I know it sounds silly when talking about Willows and too
much water but this could be the problem. If there is so much water in the ground that it is sitting in water this can produce anaerobic growing conditions that mean your Willow is waterlogged and the roots may be rotting.
If this is not the case then check for fungus at the base of the trunk. They are prone to honey fungus infection. Any plant infected will do an amazing show before showing symptoms of stress.
Have you checked for pests? The only pest that causes leaf yellowing is the lace bug. Usually found under the leaf.