Literature Review: 

Comparison of Common Watershed Models for Rainfall, Runoff 

Sediment Transport, and Contaminant Transport and Fate 

Prepared By: 

HydroQual, Inc. 

1200 MacArthur Boulevard 

Mahwah, New Jersey 07436 

United States 

November 2010 
ACKNOWLEDGEMENTS 

This report was prepared by Mark Velleux, Masatsugu (Masa) Takamatsu, and Lihong 

(Lucy) Yang. Portions of this report were derived from earlier works completed at 

Colorado State University. The ongoing support of Dr. Pierre Julien (Colorado State 

University) is gratefully acknowledged. 
LITERATURE REVIEW

TREX is a spatially distributed, physically-based model to simulate hydrology, sediment 

transport, and chemical transport and fate at the watershed scale (Velleux et al. 2008). It 

has been used to simulate runoff and sediment transport in mountain watersheds as well 

as extreme floods (Velleux et al. 2006; England et al. 2007). Brief overviews of 

watershed model frameworks are presented to give a context to the research that lead to 

TREX development and how TREX can be used to simulate a wide range of watershed 

conditions. 

WATERSHED MODEL FRAMEWORKS 

A range of watershed modeling methods and frameworks exist. Methods include unit 

hydrograph/lumped parameter, advanced lumped parameter/semi-distributed, and fully 

distributed, physically based approaches.1 Singh (1995) presents descriptions of 

numerous watershed models. Each approach has characteristic strengths and limitations 

and there are tradeoffs between the spatial and temporal detail used to represent physical 

processes and model performance. Although methods used differ, all model frameworks 

reviewed have the ability to simulate runoff. Some frameworks can simulate soil erosion. 

A few frameworks can also simulate stream sediment transport (erosion and deposition) 

processes. A small number of frameworks have specialized capabilities such as the ability 

to simulate chemical transport. 

TR-55, HEC-1, and HEC-HMS 

Unit hydrograph/lumped parameter models such as TR-55 (USDA, 1986; USDA 2009), 

HEC-1 (USACE, 1998), and HEC-HMS (USACE, 2010) simulate watershed runoff. 

However, physical processes that generate runoff are represented in a simplified manner. 

For example, runoff is calculated using an empirical curve number approach rather than 

 

1

 Absolute distinctions between approaches are somewhat subjective. For example, lumped parameter 

models that can treat a watershed as a number of sub-basins can be considered equivalent to semidistributed models while fully distributed models with limited spatial variation of parameters can 

approach the performance of lumped parameter models. 
physically-based equations that use conservation of mass principles to relate precipitation 

and infiltration rates to resultant runoff. In the case of TR-55, the maximum watershed 

area that can be simulated is limited to a maximum of 65 km2 (6,500 hectares; 25 mi2). In 

addition, these simple frameworks do not permit the extent of model parameter variation 

that is needed to account for the wide range of landscape and soil property variations that 

occur in complex watersheds. Importantly, these frameworks cannot simulate soil erosion 

and stream sediment transport. 

1.1.1 HSPF 

HSPF (Hydrologic Simulation Program–Fortran) (Johanson et al. 1980; Donigian et al. 

1984; Bicknell et al. 1997; Bicknell et al. 2001) has its origins in the Stanford Watershed 

Model (Crawford and Linsley, 1966) and can be classified as an advanced lumped 

parameter or semi-distributed model. Among its processes and state variables, HSPF 

simulates interception, infiltration, soil moisture, surface runoff, interflow, base flow, 

snowpack depth and water content, snowmelt, evapotranspiration, groundwater recharge, 

dissolved oxygen (DO) and biochemical oxygen demand (BOD), temperature, pesticides, 

fecal coliform bacteria, sediment detachment and transport, sediment routing by particle 

size, channel routing, reservoir routing, constituent routing, pH, nitrogen and phosphorus 

compounds, and plankton. Hydrologic processes are represented as flows and storages. 

Routing is performed using a modified form of the kinematic wave equation. Generally, 

flow is a one-dimensional outflow from storage, expressed as a function of the present 

storage amount and the physical characteristics of the subsystem. Although the overall 

model is physically based, many of the flows and storages are represented in a simplified 

or conceptual manner. This requires use of calibrated parameters but has the advantage of 

avoiding the need to explicitly specify all physical dimensions and characteristics of the 

flow system. 

