Introductory presentation of the drainage basin systems in the first chapter of Hydrology and Fluvial Geomorphology, suitable for AS students, consisting in the following: the global hydrological cycle, store, flows, the drainage systems, precipitation, evapotranspiration, interception, infiltration, percolation, drainage patterns, the water balance.

1. HYDROLOGY AND FLUVIAL
GEOMORPHOLOGY
The Drainage Basin System
AS GEO 1.1

2. 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:

3. 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.

5. 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.

6. 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.

7. Basic Features of a Drainage Basins Hydrological Cycle

8. The pattern (shape) of a drainage basin is dependant on its
relationship with:
– Relief
– Climate
– Vegetation
– Underlying geology

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

15. Drainage Patterns
The drainage patterns are:
TRELLISED
RECTANGULAR
PARALLEL
DENDRITIC
DERANGED
ANNULAR

16. 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

17. 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.