Drainage density is a quantity used to describe physical parameters of a drainage basin. First described by Robert E. Horton, drainage density is defined as the total length of channel in a drainage basin divided by the total area, represented by the following equation: The quantity represents the average length of channel per unit area of catchment and has units , which is often reduced to . Drainage density depends upon both climate and physical characteristics of the drainage basin. Soil permeability (infiltration difficulty) and underlying rock type affect the runoff in a watershed; impermeable ground or exposed bedrock will lead to an increase in surface water runoff and therefore to more frequent streams. Rugged regions or those with high relief will also have a higher drainage density than other drainage basins if the other characteristics of the basin are the same. When determining the total length of streams in a basin, both perennial and ephemeral streams should be considered. If a drainage basin contained only ephemeral streams, the drainage density by the equation above would be calculated to be zero if only the total length of streams was calculated using only perennial streams. Ignoring ephemeral streams in the calculations does not consider the behavior of the basin during flood events and is therefore not completely representative of the drainage characteristics of the basin. Drainage density is indicative of infiltration and permeability of a drainage basin, as well as relating to the shape of the hydrograph. Drainage density depends upon both climate and physical characteristics of the drainage basin. High drainage densities also mean a high bifurcation ratio. Inverse of drainage density as a physical quantity Drainage density can be used to approximate the average length of overland flow in a catchment. Horton (1945) used the following equation to describe the average length of overland flow as a function of drainage density: Where is the length of overland flow with units of length and is the drainage density of the catchment, expressed in units of inverse length. Considering the geometry of channels on the hillslope, Horton also proposed the following equation: Where is the channel slope and is the average slope of the ground in the area. Elementary components of drainage basins A drainage basin can be defined by three elementary quantities: channels, the hillslope area associated with those channels, and the source areas. The channels are the well-defined segments