I have found this sentence. It said 'Whatever the final answer to the water crisis may be, it is evident that within the High Plains, irrigation water will never again be the abundant, inexpensive resource it was during the agricultural boom years of the mid-twentieth century'.
2 Answers
Whatever the final answer to the water crisis may be, it is evident that within the High Plains, irrigation water will never again be the abundant, inexpensive resource [(that/which) it was during the agricultural boom years of the mid-twentieth century].
It's fine.
Square brackets surround a relative clause modifying the noun phrase "abundant inexpensive resource". Note that "that" or "which" could be inserted, as shown. Within the relative clause, "it" refers to "irrigation water".
Yes, it is correct.
You can read it like this:
...water will never again be the ... resource which it was...