In practical terms, if you're caught out in the rain and run through it, it's most likely that your hemp backpack gets splashed upon but it absorbs the water so your items inside are safe.
If the rain is heavier, then, well, run for cover!
Can you wash a hemp backpack? Yes.
The fibers in hemp become stronger in water because it helps them tighten up.
The trick to prevent hemp bags from shrinking is to wash them in cold water. Now don't go running to the next river to have your hemp back cleaned up. All you need to do is set the washing machine on cold - it's usually symbolized as a star. If you live in a place where there are some months in which the water is way too cold, go with the lowest temperatures possible.
Once the washing is ready and while still wet, the trick is to give them a good old tug to open the bag into its usual extended shape.
Try avoiding putting it in the dryer. It usually dries up pretty quick anyway.