Complex sentence: Hillsides are more prone to gullying when they are cleared of forest or grazed, where eroded soil is easily carried by the flowing water after being disloged from the ground, and when rainfall falls during short, intense storms as during thunderstorms.
Split into simpler sentences: Hillsides are more prone to gullying when they are cleared of vegetation, through deforestation, over-grazing or other means. The eroded soil is easily carried by the flowing water after being dislodged from the ground, normally when rainfall falls during short, intense storms such as during thunderstorms.