have had 一問

在你句子中的have had 無可爭議是個現在完成式的虛擬詞氣(present perfect
tense in subjunctive mood), 具' 曾經發生過' 的意思。所以整句可以改寫為
What impact irrigation may have occurred, happened or taken place on the
land-use pattern ? 問題就只是關乎到個 may 字而已。
may 是個動詞, 你也一定知是解作' 可以 ',' 可能 ' 之意, 但它可作主動詞(principal verb or main verb),亦可作助動詞(auxillary verb)。
作主動詞(may--- used as a principal verb)：
1) It may rain.=It is possible that it will rain.
2) It may be true.=It is possibly or probably true.
3) You may go = You are permitted to go.
4) She may be right.等句子中的may字有本身的意義, 所以是主要動詞。
作助動詞(may---used as an auxillary verb)：
1)He flatters that he may get promoted.
2)May you live happily together.
3)The man arrested may have had a history of child abuse. ' may '多在表達一個具目的, 願望等句子中作助動詞。
就上述的結論, may 在你原句中沒有had字就可以,雖然句子意義不大相同,但若連have也省去了就沒有主動詞, 句子亦因而解不通了。

WorldDupFinal﹕
I will have been in USA for 5 years next month.不是題主想要的have had 連在一起的例子，可否另再舉例？