Work Flow of Point Source and Reservoir Monthly/Daily Discharge File Generation

When reservoir outflow is simulated with measured daily/monthly outflow, a discharge file (.day/.mon) would be generated. The file would be overwritten when the simulation is setup with the given simulation period. For those who don’t use ArcSWAT/SWAT Editor to run the model, remember to setup simulation before running the model if there are reservoir or point source in your watershed and the daily/monthly discharge method is used.

The data flow of reservoir discharge data in ArcSWAT/SWAT Editor is described below. Point source discharge data (.DAT) also follows exactly the same data flow.

1. Prepare the original discharge data in dbf/txt format

Daily

“DATE”, “RESOUTFLOW”

1/1/1978,0.00

1/2/1978,0.00

Monthly

“Year”,”Resout1″,”Resout2″,”Resout3″,”Resout4″,”Resout5″,”Resout6″,”Resout7″,”Resout8″,”Resout9″,”Resout10″,”Resout11″,”Resout12″

1990,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0

1991,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0

Note: The comma in the column name is a must. You may expect an error message shown below if they are missing.

2. Import the discharge data in ArcSWAT/SWAT Editor. The discharge data would be imported to timeseries data in project mdb. Note that TSTypeID is 0 and the time is same as the original discharge file. The data here should cover the future simulation period.

3. Re-write .Res/.Lwq, which would re-write all RES file and possible MON/DAY file. Note that the data in the generated DAY file doesn’t started from Jan 1, 1990 (in which day the discharge is 0.07 m³/s) as the simulation period hasn’t been set. ArcSWAT/SWAT Editor may set an arbitrary starting and ending date here to make the time range big enough (like from 1/1/1000 to 1/1/3001). This is not the final version used in simulation.

.DAY

Daily Reservoir Outflow file: .day file Subbasin:40 9/25/2014 12:00:00 AM ArcSWAT 2012.10_1.15

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

4. Setup Simulation, the MON/DAY file would be overwritten again to extract discharge data fell between the starting date and ending date from timeseries datatable. Now the data in DAY file starts from Jan 1, 1990.

.DAY

Daily Reservoir Outflow file: .day file Subbasin:40 9/25/2014 12:00:00 AM ArcSWAT 2012.10_1.15

0.07

0.07

0.07

0.07

0.07

0.07

0.07

0.07

0.07

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Default CN2 in MGT

A default CN2 is defined in mgt file as shown in following examples.

49.00 | CN2: Initial SCS CN II value

Question 1: Where does this default CN2 come from?

The default CN2 comes from either crop table or urban table of SWAT2012.mdb. Four columns (CN2A, CN2B, CN2C, CN2D) are defined in table crop/urban, which are CN2 values for soils with hydrological group A, B, C and D respectively. HRU is a combination of unique landuse, soil and slope. From the soil type, the hydrological group is obtained from usersoil table. And then depending on landuse type, the CN2 value is read from table crop or urban.

Urban is quite unique compared to crop. The default CN2 is just for pervious surface. For impervious surface in urban area, URBCN in table urban would be used. Its value is usually 98.

Question 2: What’s the impact of the default CN2 on model results?

The CN2 in mgt file is the initial CN2 for the HRU. If the CNOP is NOT defined in plant/harvest/tillage operations, the curve number used in infiltration calculation would just depends on the default CN2 and soil moisture. In this case, the default CN2 would have big impact on infiltration and surface runoff and further on flow discharge. That’s why this is usually the main calibrated parameter.

However, the default CN2 could be changed to CNOP if it’s defined in any plant/harvest/tillage operation, where the default CN2 would be only the initial value and would only have impact before it’s changed. In this case, it’s the CNOP we should calibrate rather than CN2.

So, for some scenarios which focus on tillage or crop change, it’s very important to set CNOP.

Question 3: What if the default CN2 is 0?

SWAT model would check the default CN2 and make sure its value is between 35 and 98. See following codes from readmgt.f.

if (cn2(ihru) <= 35.0) cn2(ihru) = 35.0

if (cn2(ihru) >= 98.0) cn2(ihru) = 98.0