Virdas are shallow wells dug in low depressions called
jheels (tanks). They are found all over the Banni grasslands,
a part of the Great Rann of Kutch in Gujarat. They are systems
built by the nomadic Maldharis, who used to roam these grasslands.
Now settled, they persist in using virdas.
These structures harvest rainwater. The topography of the area
is undulating, with depressions on the ground. By studying the
flow of water during the monsoon, the Maldharis identify these
depressions and make their virdas there.
Essentially, the structures use a technology that helps the
Maldharis separate potable freshwater from unpotable salt water.
After rainwater infiltrates the soil, it gets stored at a level
above the salty groundwater because of the difference in their
density. A structure is built to reach down (about 1 m) to this
upper layer of accumulated rainwater. Between these two layers
of sweet and saline water, there exists a zone of brackish water.
As freshwater is removed, the brackish water moves upwards,
and accumulates towards the bottom of the virda.