Water Lecture 1- Hydrological Cycle

Question 1

If it started raining today, what would the depth of rainfall be before which the atmosphere ran out of water?

Answer 1

25 mm

Question 2

What is the hydrological cycle?

Answer 2

A conceptual model that can be evaluated at a number of levels of complexity for different purposes.

Question 3

How much water in oceans?

Answer 3

93%

Question 4

How much water in terrestrial?

Answer 4

5%

Question 5

How much water in polar ice?

Answer 5

2%

Question 6

How much water in atmosphere?

Answer 6

0.001%

Question 7

How can we directly measure atmospheric moisture?

Answer 7

Limited spatial and temporal coverage

Question 8

How can we indirectly measure atmospheric moisture?

Answer 8

Repeat survey (daily at 250m for MODIS)
Estimation based on reflectance of radiation

Question 9

What do uncertainties in quanifying the hydrological cycle mean?

Answer 9

These uncertainties mean that because estimates like these are balanced, they can lead us to think that we know more about the system than is really the case.

For example, Chahine (1992) estimates that underground water may be incorrect by a factor of 2 to 4

Question 10

What is the residence time in atmosphere?

Answer 10

< 10 days

Question 11

What is the residence time in polar ice?

Answer 11

15000 years

Question 12

What is the residence time in oceans?

Answer 12

3600 years

Question 13

What is the residence time in groundwater?

Answer 13

Up to 10,000 years

Question 14

What is the residence time in lakes?

Answer 14

10 years

Question 15

What is the residence time in biological water?

Answer 15

Hours to weeks

Question 16

What is the residence time in soil?

Answer 16

Days to years

Question 17

What is the residence time in rivers?

Answer 17

weeks

Question 18

Discuss the equation of continuity or mass conservation.

Answer 18

If the system is at steady state then flow in = flow ou.
Store/flow in = store/flow out = residence or turnover time.

Question 19

What are turnover times for continental atmosphere?

Answer 19

15 days

Question 20

What are turnover times for maritime atmosphere?

Answer 20

9 days

Question 21

What are turnover times for terrestrial water?

Answer 21

551 years

Question 22

What are turnover times for ocean water

Answer 22

3226 years

Question 23

What is the equation for evapotranspiration?

Answer 23

Precipitation - deep drainage - change in storage

Question 24

How can evapotranspiration be measured?

Answer 24

Evapo(transpi)ration is a complex process (involving net radiation, soil heat
flux, air density, air specific heat, air vapour pressure, surface and
aerodynamic resistances and partial pressure of water in air)
It is also difficult to measure! This example uses a lysimeter.
Can also be estimated by satellite.

Question 25

In terms of ET how much variance fo prections explain in observations?

Answer 25

74%

Question 26

Where does evaporation tend to be lowest?

Answer 26

NH apart from during summer

Question 27

Discuss fluxes and feedbacks in Eastern Chad.

Answer 27

Eastern Chad, August 2005 using MODIS images in false colour, and water is dark blue, clouds are light blue, bare land is a pinkish tan, and vegetation is bright green (over 16 days) Note the very rapid vegetation response to rainfall

Question 28

What is the charney hypothesis?

Answer 28

Charney (1975) suggested that changes in albedo as a function of vegetation growth has a positive feedback on rainfall in the Sahel. Vegetation has lower albedo → more surface heating → stronger land-ocean temperature gradients → enhanced monsoonal circulation in the tropics → more vegetation. Reduced vegetation → increased surface albedo → low-level cooling → increased atmospheric stability → low-level air subsidence drying → reduced vegetation

Question 29

What are issues with the Charney hypothesis?

Answer 29

Jackson and Idso (1975) suggested that albedo changes in the US with vegetation change were inconsistent with Charney’s. Wendler and Eaton (1983) found that the difference in albedo for vegetated and unvegetated sites in Tunisia was also insufficient for Charney’s model to explain patterns of precipitation change

Question 30

What is an alternative explanation to the Charney hypothesis?

Answer 30

Entekhabi et al. (1992) have suggested that reprecipitation of moisture that is evapotranspired from vegetation is more likely to lead to the feedback at regional level. Pl is locally derived pptn Pa is pptn from advection P is total precipitation Remember that although potential evaporation from a bare surface in the Sahel is high, actual evaporation is usually zero because there is no moisture available

Question 31

Discuss examples of land-surface feedbacks?

Answer 31

Snow storms in Buffalo, NY, Nov 2014 Amazonian rainfall- around 50-60% of precipitation is generated by the rain forest, rain starts 2-3 months before seasonal moist winds from ocean.

Question 32

Do global energy budgets influence the hydrological cycle?

Answer 32

Yes

Question 33

Discuss the hydrological cycle as an Engine.

Answer 33

We can understand the hydrological cycle in relation to global energy balances. Radiative heating in the atmosphere leads to: 1. Evaporation into unsaturated air- leads to latent heat flux (88 Wm-2) 2. Uplift and adiabatic cooling of water vapour (potential energy = mass x height x gravity, estimate height using adiabatic lapse rate, global mean 0.3 wm-2 but tropics 1.5 wm-2) 3. Condensational heating of air (7.3 wm-2, release of latent heat, should match energy from evap) 4. Sinking of unsaturated air (63% of this energy drived moist convective motion in the atmosphere) 5. Precipitation of condensed water All these processes in the hydrological cycle (frictional dissipation of 1-2 wm-2 as they fall and mean PE remaining for rain falling on land is around 0.08 wm-2. have feedbacks on the energy balance … and erosion and sediment transport