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You did in fact stale last turn, as did Abysia.

Income is the default 100%. If you are used to games with income increased, it can seem a little low. Also, farmland produces the most income, with plains next, then forests/hills, and finally mountain, waste, and swamp all produce very little income. On the other hoof, the rougher terrains all are more likely to have magic sites, and Arco is exceptionally good at finding magic sites.

If you had an unrest-causing event (or overtaxed without patrolling) you can get unrest in your home province, which will cut your income dramatically. It's not 100% linear, even a little unrest causes a lot of income loss. Patrolling and cutting taxes will drop unrest, but these have their own disadvantages - patrolling kills population and ties up your troops, and lowering taxes causes a short-term revenue drop.

If you have left the taxes alone, and something in your home province causes unrest, the game will automatically drop the tax rate to get unrest back to 0. That seems like the most likely cause of a temporary income loss.

Looking at the charts, your income is exceptionally low, but it wasn't always. Your income is considerably lower than it was at the start of the game, so your unrest situation must be awful.

Especially early in the game, your home province is a huge portion of your economy, so if someone is sieging your capital, you will lose most of your income.