They had planned to spray water on the track via sprinklers — it would be a controlled amount of water to simulate light, medium and heavy rain (there are two types of rain tires one for light conditions and the other called: full wet).
There are new vehicle specifications this year and the rain tires are new and untried...
Hope this helps!
Yes, I know all of that. I just don’t understand why do it in Bahrain when they had test data from the new cars in Spain too. Maybe design upgrades after Spain made Bahrain a better location to compare lap times of intermediate and wet tire designs to the dry compound tire options.