Art Sample: Mr. Bucket, the Autobucket. (Patreon)

When you install the water tower, that won't be enough to automatically water your farms (though it will give you a free Mr. Bucket to install where you like). To regulate your fields, you'll have to upgrade your Well to a Mr. Bucket, which will then make sure that water on any given tile doesn't fall below a certain amount in every field in his part of the farm. If you wish him to be able to perfectly regulate the water, even reducing it when it gets above a certain amount, then you will need to further upgrade him to Mr. Bucket Neo, which is only possible if you're in the good books of the local wizard Giev.

At the moment, the need for this is fairly trivial, because all plants have the same values in water range. Eventually though there'll be plants that grow better or worse depending on the season, the amounts of water they need varying from day to day.