Drainage Basin As An Open System

Question 1

Define Drainage Basin

Answer 1

An area of land that is drained by a river and its tributaries (simply: area surrounding the river where the rain falling on the land flows into that river)

Question 2

Define watershed

Answer 2

The boundary of a drainage basin (usually a ridge of high land). Any precipitation falling beyond the watershed enters a different drainage basin.

Question 3

Why can it be argued that the drainage basin is a closed system?

Answer 3

The principles of cause and effect are contained and do not spread outside its area. The drainage basin is contained within that basin and will not affect neighbouring basins

Question 4

Describe the Inputs of the Drainage Basin

Answer 4

Precipitation

• water enters drainage basin as precipitation in form of rain, hail, sleet or snow
• some water may be intercepted by plants and trees where it may be stored (vegetation storage) before being evaporated
• it takes time for water to drop through the leaves or down the stems (stem flow) to the ground surface
• water is either stored in puddles (small Hydrosphere store), Flows over ground as Overland Flow when ground is saturated or infiltrates into soil
• some water may be taken up by plants before being transpired

Question 5

Describe the Store of Interception in the Drainage Basin

Answer 5

• this is when precipitation lands on buildings, vegetation and concrete before it reaches the soil
• for example, vegetation cover intercepts the precipitation and a store of water may be held on leaves and branches
• interception storage is only temporary as it is often quickly evaporated
• tropical rainforests can intercept 58% of rainfall

Question 6

Describe the Store of Surface Storage in the Drainage Basin

Answer 6

• the total volume of water held on the earth’s surface in lakes, ponds and puddles
• this mainly occurs in built environments as puddles because impervious surfaces are more likely in urban areas so water can’t infiltrate through the soil
• in natural environments, Infiltration normally occurs more quickly than rainfall and there will be more surface puddles after very long periods of rainfall or on impacted surfaces/bare rock

Question 7

Describe the Store of Soil water in the Drainage Basin

Answer 7

• once in the soil, water may be stored as soil water or pass through as Throughflow, dependent on the depth and texture of the soil
• sandy soil absorbs and transfers water rapidly. This increases the likelihood of floods. These soils have a high field capacity (retain little water)
• clay soils drain and transfer more slowly and have a high field capacity (retains lots of water)

Question 8

Describe the Store of Groundwater storage in the Drainage Basin

Answer 8

The storage of water underground in permeable rock

Question 9

Describe the Store of Channel storage in the Drainage Basin

Answer 9

The water held in a river or Stream channel

Question 10

Describe the Flow/Transfer of Groundwater Flow in the Drainage Basin

Answer 10

• Water infiltrates through rocks (e.g water from precipitation) and enters Groundwater storage
• Groundwater Flow is the deeper movement of water through underlying permeable rock below the water table (slow process of transfer)
• limestone is highly permeable with lots of joints and can lead to faster Groundwater flow
• eventually, water moves out of the system as runoff, when the river flows into lakes or the sea or evapotranspiration

Question 11

Describe the Flow/Transfer of Stemflow in the Drainage Basin

Answer 11

Water flows down the stems of plants and tree trunks (slow movement)

Question 12

Describe the Flow/Transfer of Infiltration in the Drainage Basin

Answer 12

• the downward movement of water into the soil surface
• the texture, structure and organic content of soil affect the infiltration rate (infiltration capacity). The rate usually declines during an early part of a storm
• infiltration capacity is exceeded when the soil is unable to absorb water at the rate at which it is falling
• thin frozen or already saturated soils have a low infiltration capacity
• trees may promote infiltration as the roots form pathways for water to percolate underground
• water soaks into the soil by the attraction of water molecules to soil particles and gravity

Question 13

Describe the Flow/Transfer of Overland flow in the Drainage Basin

Answer 13

• rainfall flowing over the ground surface due to soil bring saturated or because rainfall is exceeding the soil infiltration capacity
• if water is unable to infiltrate, it may run off the surface as Overland flow, flowing across a large surface (sheet flow) or concentrated into small channels (rills)
• Overland flow is less likely on agricultural land as much of the land is covered by vegetation
• in urban areas, particularly on roads, water cannot infiltrate into the soils, which may cause flooding

Question 14

Describe the Flow/Transfer of Channel flow in the Drainage Basin

Answer 14

The flow of water in rivers

Question 15

Describe the Output of Evapotranspiration in the Drainage Basin

Answer 15

Combined loss of water through Evaporation and transpiration by plants
(10% of water vapour in Atmosphere is due to transpiration by plants)

Question 16

Describe the Output of Runoff in the Drainage Basin

Answer 16

The output of water from the drainage basin as it moves across the land surface either as Overland flow or channel flow

Question 17

What are the Inputs in the Drainage System?

