The Influence of the Types of Rocks & Soil on Drainage Density

Definitions

• Porosity gauges the size of rock and soil particles, as well as the amount of space that is created between them by the earth's endless shifting. Permeability is the term used to describe how quickly water moves through and between the particles, and many factors affect this process. Soil is classified according to four basic forms: sand, silt, loam and clay. Water moves most rapidly through sand and most slowly through clay. Some rocks, such as granite, have a porosity that is close to that of clay, indicating that water moves extremely slowly through both.

Soil Types

• Clay is the least porous of all soil types, followed in ascending order of permeability by silt, sand and gravel. Loam, or humus, is not mentioned, because it is actually a composite of these types of soil that contains a high level of decomposed organic matter. Depending upon the amounts of sedimentary soil found in loam, its porosity varies greatly as compared to evaluations of each of its individual components. Organic matter is most generally found on the surface of any soil profile, and plays a lesser role in determining drainage density. Once water penetrates the first few inches of the soil's surface, what lies below it influences whether it continues to be absorbed by deeper layers of sand, silt or clay, or simply runs off.

Rocks

• The porosity of rocks ranges from igneous varieties composed of molten particles that include obsidian, granite and gabbro, to the more porous made entirely from compressed sediments that include sandstone, limestone and shale. An aquifer, or groundwater reservoir, is usually composed of various amounts of sand, soil and rock. The more porous the rock and soil base are, the more water it collects. Conversely, groundwater flow is greatest if the aquifer is composed of a high level of non-porous materials such as clay or granite, representing a high level of drainage density.

Hydrologic Cycle

• Juvenile water is the term given to so-called new water that comes from the earth's core. All other water in lakes, oceans and rivers, is constantly being recycled through a series of events that include precipitation, evaporation and runoff. Of that endless supply, only a tiny fraction is usable by humans, with the bulk of that existing as groundwater, supplied primarily by runoff. According to Idaho State University, drainage density is influenced not only by rock and soil types but by land slope, climate, rainfall, saturation rates, vegetation and time.