"乾く" is dry. "渇く" is thirsty.
"乾く" means only the state change from moist(wetty) to dry.
But "渇く" means not only the state change from moist to thirsty , but also the desire to return to the original state.
For example
"乾燥" ⇒only the state change from moist(wetty) to dry.
"渇望" ⇒I don't have something. So I want it very much.