Evapotranspiration

Question 1

Evapotranspiration (Dingman, 1994)

Answer 1

• collective term for all processes by which water in the liquid or solid phase at or near the earths land surfaces becomes atmospheric water vapour
• evap from rivers, lakes, bare soil and vegetation (leaves)
• plus sublimation from snow and ice surfaces.

Question 2

John Dalton: Aerodynamic/mass transfer approach

Answer 2

• net balance between vaporisation and condensation
• temperature or air and evaporating surface
• wind speed
• water vapour capacity of air
• atmospheric pressure
• evaporating surface characteristics
• size of the surface

Question 3

Thermodynamic/Energy Balance approach

Answer 3

• energy balance of evaporating surface
• estimating the latent heat available of vaporisation (liquid to gas)
• net radiation (balance between income short & longwave radiation and that outgoing)

Question 4

Penman-Monteith

Answer 4

• summary of thermodynamic and aerodynamic approach
• sunshine hours, windspeed, specific humidity and air temperature
• addition of
  canopy conductance: measures whether the plants stomata open or closed.
  Atmospheric conductance: presents how easy it is for air to pass through the canopy

Question 5

aerodynamic approach

Answer 5

• deals with vapour flux away from evaporating surface

- concerned with the drying power of air, comprising its humidity and rate vapour can diffuse away

Question 6

CROPWAT

Answer 6

• model that calculates crop water and irrigation requirements
• reference evapotranspiration
• based on soil, climate, and crop data
• the food and agriculture organisation system allows farmer to evaluate their irrigation practices
• allows development of irrigation schedules for management conditions

Question 7

CROPWAT Advantages

Answer 7

• data is collected daily, monthly and decadal for climate input
• possible to calculate climate data in case of absence
• when climate changes model is able to adapt
• easy to import and export data, easy saving and retrieval settings
• interactive user adjustable schedules and creates daily soil water balance output tables
• overall outline crop water requirement which can help farmers

Question 8

CROPWAT Disadvantages

Answer 8

• computer may limit uses, especially farmers
• some countries that need it mat not be able to have access
• doesn’t take into account climate variations e.g. hot winds, temp and lack of precipitation
• ineffective for prediction process
• simulation if not always accurate and may give incorrect info

Question 9

Land and Water Development Division

Answer 9

• investigated the use of the FAO cropwat model in deficit irrigation studies
• concluded that cropwat can adequately simulate yield reduction
• accounting well for relative sensitivity of different growth stage

Question 10

Melakin et al. 2009

Answer 10

• evaluating the efficiency of cropwat model for determining plant water requirements in arid regions
• cropwat over predicted water consumption (irrigation)
• due to climate conditions
• Cumin plant, S.E. Iran