HSPF has been used for a wide variety of spatial scales from the Chesapeake Bay 

watershed (160,000 km2

) to small (a few acres) experimental plots near Watkinsville, 

Georgia. The model is best used for continuous simulation of hydrology and water 

quality in watersheds. To simulate chemical transport, HSPF incorporates the watershedscale NonPoint Source (NPS) (Donigian and Crawford, 1976a) Agricultural Runoff 
Management (ARM) (Donigian and Crawford, 1976b; Donigian et al. 1977), and 

Sediment and Radionuclides Transport (SERATRA) (Onishi and Wise, 1979) models 

into a framework that includes fate and transport in one dimensional stream channels. 

HSPF simulates three sediment types (sand, silt, and clay) in addition to a single organic 

chemical and transformation products of that chemical. However, it does not simulate 

metals. HSPF (Version 11.0) source code is publicly available. 

KINEROS 

KINEROS (Kinetic Runoff and Erosion) (Woolhiser et al. 1990) is a semi-distributed, 

physically based, event-oriented model that simulates rainfall, interception, infiltration, 

surface runoff and erosion. A rainfall record is used to describe the spatial and temporal 

distribution of rainfall to simulate runoff for a catchment. A watershed is represented by a 

(cascading) series of planes and channels (open channels or pipes). The kinematic wave 

approximation and the Manning equation are used for one-dimensional flow routing. 

Infiltration is determined using the Smith-Parlange (1978) equation. KINEROS does not 

include groundwater flow processes other than infiltration and overland flow is 

Hortonian. Erosion overland is represented by splash and hydraulic rate components and 

a transport capacity relation of the form summarized by Julien and Simons (1985), such 

as tractive force (Meyer and Wischmeier, 1969), unit stream power (Yang, 1973), Ackers 

and White (1973), and Engelund and Hansen (1967). Erosion in channels is similar with 

the exception that splash erosion is neglected and lateral inflow is not. Deposition is 

computed from particle fall velocity for the particle concentration in excess of the 

concentration computed from the transport capacity equation. Sediment transport is 

represented for a single effective grain size (d50) that can vary by element in the model 

domain. KINEROS (Version 2) source code is publicly available. 

SWAT 

SWAT (Soil and Water Assessment Tool) (Neistch et al. 2002) is an advanced 

lumped/semi-distributed, physically based, continuous simulation model that simulates 

rainfall, infiltration, flow routing through basin streams and reservoirs (including lateral 

flow, groundwater flow, and transmission losses), and sediment and chemical transport 

through ponds, reservoirs, and streams. Simulated watersheds can be divided into sub-
basins or hydrologic response units. SWAT was developed to predict the impact of land 

management practices on water, sediment and agricultural chemical yields in large 

complex watersheds with varying soils, land use and management conditions over long 

periods of time. Major components model include weather, hydrology, erosion, soil 

temperature, crop growth, nutrients, pesticides, subsurface flow, and agricultural 

management. Instream chemical transport in SWAT is based on algorithms adapted from 

the QUAL2E (Brown and Barnwell, 1987) stream water quality model and includes firstorder decay relationships for algae, dissolved oxygen (DO), carbonaceous biochemical 

oxygen demand (CBOD), as well as nitrogen and phosphorus compounds. However, it 

does not simulate metals. Bian et al. (1996) linked SWAT with the Arc/Info geographic 

information system (GIS). 

Soil profiles can be divided into ten layers. Infiltration is computed as precipitation minus 

runoff (SCS curve number approach) or using the Green and Ampt (1911) relationship. 

Infiltration moves into the soil profile and is routed through the soil layers. A storage 

routing coefficient is used to compute flow through each layer, with flow occurring when 

a layer exceeds field capacity. When water percolates past the bottom layer of the soil 

profile, it enters the shallow aquifer zone (Arnold et al. 1993). Channel transmission 

losses and pond/reservoir seepage replenish the shallow aquifer layer, which can interact 

directly with the stream. Flow to the deep aquifer system is lost and cannot return 

(Arnold et al. 1993). An irrigation algorithm allows water transfers from any reach or 

reservoir to any other in the watershed. Based on surface runoff calculated using the SCS 

runoff equation, excess surface runoff not lost to other pathways is delivered to the 

channel network and is routed downstream. Sediment erosion for stream transport is 

determined with the Modified Universal Soil Loss Equation (MUSLE) (Williams, 1975). 

For sediment routing, deposition is based on fall velocities for various sediment sizes. 

Channel degradation rates are determined using the Bagnold (1977) stream power 

equation. Sediment size is estimated from the primary particle size distribution (Foster et 

al. 1980) for soils the described in the STATSGO database (USDA, 1991). Stream power 

is also used in the sediment routing routine to calculate re-entrainment of loose and 

deposited material in the system until all of the material has been removed. SWAT is a 

continuous time (long-term yield) model and is not designed to simulate detailed, single-
event flood routing (Neistch et al. 2002). SWAT (Version 2000) source code is publicly 

available. 

