How did the wicked witch of the west become to be on fire?

I have checked the script for the answer and it seems the Witch hurls a fire ball at the scarecrow and then Dorothy throws a bucket of water over him also covering the witch which melts her.

Please see the script below for this dialog:

Scarecrow

Hurry, we've got no time to lose!

Wicked Witch

Ha, ha, ha! Going so soon? I wouldn't hear of it. Why, my little party's just beginning.

Ha, ha, ha, eh, eh!Ring around the Rosey! A pocket full of spears! Well,

the last to go will see the first three go before her! And your mangy little dog, too!

How about a little fire, Scarecrow? Huh?

Scarecrow

No! No! No! No! Help! I'm burning! I'm burning! I'm burning!

Wicked Witch

No, don't throw that water! Ohhhhh! You cursed brat! Look what you've done! I'm melting!

Melting!

Leader of Winkies

She's - she's dead. You've killed her. Hail to Dorothy! The Wicked Witch is dead!