Changing the way the water make arable land.

It may be too complicated to do, would unbalanced the game during drough or too much for the CPU. Idk.

For now if a tile is up to 16 tiles from a tile with water at the same level, it goes green. Some malus with height.

The idea :

If a dirt tile is touching water (in the corners too), it go green (so 9 tiles green around 1 tile of water).
Each dirt tile would have a new "moisture" attribute, 0 to 100%.
A dirt tile touching a water tile will have its moisture % going up instantly to 100%.

For the other dirt tiles :
When the moisture % reach a threshold, say 20%, it goes green.
When reaching another threshold, say 50%, it allows adjacent dirt tiles to gain moisture.
And so on.
You could set up an evaporation rate for the moisture.
You could set up different moisture transfer rate between dirt tiles, depending on tiles moisture level.
When the evaporation rate is greater than the transfer rate, you'll have your green limit.