From the actual movie script, not a secondary source:
1 EXT. DESERT - DAY 1
Shimmering waves of heat. A flat unforgiving landscape
stretches to infinity. A storm of dust gathers on the
horizon, small at first, increasing in size as ...
He is digging for the thing you need the most in a desert: water.
From the film's official novelization:
A STORM GATHERED on the desert horizon.
J'Onn paused in his work to stare beyond the wavering black bands of heat at the growing cloud of dust. Normally he would have headed back for the ramshackle shelter that served as home and waited ...