Fluxes of water (W2)

Question 1

Define precipitation

Answer 1

Precipitation is any product of the condensation of atmospheric water vapour that falls under gravity.

Question 2

How does precipitation occur?

Answer 2

Precipitation occurs when moist air is cooled below its dew point and the air becomes saturate.
Masses of air are cooled quickly when they are lifted to higher elevations rapidly.
As air is forced to rise, it’s pressure decreases and it expands and cools.

Question 3

Explain factors controlling precipitation(3)

Answer 3

• Stochastic variations: arising from probalistic or random nature of precipitation occurrences.
• Periodic (cyclic – e.g. diurnal, annual) variations related, for example, to astronomical cycles
• Secular (long term) variations – due to climate change.

Question 4

Define dew point

Answer 4

Temperature below which water condenses and dew can form

Question 5

How is precipitation measured?

Answer 5

Using a raingauge. automatically registers intensity or rate of accumulation

Question 6

Explain factors controlling interception

Answer 6

-Vegetation characteristics: Shape, size, roughness of leaves control the ‘crown capacity’
-Rainfall intensity/duration: Controls speed at which crown capacity is reached
Windspeed and turbulence: Wind increases evaporation and hence interception loss However, high windspeeds may dislodge water from leaf surface
- Energy balance: Forests have lower albedo than grassland hence a higher net energy balance leading to increased losses

Question 7

Define interception

Answer 7

Canopy interception is the precipitation retained by the vegetation as surface storage and subsequently evaporated

Question 8

Define Evaporation

Answer 8

The process by which a liquid (water) is turned into a gas (water vapour) and its transfer away from the surface into the atmosphere

Question 9

How is evaporation measured?

Answer 9

• Evaporation pans and tanks.
• NB. These are not measuring actual evaporation from a soil
• hard to measure so use mathematical models

Question 10

Define potential evaporation

Answer 10

‘evaporation from an extended surface of short, green crop, actively growing, completely shading the ground, of uniform height and not short of water’

Question 11

Define transpiration in plants

Answer 11

Evaporation of water that has passed through plants, particularly their stomata. open and close due to environmental conditions

Question 12

Define evapotranspiration

Answer 12

The direct evaporation of moisture from the soil and various water bodies, plus transpiration from plant leaves

Question 13

factors controlling evapotranspiration

Answer 13

1. Evaporative potential of air
2. Water supply from soil
3. Regulation of water loss by leaves using stomata

Question 14

factors controlling transpiration

Answer 14

• Types of vegetation:
  Size, density and location of plant stomata
• Season:
  Determines leaf cover (deciduous trees)
• Time of day: Alters radiation balance, photosynthesis and stomatal activity
• Stage of growth:
  Plants use more water during times of active growth
• Meteorological factors:
  Affects opening and closing of stomata
• Soil properties:
  Affects ease of extraction of water

Question 15

Components of evapotranspiration:

Answer 15

1. transpiration
2. interception evaporation
3. bare soil evaporation