Drainage basins as open systems

Drainage basin

An area of land drained by a river and its tributaries

evapotranspiration

The total water output from the drainage basin back into the atmosphere

Infiltration

The downward movement of water from the surface to the soil

interception storage

Precipitation that falls on the vegetation surfaces or human made cover and is temporarily stored on these surfaces

overland flow

The tendency of water to flow horizontally across land surfaces when rainfall has exceeded the infiltration capacity.

percolation

The downward movement of water within the rock under the soil surface. Rates vary depending on the nature of the rock.

run-off

All the water that enters a river channel and eventually flows out of the drainage basin

stemflow

The portion of precipitation intercepted by the canopy that reaches the ground by flowing down the stems

throughfall

The portion of the precipitation that reaches the ground directly through gaps in the canopy and drips from leaves twigs and stems. This occurs when the canopy exceeds its storage capacity. 

throughflow

The movement of water down-slope through the subsoil under the influence of gravity. Particularly effective when the underlying permeable rock prevents further downward movement 

water balance

The balance between inputs and outputs in a drainage basin.

what effects the interception store

type of vegetation, density of vegetation

what happens to water captured by vegetation

lots is evaporated back into the atmosphere. Some reaches the ground by the process of through-fall. and some by stemflow

what happens when water reaches the ground

water soaks into the soil by infiltration (affected by soil porosity)

what kind of soil holds the most water

Coarse textured soils have larger pores than fine-grained soils and therefore allow for more water flow

what happens when soil becomes saturated and where is this most common

water builds up as surface storage. Most common in man made environments, very uncommon in natural environments other than during storms

what happens to surface storage

Much of this surface storage then evaporates back into the atmosphere. When the surface stores are full overland flow begins on slopes. This is very fast flow, rapidly reaching the nearest channel.

what happens to the rate of throughflow in a vegetated area

the rate of throughflow is higher, as the roots aid the process

in what twao ways does discharge vary

Discharge levels rise and fall, often showing an annual pattern (Called the rivers regime). They also vary in the short term following heavy rainfall.

water balance formula

precipitation = discharge + evapotranspiration ± change in storage