A good margrave will always have stockpiles. Assuming a 10 day margin. Since no one else seems to be doing the math, I'll show you what I mean. Seasons last 21 days. Let us consider rot for 1 year, 84 days. Let's assume that rot is 1% per day, we're a city with 20000 peasants, so a consumption of 40 per day. Each day new food arrives in proportion to the season. Thus with no season we assume 40 bushels arrives every day. With seasons, spring 30 bushels, summer 40 bushels, autumn 80 bushels, and winter, 10 bushels.
Total consumption over a year is 3360 bushels.
Without seasons:
We start with 400 bushels.
Every day we keep 10 * 40 + 40 - 40 = 400 bushels after consumption. 4 bushels rot every day for a total of 4 * 84 = 336 bushels.
With seasons:
We start with 400 bushels.
Spring - For 21 days, first day: 400 * 1% = 4 bushels rotten, 400 + 30 - 40 = 390 left. Similarly, after 21 days 190 bushels remain, and a total of 63 bushels lost to rot.
Summer - For 21 days, 190 bushels left, 40 production 40 consumption, 190 * 1% * 21 = 40 bushels rotten.
Autumn - For 21 days, first day: 190 * 1% = 1.9 bushels rotten, 190 + 80 - 40 = 230 left. Similarly after 21 days, 1030 bushels remain, and a total of 128 bushels lost to rot.
Winter - For 21 days, first day: 1030 * 1% = 10.3 bushels rotten, 1030 + 10 - 40 = 1000 left. Similarly after 21 days, 400 bushels remain, and a total of 150 bushels lost to rot.
63 + 40 + 128 + 150 = 381 bushels lost to rot, 45 bushels more than with seasons, over an entire game year, almost 3 months of real life time. A whopping 13% more rot than without seasons, and 1.3% of the year's consumption.
Please tell me how 1.3% more consumption averaged per day cripples your city.
Note: If you increase the original parameters by any factor, like multiply by 1.5 since your city has 30000 population, you'll get the same result. If you feel more comfortable to start out with a bigger stockpile, be my guest, and multiply that 400 by anything like 2, or 0.5 if you feel you don't need the stockpile, if you prefer 20 days of stockpile, you'll get the same result. It's 1.3% more consumption on average due to rot by seasons however you calculate it, for those interested.