Hyper-dry conditions provide new insights into the cause of extreme floods after wildfire

► Demonstrates that hyper-dry conditions can exist after wildfire. ► Soil properties were measured before substantial rain altered the soil conditions. ► Soil retention curve was measured in situ in the hyper-dry region. ► Diffusion-adsorption of water-vapor controls the critical rewetting process. ► Hyper-dry conditions can explain increased flooding during the first rainstorms.