"The water is on top of the refrigerator."

Translation: 水在冰箱上。

[marchionesselle]

What's the difference between using just 上 or 上面？

[cagprado]

Quoting from https://www.digmandarin.com/the-usage-of-chinese-location-words.html (note 1)

When 上边/面, 下边/面, 里边/面 ,外边/面and 旁边 are modified by other nouns, the character 边/面 can be omitted. But never 左,右 ,对,中 + suffix, the suffix can be omitted if 的 is omitted.

[MauroEzequ6]

Or any other preposition +边or 面.

[isabel399732]

Just the same

[steve9788]

上means "up" and上面means "on" correct me if im wrong

[P5zG5]

Should be 水在冰箱的上