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.

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).