Warming has brought extreme rain to the parched Sahel, which is seeing aquifers refill as monsoons grow stronger. But heavier downpours alone cannot explain the groundwater revival, say hydrologists, who believe efforts to harvest rainfall may also be paying dividends.

Near-extinct oryx are returning. Farms are prospering as irrigation water reaches fields for the first time in decades. Farmers are even nurturing new trees on their land. Once a byword for drought and famine, the African Sahel region on the southern flank of the Sahara Desert now has more water than it has for decades. Wells are filling as water tables have risen by 13 feet or more in places. Lake Chad, which was one of Africa’s largest expanses of water before shriveling during the droughts, is recovering.

Over years of drought in the late 20th century, the sun hard baked the soils of the Sahel.…