You were never really famous for it, though. When I think Scandinavia I think cod rather than cabbage

Anyway, to continue on the whole irrigation thing.
Every plant has certain optimal growing conditions. They vary from plant to plant, and even in the same plant depending on where it is in it's life cycle, or even on the time of day (their metabolism changes according to presence of sunlight). Any deviation from these optimal conditions will naturally have a negative impact on the plant, though it's usually mild since most crops are fairly sturdy. In nature, all the conditions fluctuate, thus while they may always be close to the optimum they are not *at* the optimum constantly, so there is room for improvement of the growing conditions. Water levels in the soil are one of the more important variables. Irrigation ditches serve to both add and take away water, depending on what's needed. This is accomplished simply by maintaining constant water level in the ditches - if the water in the soil is higher it'll drain into the ditches, and if it's lower the water from the ditches will flow into the soil.
Now, while extensive irrigation may never have been present in Scandinavia, neither were top hats, so let's just ignore realism arguments. The real question is whether large-scale irrigation projects would be good gameplay, and IMO the answer is yes.