Water in Serpentine Geoecosystems

Water is continuously cycled from the atmosphere through geoecosystems to water bodies and, by evaporation and evapotranspiration, back to the atmosphere. Water is commonly transported long distances in the atmosphere. Eventually, it forms clouds that drop rain, snow, or dew on plants or the ground. The contributions of fog and dew to geoecosystems are generally minor, but they can be important factors along some coastlines. Water from most of the precipitation that falls to the ground infiltrates soils. Some is intercepted by plants and evaporates before it can reach soils, and some runs overland to streams without entering soils. Soils are important stores of water for plants. Excess water in soils and permeable substrata drains gradually. This gradual draining of infiltrated water diminishes flooding from storms and supplies water to streams between rainfall events, helping maintain more constant stream levels. The study of meteoric water, or water that is cycled through the atmosphere, is called “hydrology.” Watersheds are basic units of hydrological investigations. A watershed is a drainage basin—an area from which water drains to a common point. All water falling on a watershed (and not lost by evapotranspiration) leaves through a single, joint location that can be monitored with a stream gauge. There are exceptions, however, in which water drains from watersheds through permeable substrata, rather than at the lowest point in the ground surface topography. These “leaky” watersheds are common in basalt, poorly consolidated sandstone, and limestone terrains. We can examine some of the data from watersheds that are not known to be leaky to learn about the runoff characteristics of serpentine streams and their chemistry. Watersheds range in size from less than a hectare to large portions of continents (e.g., the Amazon River drains 6,475,000km2, about 35% of the South American continent). The smaller watersheds are drained by headwater streams with no tributaries, and the larger ones are drained by streams with many tributaries. Some of the most useful information can be gained from small watersheds because they have more uniform lithology, topography, soils, climate, and vegetation than larger ones.