What is the long vowel in dehydrated?

The long vowel in "dehydrated" is "e" in the first syllable, "de".

Here's why:

* "e" in "de" has the long "e" sound, as in the word "see."

* The other vowels in "dehydrated" are short vowels:

* "i" in "hy" has the short "i" sound, as in "hit."

* "a" in "dra" has the short "a" sound, as in "cat."

* "e" at the end is silent.