What is passive irrigation?
Passive water harvesting or irrigation is the practice of slowing water down to allow it to soak into the ground. With simple terrain adjustments (or “earthworks”) that catch and direct rainwater runoff, rainwater can be used beneficially, encouraging plant growth in landscapes and natural areas, healing eroded areas and can even reduce the need to irrigate with tap water. 
Goals of passive irrigation:
Slow down runoff
Spread the water out
Soak the water in