Model Formulation:Processes Simulated

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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.

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
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
Penman-Monteith with seasonal canopy resistance
Soil moisture in the Vadose zone
Two layer model,
Lateral groundwater flow
2-D vertically averaged
Stream/groundwater interaction
Darcy’s law
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
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).

GSSHA User's Manual

2 Model Formulation
2.1     Processes Simulated
2.2     Time Steps and Process Updates
2.3     Inputs