Surface Hydrology: Evaporation

Question 1

What are the main factors determining evaporation?

Answer 1

Meteorological factors, soil moisture

Question 2

What is evapotranspiration?

Answer 2

The combined process of evaporation and plant transpiration.

Question 3

What are the forces involved in transpiration by plants?

Answer 3

Stomatal resistance, epidermis permeability

Question 4

State the energy balance equation for the Earth’s surface.

Answer 4

λE = RN ± C ± V ± G − PS

Question 5

What does λ represent in the energy balance?

Answer 5

Latent heat of vaporization of water (~2470 kJ/kg).

Question 6

What is the Bowen ratio?

Answer 6

β = C / λE

Question 7

List two meteorological factors affecting evaporation from open surfaces.

Answer 7

Solar radiation, relative humidity.

Question 8

List two physical factors affecting evaporation from water surfaces.

Answer 8

Salinity, water depth.

Question 9

What physical factors affect evaporation from soils?

Answer 9

Soil moisture content, capillary characteristics

Question 10

What are the two main impacts of vegetation on evaporation?

Answer 10

Interception and transpiration.

Question 11

Name the three components of interception by vegetation.

Answer 11

Interception loss, throughfall, stem flow

Question 12

What is the formula for interception loss?

Answer 12

Interception loss = precipitation - throughfall - stemflow.

Question 13

List two factors affecting interception loss.

Answer 13

Interception storage capacity type and morphology of vegetation cover.

Question 14

What is the range of typical interception loss for trees?

Answer 14

20% to 95%.

Question 15

What is fog drip?

Answer 15

Fog intercepted by forests contributing up to 20% of river discharge.

Question 16

What are the two factors limiting actual evapotranspiration (Et)?

Answer 16

Available water and available energy.

Question 17

What is potential evapotranspiration (Ep)?

Answer 17

Evapotranspiration under unlimited water supply.

Question 18

What is actual evapotranspiration (Et or Ea)?

Answer 18

Evapotranspiration limited by available water and energy. May be lower due to water stress.

Question 19

What is reference evapotranspiration (Et0)?

Answer 19

Evapotranspiration of a well-watered reference crop (e.g., grass).

Question 20

Through which plant structures does transpiration occur?

Answer 20

Primarily stomata less through the epidermis.

Question 21

Name factors influencing plant transpiration.

Answer 21

• Number and size of stomata
• Epidermis permeability
• The area of internal surfaces exposed to intracellular spaces
• Arrangement of vascular tissue
• Solar radiation

Question 22

What is the Penman-Monteith method used for?

Answer 22

Calculating reference evapotranspiration (Et0).

Question 23

State the Penman-Monteith equation.

Answer 23

Et0 = (0.408Δ(Rn - G) + γ 900/(T+273) u2 (es - ea)) / (Δ + γ(1 + 0.34u2))

Question 24

What does Δ represent in the Penman-Monteith equation?

Answer 24

Slope of the vapor pressure curve [kPa/°C].

Question 25

What does γ represent?

Answer 25

Psychrometric constant [kPa/°C].

Question 26

What is the vegetation coefficient (Kc)?

Answer 26

Ratio between crop evapotranspiration and reference evapotranspiration.

Question 27

What is the formula linking Ep and Et0 using Kc?

Answer 27

Ep = Kc × Et0

Question 28

What is the formula linking Et and Et0 using Kc and Ks?

Answer 28

Et = Kc × Ks × Et0

Question 29

What does Ks represent?

Answer 29

Water stress coefficient.

Question 30

What is a typical Kc value for watermelon at mid-stage?

Answer 30

1.00

Question 31

What is a typical Kc value for rice at mid-stage?

Answer 31

1.20

Question 32

What is a typical Kc value for tomatoes at mid-stage?

Answer 32

1.15

Question 33

How can stemflow be measured?

Answer 33

Simple setups with a rain gauge and water collection system.

Question 34

How is throughfall typically measured?

Answer 34

Using multiple rain gauges under the canopy.

Question 35

How is evapotranspiration (Et) measured experimentally?

Answer 35

Using lysimeter experiments with a known soil-plant volume.

Question 36

Why are lysimeter experiments challenging?

Answer 36

Because they require careful monitoring of precipitation and mass changes.

Question 37

How is Et typically estimated in practice?

Answer 37

Using hydrometeorological measurements combined with empirical crop characteristics.

Question 38

What does it indicate if sugarcane's Et is lower than Et0?

Answer 38

The crop experienced water stress.