Before creating and running continuous simulations, it may be necessary to change the global parameters. To change these parameters, click the Engine Options button in Simulation tab. The Simulation Engine window will appear.
In the Simulation Engine window, the parameters are grouped to four (4) categories, generally based on the hydrological process. Each group has a corresponding tab in the window. There is only one parameter in the Time Step group, which is the simulation time step in minutes. The default time step is 5 minutes, which may need to be changed for long-term simulations. All other parameters are described in Table 9-1, Table 9-2 and Table 9-3 for snow, initial abstraction, and soil.
TABLE 9-1 SNOW PARAMETERS FOR CONTINUOUS SIMULATION
|Snowfall Temperature||°C||The dividing temperature for snowfall and rainfall||0|
|*Rel. Density||vol/vol||Relative density of new snow which varies from 0.02 to 0.15||0.1|
|Fraction of Liquid Water||N/A||The fraction of liquid water in new snow||0|
|Compaction Coef. A||N/A|| Compaction coefficient A used in
KC = B * EXP (-A * TAIR)
|Compaction Coef. B||N/A|| Compaction coefficient B used in
KC = B * EXP (-A * TAIR)
|Max. Rel. Dry Density||vol/vol||The snow pack maximum relative dry density||0.35|
|Snowmelt Temperature||°C||The snowmelt base temperature||-0.2|
|Snowmelt Factor||mm/(day.°C)||The monthly snowmelt factor||N/A|
|Fraction Free Water Capacity||N/A||Irreducible water saturation fraction of total pore volume||0.05|
|Rel. Density of Ice||vol/vol||The relative density of ice||0.92|
TABLE 9-2 INITIAL ABSTRACTION PARAMETERS FOR CONTINUOUS SIMULATION
|Infiltration Ration in Per.||N/A||The infiltration ration in pervious area||0|
|Fraction Runoff Indirectly Imp. to Per.||N/A||The fraction of runoff from indirectly con-nected area flow to pervious area||1|
TABLE 9-3 SOIL PARAMETERS FOR CONTINUOUS SIMULATION
|Lake Evap. to Pan Eva. Ratio||N/A||Lake evaporation to pan evaporation ratio, 0.6 – 0.8||0.8|
|PET Reduction Ratio Due to Rain||N/A||PET reduction ratio to account for rain||0.1|
|Soil Storage Capacity Reduction Ratio Due to Frozen Soil||N/A||Soil storage capacity reduction ratio due to frozen soil conditions||0.12|
|Monthly Evaporation||mm/month||Monthly evaporation||N/A|
|Monthly Growth Index||N/A||Monthly growth index||N/A|
The snow parameters, Snowmelt Temperature, Snowmelt Factor and Max. Rel. Dry Density, are assumed to change in a year between a given minimum value and maximum value. The minimum value appears in each month. The change is described using a sinusoidal curve as shown in the figure below for a snowmelt factor. To edit the minimum, maximum value and the month for the minimum value, click the button on the right side of each parameter. The preview curve will be updated automatically once the values are changed. By default, a fixed value is used for Snowmelt Temperature and Max. Rel. Dry Density but Snowmelt Factor changes.
To edit the monthly evaporation and growth index, click the Edit button after the parameter name as shown in the figure below.
The monthly data editor will appear as shown in the figure below. New values can be entered in the table on the left and the plot will be automatically updated.