The Curve Number (CN) Value calculated by the model for this particular input is based on the assumption that the pervious area is 'grass' in 'fair' condition.

In case the pervious area is agricultural or forest for example the CN value and consequently the runoff generated will be very different. Land uses such as agriculture for eg. also generate runoff and impact the long-term hydrology of the watershed. Converting agricultural land to urban use generates less difference in the amount of runoff than if grassland or forest is converted to higher intensity uses such as commercial or high density residential.

For more details refer to the following documents: