A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
A player, 2 August 2013
Wide Valley: Before an action's effect destroys a card in a unit: Negate the destruction.
Is the trait of Wide Valley limited to its own battlefield?
If Wide Valley has been played, followed by Strength in the Earth, No Hiding Place, or a similar effect that allows you to generate a destruction effect against a card in a unit away from the current battlefield, such as in the Target's home, will the Target be destroyed? Or will Wide Valley prevent the destruction despite the Target existing at a different location?
A player, 2 August 2013
The trait in a Terrain isn't per se restricted to the battlefield (though some may be per card text). Wide Valley can protect cards in units at other locations.