From PathfinderWiki

Sabkha is a shallow basin filled with gritty salt-laden sand. It is the primary source for the nagri salt that farmers use to make pesh. Farmers travel to Sabkha once a season to sift salt out of the dirt and carry sacks of it home. Merchants flock to Sabkha to collect nagri salt, which they then sell in the markets of Katapesh. Once a month or so, heavy rains fill Sabkha's shallow basin with trapped rainwater. After several days, the water drains out through the rock, leaving new salt deposits on the surface.1