HYDROLOGY AND FLUVIAL GEOMORPHOLOGY

The Drainage Basin System

AS GEO 1.1

The Global Hydrological Cycle

Hydrology • The scientific study of water, both surface and underground,

including its properties, distribution, movement and human use.Inputs• Energy or matter that enters the system.Outputs• Energy or matter that leaves the system.Open System• A system where there is movement of energy and/or matter in and

out of the system e.g. a drainage basin.Closed System• A system where there is no exchanges of matter in and out of the

system (only energy) e.g. the global hydrological system.

Key Terms:

Stores • Natural reservoirs of water in the environment, such as rivers,

lakes, soil, vegetation, the atmosphere and the water table.Flows• Paths that water flows in the land based part of the hydrological

cycle, i.e. flows through soil and flows in rivers.

Important facts:Important facts:1. The global hydrological system is a closed system as there are no

exchanges of matter into or out of the system, only energy (solar energy).

2. Water is held in stores and moves between these stores through the processes of precipitation, evaporation and runoff (transfers). With huge variations in storage location and time.

3. 97% of water in the hydrological system is salt water held in the oceans. While 77% of freshwater is stored in the polar ice caps.

The Global Hydrological cycle has three major pathways: precipitation, evaporation/transpiration and vapor transport.

Water precipitates from the sky as rain or snow, most of which (385,000 cubic kilometers per year) falls into the oceans. It returns to the atmosphere by

evaporation.

Some flows from the land to the sea as runoff or groundwater; in the other direction, water vapor is carried by atmospheric currents from the sea to the land.

Net flow is measured in thousands of cubic kilometers per year.

The Drainage Basin System

• To help understand the terrestrial movement of water we break up the cycle into surface units called drainage basins. These are ‘open systems’.

• A ‘Drainage Basin’ is an area of land drained by a river and its tributaries, bounded by a watershed (boundary).

• It is sometimes called the ‘catchment area’ of a river because it is the bowl which catches the rain.

• The water table is the level of water in the rocks below the surface.

• Gauging stations are used to measure the relationship between rainfall intensity / duration and discharge outflow characteristics of a drainage basin.

Basic Features of a Drainage Basins Hydrological Cycle

The pattern (shape) of a drainage basin is dependant on its relationship with:

– Relief

– Climate

– Vegetation

– Underlying geology

PRECIPITATION

It is a major input into the system, but amounts can vary over time and space (rain and snow).

The greater the intensity of the storm, the shorter its duration. Convectional thunderstorms are short, heavy and may be confined to small areas.

EVAPOTRANSPIRATION

When the water is lost from the system either by the river carrying it to the sea or through evapotranspiration (the loss of water directly from the ground, water surfaces and vegetation).

The two components of evapotranspiration are outputs of the system.

Evaporation is the physical process by which moisture is lost directly into the atmosphere from water surface, including vegetation and the soil, due to the sun heat and air movement.

Transpiration is the biological process by which water is lost from a plant through the pores in the leaves.

INTERCEPTION

The first raindrops of a rainfall event will fall on vegetation. This is called interception storage.

If rainfall persists, water begins to reach the ground by three possible routes: dropping of the leaves, or throughfall; flowing down the trunk, or stemflow, and by undergoing secondary interception by undergrowth.

INFILTRATION

The maximum rate at which water can pass through the soil is called its infiltration capacity and is expressed in mm/hr.

The rate of infiltration depends upon the amount of water already in the soil (antecedent precipitation), the porosity and the structure and nature of the soil.

PERCOLATION

Percolation = as water reaches the underlying soil or rock layers, which tend to be more compact, its progress is slowed.

This constant movement, called percolation creates groundwater storage.

Drainage Patterns

The drainage patterns are:

TRELLISED

RECTANGULAR

PARALLEL

DENDRITIC

DERANGED

ANNULAR

Create this table, assign the following terms to the relevant headings.

Inputs Stores Flows Outputs

• Precipitation• Interception• Throughfall• Stemflow• Runoff/overland flow• Discharge• Infiltration• Throughflow• Percolation• Baseflow• Groundwater• Recharge• Water tables• Springs• Evaporation• Evapotranspiration

The Water Balance (Drainage Basins)

• As drainage basins are ‘open’ systems the balance of water in the basin (inputs vs outputs) can be written as a simple equation accounting for water within the system.

• It can be expressed as:

P = Q + E (+/- S)Where:

P = Precipitation (measured using rain gauges)Q = Runoff (measured using river channel gauging stations)E = Evapotranspiration (difficult to measure)

S= Change in storage within the basin

• This simple equation can be used by hydrologists to manage the water supply of a catchment area to minimize drought or flooding.