Thanks, therother. I am going to suggest the following topics to explore in this thread (and encourage anyone to suggest more.) Some of you out there might already know the answers to some or all of these.

1. Farming income effect of base region farming level
2. Farming income with farming upgrades
3. Harvest level effect on farming income & how to influence harvest level?
4. Culture/terrain fertility type effects on farming? (Haven't found clear evidence that any of these exist--fertility seems to apply to tile descriptions only, not regions.)
5. Strategic game difficulty level impact on farming income.
6. Oddities about build order, such as farming temple before land clearance?
7. Population size, govt. building or other unknown effects on farm level/income?
8. Total farm level impact on population growth