I had first heard about the claim that more people drown in deserts than die of thirst in school maybe 30 years ago. The explanation is that sudden rains can create dangerous flooding in normally dry valleys (e.g. wadis).
In the Internet, the claim is widespread. See for example on a website of the U.S. Geological Survey here:
Rain does fall occasionally in deserts, and desert storms are often violent. A record 44 millimeters of rain once fell within 3 hours in the Sahara. Large Saharan storms may deliver up to 1 millimeter per minute. Normally dry stream channels, called arroyos or wadis, can quickly fill after heavy rains, and flash floods make these channels dangerous. More people drown in deserts than die of thirst.
However, is this really true? Can this claim be substantiated with credible numbers?