1. WATER CYCLE 2: drainage basin systems
Outputs
Evaporation
Transpiration
Channel discharge to oceans
Stores
Interception
Vegetation store
Surface store
Channel
Soil moisture store
Groundwater store
Flows
Throughfall
Stemflow
Infiltration
Throughflow
Overland flow(s)
Channel flow
Percolation
Groundwater flow
Inputs
Precipitation
We are grateful to Hodder for their permission to use the diagrams in this resource.
WJEC Focus Box 3.1.2
Eduqas Focus Box 2.1.2

2. Water Cycle 2 The drainage basin system
The drainage basin is a subset of the global hydrological cycle
It is defined as a catchment area forming part of the Earth's surface area which is drained by a particular stream or river
It is an open system which allows energy and matter to be transferred across its boundary (the watershed or drainage divide) from external areas
Inquiry question: What are the main hydrological inputs in a drainage basin and how do they operate?
Class activity
Why is a river drainage basin an open system, whereas the global water cycle is a closed system?

3. Water Cycle 2 Drainage basin inputs
There is only one physical input into a drainage basin: precipitation
Precipitation types are varied: they includes rain, snow, sleet hail and frost
Inquiry question: What are the main hydrological inputs in a drainage basin and how do they operate?
The idea of variations might include: types, amount, duration and density. Spatial variations depend on climate and relief; temporal variations relate to seasonality
Class activity
Why do drainage basin inputs vary from place to place or from time to time?
(c) Hodder Education [Edexcel A-level Geography Book 2 (Dunn)]

4. Water Cycle 2 Precipitation variations
The duration and intensity of precipitation both affect how a drainage basin system responds
Very high-intensity rainfall of millimetres per hour is rare; it can result in flash flooding
In Boscastle, 185mm rainfall fell in just five hours in 2004 (equivalent to two months of normal rainfall)
Low intensity but relatively long duration rainfall is far more common in the UK
Inquiry question: What are the main hydrological inputs in a drainage basin and how do they operate?
The link can be explained by anticipating the kinds of processes which will operate: if precipitation exceeds infiltration capacity, overland flow and a flashy river response can be exppected.
Class activity
What processes could explain the link between high-intensity rainfall and flash flooding?

5. Water Cycle 2 Drainage basin flows (1)
Class activity
How many water flows are shown in this diagram?
Inquiry question: What are the main hydrological flows in a drainage basin and how do they operate?
This topic is covered in most A-level textbooks, for both past and present courses
Another PowerPoint containing relevant slides is available at:
(c) Hodder Education [Edexcel A-level Geography Book 2 (Dunn)]

6. Water Cycle 2 Drainage basin flows (2)
Rainwater dripping from leaves and branches towards the ground is called throughfall
Water which flows to the ground via vegetation stems and trunks is stemflow
Infiltration is the movement of water from the ground surface into the soil
Throughflow is the movement of water laterally (sideways) through the soil, via (1) a matrix of pore spaces, fissures and (2) pipes (animal burrows)
Inquiry question: What are the main hydrological flows in a drainage basin and how do they operate?
The spatial and temporal variations in flow operate will depend on environmental factors: climate and weather; vegetation and season; relief and geology; soil type; human factors e.g. land use changes and urbanisation
Class activity
Why might drainage basin water flows vary from place to place or from time to time?

7. Water Cycle 2 Drainage basin flows (3)
Percolation is the transfer of water from the soil into the underlying bedrock
Groundwater flow is the vertical and lateral movement of water through a drainage basin’s underlying rock due to gravity and pressure
Coarse-grained sedimentary rocks show high permeability and permit high groundwater flow; fine-grained igneous rocks are relatively impermeable
Inquiry question: What are the main hydrological flows in a drainage basin and how do they operate?
Because groundwater flow is a very slow process, it can supply river channels with water during dry seasons in places where the geology allows for this.
Class activity
What are the implications of the varying geology of different drainage basins for river flow in dry seasons?

8. Water Cycle 2 Overland flow(s)
Overland flow is the movement of a sheet of water across the ground
Infiltration-excess overland flow occurs when rainfall intensity is so great that not all water can infiltrate, irrespective of how dry or wet the soil is
Saturation-excess overland flow happens if rainfall continues for a long time. The entire soil becomes saturated; overland flow begins
Inquiry question: What are the main hydrological stores in a drainage basin and how do they operate?
(c) Hodder Education (Geography Review)

9. Water Cycle 2 Overland flow(s)
Looking ahead to work on storm hydrographs, it is usually the case that infiltration-excess O.F. results in a very short lag time; saturation OF tends to occur later in a storm event.
Class activity
Why would different types of overland flow result in differing lag times shown on storm hydrographs?
(c) Hodder Education (Geography Review)

10. Water Cycle 2 Drainage basin stores
interception store - leaf and plant surfaces
vegetation store - water held in the biomass itself
surface store - water collected on the ground in depressions and hollows, and also snow cover
soil moisture store - water held in soil pores space
channel store - water held in the river channel itself
groundwater store - water stored in solid rock and in any superficial deposits e.g. gravels below the soil
Inquiry question: What are the main hydrological stores in a drainage basin and how do they operate?
Class activity
Why might drainage basin water storage vary from place to place or from time to time?

11. Water Cycle 2 Drainage basin stores and limits
Inquiry question: What are the main hydrological stores in a drainage basin and how do they operate?
In some local contexts, interception may prevent almost all of the precipitation falling on an area from ever reaching the ground surface. Interception varies with the duration of precipitation and with the character of the vegetation. If a storm lasts a short time then a considerable proportion of the rainfall remains caught on the leaves and branches. All of this water may be evaporated eventually back to the atmosphere. However, during a longer storm these myriad small reservoirs will overflow and the rainfall drips to the surface as throughflow or flows along vegetation surfaces to the ground as stemflow. As the duration of the storm increases so too does the proportion of rainfall reaching the surface beneath the plants.
Class activity
Describe and explain the changes in water storage and flows during the storm shown in the diagram
(c) Hodder Education [Edexcel A-level Geography Book 2 (Dunn)]

12. Water Cycle 2 Drainage basin outputs
Channel discharge is the water leaving a drainage basin via its main river during a unit of time
Evaporation is the change in state of water from a liquid to a gas. Meteorological factors influence the rate of evaporation (temperature and wind speed)
Transpiration is the diffusion of water from vegetation into the atmosphere. Water vapour is lost through the stomata (pores) of leaves
Inquiry question: What are the main hydrological outputs in a drainage basin and how do they operate?
Class activity
Why might drainage basin water outputs vary from place to place or from time to time?

13. Water Cycle 2 Drainage basin outputs
Precipitation sometimes falls as snow in Winter
It can be stored on the ground for days or months before melting
This delays the loss of water from the drainage basin
Inquiry question: What are the main hydrological outputs in a drainage basin and how do they operate?
The diagram suggests snowfall in February and March; snowmelt and high river flow during a warm and dry April-May period
Class activity
Describe and explain the variations in precipitation (blue) and river discharge (red) shown in the diagram

14. Precipitation varies according to type, amount, duration and intensity
Water Cycle 2 Plenary
Precipitation varies according to type, amount, duration and intensity
Flows include different types of overland flow, in addition to stemflow, throughflow, percolation, groundwater flow and other water transfers
Interception plays an important role in preventing or allowing rainwater to move to other stores
Local variations in soil and rock type play an important role in river catchment hydrology
Water leaves the drainage basin as evaporation, transpiration and channel discharge