What is the meaning of the word ‘depletion’ in line 2?

Directions: Read the passage and do as directed.

1. `Deficient rainfall over a long time causes drought, which is characterised ---21----- water shortage,
2. depletion of groundwater and crop damage. Apart from deficient rainfall, unscientific agricultural
3. practices,  the uncontrolled drawing of water from existing sources and deforestation contribute
4. to droughts. Their ill effects are felt only after sometime. Droughts develop gradually.
5. Droughts can cause death by starvation and -----24----- people to migrate to other countries.