Agricultural fields used as rain-fed storage structures are referred to as:

Rainwater harvesting (RWH) is the collection and storage of rain, rather than allowing it to run off. Johads are rain-fed storage structures for water storage. They are used in semi-arid regions like Rajasthan. 

They play a vital role in improving water security and preserving the local ecosystem by recharging groundwater, reducing soil erosion, and conserving biodiversity. They are an example of traditional water management practices adapted to local conditions and are still in use today, particularly in rural communities.