Erosion is the removal of soil particles by the forces of wind and water. Wind and water erosion can threaten the long-term productivity of soils through the loss of valuable, nutrient-rich topsoil. Erosion occurs naturally, but some land use practices can accelerate its rate. Soil erosion is estimated to reduce annual net profits by an average of $12 per cropped hectare. Where soils are deep enough, topsoil can be rebuilt, but only at a very slow rate (generally much slower than the rate of soil erosion).