This etymology is simple from the formal point of view and implies only one formal manipulation, the addition of a phoneme. As a matter of fact, δεύω "to need, to miss" is the epic variant of δέω (as χεύω is the epic variant of χέω), so that this δεύω "to need" was homophonous with δεύω "to wet". Deriving the one from the other implies starting from the passive form δεύομαι "to be wet", not from the active form, and this can be assimilated to an etymology a contrario: the earth needs what it does not have, rain. However, this point was not taken into account by Greek etymologists, who generally speaking take the active form as the default form