How to prevent water from freezing

In one of my worlds I built a house on the top of a mountain. When starting to build a wheat farm, I found out that the water keeps freezing, and I have to break the ice blocks:

Does wheat grow even if it is near ice instead of water? If not, how can I prevent the water from freezing?

Any form of light except sunlight will melt nearby ice. Specifically, ice will melt in a light level of over 11, so torches will melt ice 3 blocks away.

Placing a block anywhere above the water will also stop it from freezing (although not melt it if it is already frozen), so long as the water is not exposed directly to the sky.

Ice will not hydrate farmland. However water has a 9*9 area in which it hydrates farmland, so you don't need a water source next to each crop.

Also, crops can grow even on dry farmland, just at a slower rate.

Only full water blocks will freeze. However, flowing water will still hydrate nearby crops. So you can place a single block of water at one end of your trench, and it will hydrate all your crops, and it will never freeze because of the torch.

A pumpkin torch placed under a water block will also prevent it from freezing.

An added benefit of this system is that any items which fall in the water will be swept to one end for easy collection.

Taken to the logical conclusion you get this:

Only the four full water blocks (two in the middle of each row) can possibly freeze, and the pumpkin torches underneath them prevent that. The normal torches are only to prevent monsters spawning. The stone paths prevent animal spawns and are sunken so that you can wall-strafe to quickly harvest without accidentally walking on the crops.

This isn't a space-efficient or material-efficient farm layout, but once built it is very time-efficient and low-maintenance.