SHETRAN and MIKE SHE 

SHETRAN (Ewen et al. 2000; Ewen et al. 2002) has its origins in the Système 

Hydrologique Europèen (SHE) (Abbott et al. 1986a,b) and is a fully-distributed, 

physically-based model. Among its processes and state variables are interception, 

infiltration, surface runoff, groundwater flow in saturated and unsaturated zones 

including aquifers, snowpack and snowmelt, evapotranspiration, channel transmission 

losses, sediment erosion and deposition for multiple particle classes, and chemical 

transport and fate including advection, dispersion, erosion, deposition, partitioning, and 

decay. For surface waters, flow routing is performed using the diffusive wave 

approximation and is two-dimensional overland and one-dimensional in channels. For the 

subsurface, flow is variably saturated and three-dimensional. 

SHETRAN can be applied to a wide variety of spatial scales from large (2,500 km2

) 

contiguous river basins with multiple sub-basins, to single or partial watersheds, to 

individual hillslopes and catchments (0.94 km2) (Bathurst et al. 2004). However, most 

applications summarized in the literature are for small catchment areas (<2 km2

). 

Sediment transport processes in SHETRAN are derived from those in the earlier 

SHESED framework (Wicks and Bathurst, 1996). Overland erosion is computed based 

on raindrop impact and hydraulic erosion rates similar to the approach used in 

KINEROS. Channel erosion is computed using the Ackers and White (1973), Engelund 

and Hansen (1967), and Day (1980) (a modification to the Ackers and White approach) 

methods. Unfortunately, SHETRAN model source code is not publicly available. 

MIKE SHE is a commercial software package that is licensed and distributed by DHI 

(2007). It is based on the SHE family of watershed model frameworks. In concept, MIKE 

SHE allows simulation of the full suite of hydrologic processes in a spatially-distributed, 

physically-based modeling system. The basic framework includes MIKE SHE WM 

(Water Movement). Additional add-on modules are available to simulate soil erosion and 

water quality, including: SE (soil erosion), AD (advection and dispersion of solutes), GC 

(geochemical processes), and CN (crop growth and nitrogen processes in the root zone). 
Because it is commercial software, MIKE SHE has a very high initial purchase price 

(approximately $13,000 to $22,000 USD) with additional annual licensing fees. Add-on 

modules increase initial purchase and annual licensing costs. 

SMDR 

SMDR (Soil Moisture Distribution and Routing) is a spatially-distributed, physicallybased model designed to simulate the location and spatiotemporal evolution of variable 

source areas that generate saturation excess runoff (SER) (Cornell, 2003). SER occurs 

when the topmost soil layer becomes saturated and water cannot infiltrate. This process is 

believed to be an important flow-generating process in humid, well-vegetated regions that 

have steep to moderate slope terrains with shallow, high infiltration capacity soils. SMDR 

uses daily precipitation as input and includes snowmelt and evapotranspiration as model 

processes. Drainage (vertical) in soil layers is calculated using the 1-D Richards equation 

in which hydraulic conductivity varies as a function of water content and where capillary 

effects are neglected. Subsurface (lateral) flow in soil layers is calculated using Darcy’s 

Law in which transmissivity varies over time as a function of water saturation in each soil 

layer. However, surface runoff routing is not physically based; overland flow is assumed 

to move from each cell where runoff occurs to the watershed outlet in a single time step 

(Cornell, 2003; Veith et al. 2003). This overland flow routing approach neglects physical 

processes such as depression storage, the possibility of infiltration along the runoff flow 

path, and overland flow resistance (Veith et al. 2003). 

CASC2D 

CASC2D (including CASC2D-SED) (Julien and Saghafian, 1991; Julien et al. 1995; 

Johnson et al. 2000; Ogden and Julien, 2002; Rojas, 2002; Julien and Rojas, 2002) is a 

fully distributed, physically based, event-oriented model that simulates rainfall, 

interception, infiltration, overland flow, channel flow, as well as sediment erosion and 

deposition. For surface waters, flow routing is performed using the diffusive wave 

approximation and is two-dimensional overland and one-dimensional in channels. 

