Soil and water engineers study the interaction of plants, animals, and humans with soil and water. They seek ways to control soil erosion and reduce the effects of sediment and surface runoff on water quality. They design, build, and maintain drainage and irrigation systems for agricultural operations as well as water control structures for reservoirs, floodways, and channels. They also design equipment for applying fertilizers and pesticides. Soil and water engineers are experts in the principles of hydrology. They must also understand chemistry, environmental science, and the mechanics of water and wind erosion.