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