CASC2D does not include groundwater flow processes other than infiltration and 

overland flow is Hortonian. However, it can be directly coupled with GIS-based site 

characterization data obtained from remote sensing sources. 
CASC2D has been applied at a wide variety of spatial scales from large river basins 

(12,000 km2) to moderate watersheds (560 km2) (Molnár and Julien, 2000) to small 

watersheds (20-30 km2) (Rojas, 2002). Overland erosion is computed using a modified 

form of the Kilinc-Richardson (1973) method (Johnson et al. 2000). Channel erosion is 

computed using the Engelund and Hansen (1967) method. Up to three solids classes can 

be simulated (Rojas, 2002). Chemical transport and fate is not simulated. CASC2D 

source code is publicly available. 

GSSHA 

GSSHA (Gridded Surface/Subsurface Hydrologic Analysis) is a spatially-distributed, 

physically-based hydrologic model (Downer and Ogden, 2004). It is derived from 

CASC2D. GHSSA simulates stream flow generated by both infiltration-excess (rainfallexcess) and saturation-excess mechanisms, as well as exfiltration and groundwater 

discharge to streams. To represent these additional flow-generating mechanisms, GHSSA 

includes hydrologic processes beyond those found in CASC2D. A 1-D (vertical) headbased form of Richards Equation (Richards, 1931) is used to represent downward water 

movement through the vadose (unsaturated) zone and a 2-D (lateral) free surface 

(unconfined) groundwater model to represent water movement in the saturated zone. This 

allows the rise and fall of the water table to be simulated and coupled to infiltration at the 

land surface. Groundwater exfiltration and discharge to streams are calculated using 

Darcy’s Law. Sediment transport capabilities implemented in GSSHA are essentially the 

same as those in CASC2D (Downer and Ogden, 2008). A nutrient submodel to permit 

simulation of nitrogen, phosphorus, and carbon cycling also exists (Johnson and Gerald, 

2006). However, source code for GSSHA is not publicly available. 

TREX 

TREX (Two-dimensional Runoff, Erosion, and Export) is a spatially-distributed, 

physically-based model that can be used to simulate precipitation, overland runoff, 

channel flow, soil erosion, stream sediment transport and chemical transport and fate at 

the watershed scale (Velleux et al. 2008; England et al. 2007; Velleux et al. 2006; 

Velleux, 2005). TREX combines surface hydrology and sediment transport features from 

the CASC2D watershed model (Johnson et al. 2000; Rojas, 2002; Julien and Rojas, 2002) 
with chemical transport features from the WASP/IPX series of water quality models 

(Ambrose et al. 1993; Velleux et al. 2001). 

Hydrologic processes simulated are: (1) rainfall and snowfall (precipitation), interception, 

snowmelt, and surface storage; (2) infiltration and transmission loss; and (3) overland and 

channel flow. Model state variables are water depth in the overland plane and stream 

channels. Precipitation can be uniform or distributed in both time and space and can also 

be specified using several grid-based formats to facilitate radar precipitation data use. 

When spatially distributed precipitation is simulated, areal estimates are interpolated 

from point gage data using an inverse distance weighting approach. Interception and 

surface storage are simulated as equivalent depths. Infiltration and transmission loss rates 

are simulated using the Green and Ampt (1911) relationship. Overland flow is twodimensional and simulated using the diffusive wave approximation. Channel flow is onedimensional and is also simulated using the diffusive wave approximation. 

Sediment transport processes simulated are: (1) advection-diffusion; (2) erosion and 

deposition; and (3) bed elevation adjustment. All processes are simulated in the overland 

plane and stream channels. Model state variables are solid concentrations in overland 

runoff, soil, stream flow, and stream bed sediment. Any number of particle size classes 

can be simulated. 

Chemical transport and fate processes simulated are: (1) chemical partitioning and phase 

distribution; (2) advection-diffusion; (3) erosion and deposition; (4) infiltration and 

transmission loss; and (5) mass transfer and transformation (chemical reactions). Model 

state variables are chemical concentrations in overland runoff, soil, stream flow, and 

stream bed sediment. Any number of chemicals can be simulated. 

In floodplain areas, water and transported constituents are transferred between the 

overland plane and channel network based on the difference in water surface elevations. 

Floodplain transfers are bi-directional. Water and transported sediments and chemicals 

move into stream channels by overland flow and can return to the overland plane when 

water levels in the stream exceed bank heights. Similarly, materials can be moved from 

the sediment bed and can be delivered to the land surface by floodwaters. 
TREX source code, a user manual, reference material, and example files are freely 

available and can be downloaded from the following URL: 

http://www.engr.colostate.edu/~pierre/ce_old/Projects/TREX%20Web%20Pages/TREXHome.html
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