But I don't see how it would be a wise policy to let your surplus decrease down to zero. It seems to me that keeping your granaries full and topping them when they decrease to less than, let's say, three weeks of storage, would make sense.
I wonder how many historical medieval cities had such low stores of food. We have stories of year-long sieges, so clearly it wasn't impossible.
Obviously, the plan is not to purchase food once a year, for the whole year. But a capacity for a year's worth without imports is an arbitrary safezone that would allow to easily overcome embargoes and major food loss from trade partners due to conflict or bad harvests.
