In this time frame it would be better to work attrition as infrastructure damage (thinking "Logistics Strike" mission from HOI)
I'd work it like this:
Unit Limit (Base); as now represents the tipping point between how many troops the province can maintain on its own with regular harvests, etc, etc.
In addition you have the Yearly Unit Limit (which is 12*Base Unit Limit)...every month you subtract the unit strength present in the province...if you use up the Yearly Unit Limit you start taking attrition.
Additionally, the province would get the "Looted" attribute (as indeed, this reduction of Yearly Base would occur in looted situations from other actions/events)
The Yearly Base would get "refilled" to max in the harvest months (August/September/October...33% each...or just 100% in September)...if the province still carries the "looted" token then refill would be halved. Otherwise on non-harvest months the amount would refill at a lesser rate based on game, etc. of lets say 10% of base (doubled for those provinces with animal resources)
This could be factored into calculating max length of a siege also...assume a garrison has the equiv. of three to six months (randomly determined) of the Yearly Unit Limit stored for their garrison size; if the province is unlooted before the siege begins assume that they gather another zero to six months (random and dependent on how much remains of the province's yearly supply)...you can assume they destroy an equal random amount in the process. Assume each population point is equivalent to 10k soldiers, and then add the garrison size...divide into the calculated amount of food taken into stores and round up...this is the number of months the garrison and population will hold out.
Adding some military technologies, such as fortress depots which could provide a monthly bonus in that province and adjacent province (especially cool if the bonus is based off the capacity of the province the depot is located in...and increasing technology increases the bonus (i.e., start at 5% and work up to 15% of the located province's base yearly limit)
Add a similar naval technology which would provide an absolute number base amount of supply bonus (i.e., 1k supply for 1 ship)...drawn from the nearest friendly provinces (the technology would increase the distance allowed...at the first step it would require your ship to be next to a friendly province to draw from it...at a later step it could be 1 sea province away from your friendly province)...this "transfer" would be given to friendly units in land provinces adjacent to your fleet(s.) This would then tie the fleets into their roles of the time to parallel and help supply your land units in the field.
While this may all sound very complex, most of it would be "hidden" from normal play view. The addition of the yearly amount after the base amount of Unit Limit in the province display (i.e. 10/120, a 10 base with max of 120 available.) --- a "Supply Overlay" with graphical representation of the current (or known current) supply status of provinces would be good too.
