You probably want to have your thirst variable in your player controller, player state or game state, depending on what you have already set up.
The level blueprint is a bit of a black box to the other blueprints, it would have to push the variable reference at the start up of the widget etc. it’s messy.
To link the thirst variable from your class (e.g. player controller) onto the widget, there is a neat Binding technique, as follow.
In the UMG you create the widget to represent thirst and then you Bind it to the, e.g., Player controller’s variable.