Or HaChaim on Deuteronomy 11:28/אור החיים על דברים י״א:כ״ח

1 והקללה אם לא תשמעו, "And the curse in case you will fail to hearken, etc." Moses means that anyone who prevents himself from hearkening to the voice of G'd has already cursed himself. This is why the word והקללה appears before the words "if you will not hearken." Moses adds that in addition you will have demonstrated that you depart from the normal path of life by preparing for yourselves alternate but leaking water cisterns. In effect this is what the words: "you will depart from the way and follow alien deities" are all about.