as it was in 1966 是一个比较状语从句.it 指the output.这里的it was 可以省略.
the irrigated area in this place is four times bigger than in 1978.这里than后面省略了it was(即省略了主语和系动词).
省略不省略it was意义没有舒适区别,不省略显得更正规,省略则显得更口语化.又如:
She is as beautiful as (she was) five years ago. 她跟五年前一样美丽.
Whe is as beautiful as (she was ) in 2003. 她跟2003年时的样子一样美丽.