I thought on the drought/dry climates.. Perhaps you could have another type of farm that is to grow drought resistant crops.. Where you get more food when the water is lower.. and they don't do as well when there is to much ground water..
Perhaps a fruit tree?.. These apparently are naturally drought resistant:
Fruits: apricots, date plams, avocado, carob, mulberries, figs, grapes, peaches, pomegranates, olives, goji berries and prickly pear
Maybe this would let you have dry climates with out meaning farms in those climates are useless..
This is a good idea. I had thought about something similar before; make wheat and vegetables have lower ground water levels. Rice likes to be literally covered in water, like a overflowing marsh, so I assume that would be 90-100% ground water. I read http://www.fao.org/nr/water/cropinfo_wheat.html
and it seems like 15-90% would be good. After all I though wheat was supposed to grown in 'dry' fields, so they shouldn't be tolerate mud.
Expanding upon what Caprontos said, you could add two more farm types for very dry areas that don't yield as much but need less water. That way there are three tiers of crops. I know dates are widely grown in the middle east and I think that they are a great suggestion. After looking around I found sorghum and maize are two widely grown desert crops, but that sorghum was more natural. People know what maize (corn) is, and using sorghum might confuse some.
While writing the article, I came across an article http://www.fao.org/nr/water/cropinfo_soybean.html
that said "Soybean is often grown as a rotation crop in combination with cotton, maize and sorghum" and continued to talk about how soybean should not be overly exposed to water. Because of this, I don't think that soybeans is a very good option for the wet alternate farm type.
Caprontos said fruit trees could be the dryer replacement, and although his examples were excellent, when I think of fruit trees I think of citric fruits which require lots of water. Mangrove
So maybe the crops could be:
Dates yield 12 food and can be grown in 5-55% ground water
Maize/Sorghum yields 8 food and can be grown in 5-45% ground water
Soybean (because you already had the image for it, it could go under this and damp) yields 8 food and can be grown in 15-60% ground water
Wheat yields 16 food and can be grown in 15-90% ground water
Potatoes yield 10 food and can be grown in 30-80% ground water
Rice yields 16 food and can be grown in 55-100% ground water
Citrus Fruit yields 8 food and can be grown 65-100% ground water
I have tried to make the food growing balanced, yet it would probably need a different groundwater system to work. Also, you should be able to plant all types of plants, no matter where it is. You can plant rice in the desert in real life, you should be able to in the game. Either way it shouldn't grow anything, but you can still try. This would make ground water a more important part of the game (and add more of the options I was talking about earlier). I will think about what an updated ground water program would be like, but I don't really program so I am not sure how to do this.