Answer 17

Precipitation

Question 18

What are the Stores/Components in the Drainage System?

Answer 18

• interception
• surface storage
• soil water
• groundwater storage
• channel storage

Question 19

What are the Flows/Transfers in the Drainage System?

Answer 19

• Groundwater flow
• stemflow
• infiltration
• Overland flow
• channel flow

Question 20

What are the Outputs in the Drainage System?

Answer 20

• Evapotranspiration

- Runoff

Question 21

Describe the Water Balance of a Drainage Basin

Answer 21

The water balance is when the inputs and outputs of the Drainage system are balanced, and is said to be in dynamic equilibrium
- The water balance describes how much precipitation (input) compares with the water leaving the system as Runoff or as evapotranspiration (output)

Question 22

How is the water balance expressed?

Answer 22

Precipitation (input) = Total Runoff (Stream flow) + Evapotranspiration (+/- Changes in storage (in soil and rock))

P = O + E (+/- S)

Question 23

What does a positive balance indicate in the water balance?

Answer 23

A positive balance indicates that inputs are greater than Outputs and water will be stored in the system
- for example, in wet seasons, precipitation is greater than evapotranspiration which creates a water surplus. Ground stores fill with water via infiltration which increases surface runoff, higher discharge and higher river levels. This means there is a positive water balance.

Question 24

What does a negative balance indicate in the water balance?

Answer 24

A negative balance indicates that Outputs are greater than inputs, and stored will deplete and be used up (water taken out of store)

• for example, in drier seasons, evapotranspiration exceeds precipitation, as plants absorb water, ground stores become depleted (used up). This means there is a water deficit at the end of a dry season, so there is a negative water balance

Question 25

What does a water surplus result in?

Answer 25

A water surplus can result in wet soils, high river levels, and additional run-off

Question 26

What does a water deficit result in?

Answer 26

A water deficit leads to dry soils, falling river levels, and a possibly drier micro-climate

Question 27

Define Water Surplus (in Water Balance)

Answer 27

There is excess water avaliable to the system. This occurs when precipitation exceeds evapotranspiration and the excess is not being used by plants.

Question 28

Define Water Deficit (in Water Balance)

Answer 28

There is a reduction of water avaliable within this system. This occurs when evapotranspiration exceeds precipitation.

Question 29

Describe Recharge (in water balance)

Answer 29

After a period of deficiency, precipitation will occur and replace the lost water in the soil. This needs to occur before a period of Surplus can reoccur.

Question 30

Define Field Capacity

Answer 30

The maximum amount of water that soil can hold before it becomes saturated

Question 31

What causes Variation in Runoff? (changing water balance)

Answer 31

Total Runoff is the measure of the proportion of the total precipitation that makes its way into streams and rivers.

Changes in Runoff are caused by:
• differences in soil water
- sandy soils retain less water meaning they have a low field capacity = so absorb and transfer water rapidly, creating floods
- clay soils retain lots of water (high field capacity) = drain and transfer water more slowly

• differences in rock type
- impermeable rock e.g limestone - less water flows through groundwater flow + less water stored + more Overland flow = higher Runoff (flooding more likely)

• differences in vegetation cover
- more vegetation = more interception = more water stored in plants and flows slowly by stemflow + some evapotranspiration = less Runoff (less likely flooding)

• differences in time of year

• summer = vegetation grows and temperatures increase = more evapotranspiration and more interception = less runoff (less flooding likely)
• winter = less veg, temp decreases = less evapotranspiration and less interception = more runoff (more flooding)

• intensity of precipitation

• intense rainfall = more Overland flow = pass quickly into rivers = higher amount of runoff
• less rainfall / drizzle = most stored in trees and grass which will evaporate = less Runoff
• snow = delays Runoff = when frozen soils melt, Runoff values might be high