Desert riparian is a North American desert vegetation type (or biome) occurring in the bottoms of canyons and drainages that have water at or near the surface most of the year. It is contrasted with the desert dry wash vegetation type in which water at or near the surface is lacking most of the year. The visual character is of large, lush, perennial green trees surrounded by dry desert vegetation and soil coloration. The area may be in a patch surrounding a spring (oasis), or in a strand following the course of water flow. The soil in this biome is typically moist and ranges from rocky and sandy to silty alluvial. This biome has seasonal variation, with hot, dry summers and cool, moist winters. Precipitation mostly occurs during the winter, and ranges from 8 to 25 cm each year.