routing for Fines
|
: Completely mixed reactor
|-
| Reservoir Routing for Sands
| 
: Overland sources deposit in reservoir boundary cells
: Stream sources deposit in reservoir bottom
|-
| Reservoir Fines Deposition
| 
: Uniform deposition over submerged overland cells
: Deposition according to Shield's equation
|-
| Overland Constituent Loading
| 
: Specified rainfall concentration
: Specified groundwater concentration
: Specified loading on soil surface
: Specified loading in top soil layer
: Point source loadings
|-
| Overland Constituent Uptake
| 
: First order reaction with materials on surface
: First order reaction with materials in top soil layer
: NSM reactions with top soil layer
|-
| Overland Constituent Transport
| 
: 2-D Advection-Dispersion
|-
| Overland Reactions
| 
: First Order Decay
: NSM reactions
|-
| Channel Constituent Loading
| 
: Lateral inflow from overland
: Interaction with groundwater - specified groundwater concentration
: Point source loadings
|-
| Channel Constituent Transport
| 
: 1-D Advection-Dispersion
|-
| Channel Reactions
| 
: First Order Decay
: NSM reactions
|-
| Reservoir Constituent Loading
| 
: Precipitation
: Lateral inflow from overland
: Interaction with groundwater - specified groundwater concentration
: Point source loadings
|-
| Reservoir Constituent Transport
| 
: Completely Mixed Reactor
|-
| Reservoir Reactions
| 
: First Order Decay
: NSM reactions
|}


'''Table 1''' – Processes and approximation techniques in the GSSHA model


'''GA''' – Green and Ampt (1911), GAR – Green and Ampt with Redistribution (Ogden and Saghafian, 1997), RE – Richards’ equation (1931), ADE – Alternating Direction Explicit (Downer et al., 2000), ADEPC – Alternating Direction Explicit, Predictor-Corrector (Downer et al, 2000).

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