Hydrology Modeling in Alaska: Model Documentation Template
(please fill out as much as possible)
Your name: Anna Liljedahl
Model name: WaSiM-ETH (Water balance Simulation Model –ETH)
Authors: Jörg Schulla
Source code location (if public):
http://www.wasim.ch/en/index.html
Citations and URLs for basic documentation:
Schulla, J. and K. Jasper (2007), Model description WaSiM-ETH, IAC ETH
Zurich, 181 pp. Available at www.wasim.ch/products/wasim_description.htm
Schulla, J. (1997), Hydrologische Modellierung von Flussgebieten zur
Abschatzung der Folgen von Klimaanderungen. Zurcher
Geographische Schriften, Heft 69, ETH Zurich, pp. 187 (PhD Thesis)
Source code language:
C++
For the following section, you may wish to use appropriate keywords such as:
Physically-based, statistical, lumped parameter, spatially distributed, transitive
model, equilibrium model, implicit, semi-implicit, explicit, TOPMODEL based,
finite element, finite differences, routing, bottom boundary condition, parallel
code, Richardson equation, optimization, forecast, etc
Model type and/or conceptual framework:
WaSiM-ETH version 1: Based on a modified TOPMODEL approach; semiphysically-based; semi-distributed; parallel code
WaSiM-ETH version 2: Richardson equation for the unsaturated zone integrated
with a multi-layered 2D-groundwater model; physically based; spatially
distributed; parallel code (open MP).
Data needed to run the model (inputs):
Required: Air temperature, precipitation, sunshine duration (unless solar radiation
or net radiation is provided), digital elevation model, vegetation and soil maps.
Optional: Solar radiation/net radiation, humidity/vapor pressure and wind speed.
Parameters and how they are derived:
>20, for example; albedo, leaf area index, precipitation correction, threshold
temperature for snowmelt, and resistances of given vegetation types. Many
parameters can be derived from literature or be fitted from field measurements.
In WaSiM version 2 the hydraulic properties of soil can either be defined by a)
“look-up-tables” which only requires information of saturated hydraulic
conductivity or b) by applying the Van Genuchten [1976] method.
Spatial element used to lump inputs and outputs:
A grid, which resolution is based upon the resolution of the DEM or the network
density of the meteorological stations. The soil map and vegetation cover grid
must have the same grid cell size as the DEM.
Sub-models (i.e. snow or ground thermodynamics):
Static active layer algorithm, snow accumulation (1-D), snow melt (temperature
index or temperature-wind-index or combination approach based upon Anderson
[1973] and Braun [1985]). The active layer model is described by:
The number of snow free days is calculated on a cell by cell basis. Snow-free
days are defined by a snow cover below a threshold snow water equivalent
(SWEmin). To account for short periods of snow accumulation during the thawed
period, a period of up to nsc,min days will not be considered for refreezing but also
not for thawing. If the number of consecutive days with a snow cover above the
threshold SWEmin is longer than the number given in nsc,min then the soil is
assumed to be frozen again and dthaw and nsf is reset to 0.
Additional sub-models: Radiation correction, precipitation correction, input data
interpolation, evapotranspiration (soil evaporation and plant transpiration), glacier
melt and runoff (incl. routing within the glacier), interception, performance criteria
runoff statistics, lake, irrigation and pumping, tracer and salt transport, multilayered vegetation, plant phenology, macropore runoff, and silting-up. A dynamic
time step control algorithm is implemented in the Richard’s model. Also, WaSiM
can be one-way-coupled to a climate model and downscale the climate model
simulations. WaSiM can be coupled to an external model allowing online data
exchange. An example of the latter was a coupling to the groundwater model
PCGEOFIM.
Rainfall/runoff transformation mechanism:
See below
Runoff routing within spatial elements and to basin outlet:
WaSiM-ETH version 1 (TOPMODEL approach):
The modeling of the soil water balance and runoff generation is done using a
modified variable saturated area approach after Beven and Kirkby [1979]
extended by capillary rise and interflow. Baseflow - generated for an entire basin
as an average value based upon topographic index, saturation deficit and a
recession parameter; Interflow - generated for each grid cell separately and then
averaged over space and requires a sufficient slope and a completely saturated
soil; Surface runoff - routed to the sub-basin outlet using a subdivision of the
basin into flow time zones. Surface runoff is the sum of snowmelt runoff, runoff
due to infiltration excess and runoff from saturated areas. The discharge routing
is based on hydraulic calculation of flow velocities using the kinematic wave
approach.
WaSiM-ETH version 2 (Richard’s approach):
Numerical solutions are used to represent vertical fluxes, interflow and
infiltration/exfiltration in rivers. Vertical fluxes - A 1-D finite difference scheme is
applied to the unsaturated zone, where the upper boundary condition is
estimated after Green and Ampt [1911] and the lower boundary is the
groundwater table that is constant for each time step but variable in time.
Surface routing - Four options; a) multiple/single flow path(s) based upon DEM
(static flow direction) and b) multiple/single flow path(s) based upon water table
(dynamic flow direction).
Channel routing: Translation-retention approach using hydraulic parameters for
pre-defined channel cells.
Method for including sub-grid scale processes:
WaSiM-ETH version 1 (TOPMODEL approach):
Spatially distributed soil moisture is calculated using the topographic index for
each grid cell opposed to modeling classes of similar indices as in the original
TOPMODEL.
Resolution (possible & prudent):
Meter to kilometers (geographical), down to centimeters (soil column), hourly to
monthly.
Method of deriving topography:
Digital elevation model
Calibration approaches:
The most important parameters are usually the parameters of the soil model,
where Version 1 (TOPMODEL approach) requires calibration of nine while
Version 2 may be calibrated on three soil parameters. It is recommended to use
at least measured runoff for the model calibration. In addition, soil moisture,
groundwater tables or snow water equivalents is advantageous in the calibration
process. A detailed description of how to approach the calibration is given in
Schulla and Jasper [2007].
Treatment of frozen ground:
Reduced/limited hydraulic conductivity.
Publications using this model:
Over 30 peer-reviewed publications.
Publications, dissertations and reports prior to 2008 are available at
http://www.wasim.ch/en/dialog/publications.htm
Strengths and Weaknesses in Alaska applications:
- No published applications to permafrost regions
- No dynamic representation of soil thermal regime (fixed ground heat flux)
- Requires multiple parameters
+ Multiple options of varying complexity in representing a hydrological process
(i.e. evapotranspiration, snow melt, sub-surface hydrology)
+ Air temperature and precipitation is the only required meteorological input data
+ Plant physiological control on evapotranspiration
+ The unsaturated zone is represented by the Richard’s equation
+ Can represent storage and spilling from lakes
+ Includes a simple active layer algorithm
+ Includes a glacier hydrology model
+ Could be coupled to a snow distribution model and a soil thermal regime model