Soil erosion begins with raindrop impact — a single drop strikes bare soil at 9 m/s, detaching particles and splashing them up to 1.5m. On slopes, splash displacement is net downhill. Concentrated flow carves rills and eventually gullies. The USLE (Universal Soil Loss Equation): A = R·K·LS·C·P, where R is rainfall erosivity, K is soil erodibility, LS is slope length-steepness, C is cover-management, P is erosion control practices. A 10% slope erodes 3× faster than a 5% slope; bare soil erodes 100–500× faster than forest. No-till farming reduces erosion by up to 90% by maintaining crop residue and soil structure.