Final answer:
The term for the round hollows that are only present seasonally in the Southern High Plains of the United States is 'playas.' These features are significant for collecting runoff water during wet periods and have been impacted by events like the Dust Bowl.
Step-by-step explanation:
The round hollows in the ground in the Southern High Plains of the United States, which are only present at certain times of the year, are known as playas. These ephemeral features form in arid and semi-arid regions, collecting runoff water from surrounding uplands during wet periods. The Southern High Plains is an area that has been significantly affected by natural events and human activities, with ephemeral water bodies like playas and the historical impacts of events such as the Dust Bowl that have shaped the landscape.