1. Key points from last lecture:
- Basic Laws:
Unit Conversion:
Properties of Water:
Watersheds:
Regional Water Balance:

2. A ATM301, Lecture #3 (section 2.2) The Global Hydrologic Cycle
Instructor: Aiguo Dai

3. Earth – The Blue Planet
Water covers 71%
of the Earth’s surface!

4. The Key Questions: What is the global hydrologic cycle?
Why do we care about the hydrologic cycle?
What drives the global hydrologic cycle?
What are the main reservoirs of the hydrologic cycle?
What are the main fluxes of the hydrologic cycle?
What are the physical processes of the hydrologic cycle?

5. The Global Hydrologic Cycle
Describe the global water cycle, and then point out that my research covers many aspects of the global water cycle, such as the variability and long-term changes in atmospheric water vapor and clouds, precipitation, evaporation, streamflow, runoff, continental freshwater discharge, and soil moisture. When the water cycle is out of balance over a region on land, floods and droughts occur. This leads the topic of my talk Today – drought and how it responds to global warming.
(From Trenberth et al., J. Hydrometeorol., 2007)

6. Summary of the Global Hydrologic Cycle
Definition:
It refers to the movement of water on, above or below the surface of the Earth. Also known as the global water cycle. It helps re-distribute water and energy on Earth, greatly affects surface temperature, forms clouds and precipitation, thus playing a critical role for the weather, climate and biosphere on Earth.
Main Reservoirs:
- Oceans (96%, t=1000yrs), freshwater on land (2.5%, t=weeks to years ), the atmosphere (<0.01%, t =10days ), the biosphere (<0.01%, t=1week)
- Freshwater: 68.6% in glaciers and ice caps, 30% in groundwater, 2.5%in surface water (ice and snow, lakes, soil moisture, etc.)
Main Fluxes:
- Surface to the atmosphere: evaporation (Ocean: 413,000km3/yr; land: 73,000km3/yr)
- The atmosphere to the surface: precipitation (Ocean: 373,000km3/yr; land: 113,000km3/yr)
- Atmospheric transport: Ocean to land (40,000km3/yr)
- Land to ocean runoff: 40,000km3/yr.
Main Physical Processes:
- Evaporation (absorbs heat)
- Condensation/precipitation (releases heat)
- Infiltration : flow of water from the surface into the ground (soils).
- Runoff : lateral movements of water on land
Linked to the Global Energy Cycle: evaporation brings about 80W/m2 of latent heat into the atmosphere. Water vapor and clouds affect atmospheric energy budgets.

7. Why Do We Care about the Hydrologic Cycle?
The global hydrologic cycle determines the available freshwater resources to our society.
The global hydrologic cycle is an important part of the Earth’s Climate System.
Global warming and climate change are expected to affect the global hydrologic cycle and our freshwater resources.
Any Examples?

8. Source: http://ga.water.usgs.gov/edu/watercyclesummary.html
Describe the global water cycle, and then point out that my research covers many aspects of the global water cycle, such as the variability and long-term changes in atmospheric water vapor and clouds, precipitation, evaporation, streamflow, runoff, continental freshwater discharge, and soil moisture. When the water cycle is out of balance over a region on land, floods and droughts occur. This leads the topic of my talk Today – drought and how it responds to global warming.
Source:

9. From http://ga.water.usgs.gov/edu/watercyclesummary.html

10. Major Water Storages

11. Major Water Stocks and Fluxes

13. The Hydrologic Cycle is Linked to the Energy Cycle
Describe the global water cycle, and then point out that my research covers many aspects of the global water cycle, such as the variability and long-term changes in atmospheric water vapor and clouds, precipitation, evaporation, streamflow, runoff, continental freshwater discharge, and soil moisture. When the water cycle is out of balance over a region on land, floods and droughts occur. This leads the topic of my talk Today – drought and how it responds to global warming.
Trenberth et al. (2009, BAMS)

14. Surface Water Balance ET P R dS/dt = P- ET - R S Water Storage
This applies to local, regional and global scales
For long-term mean, dW/dt0, so
P - E  R
How these terms may change under global warming is a major research area. P ET R S
Water Storage
dS/dt = P- ET - R

15. Main Water Cycle Fluxes:
Precipitation
Evaporation
Runoff

16. Precipitation Formation:
When air cools (e.g., due to ascending), water vapor can become saturated and condense on tiny particles (condensation nuclei) such as dust, ice, or salt. This forms clouds
Small cloud droplets combine to form larger droplets (coalescence)
Some of the droplets collide to form even larger raindrops that can fall out of the clouds
Raindrops have sizes ranging from 0.1 mm to 9 mm in diameter.
Hail forms in storm clouds when super-cooled (T<0oC) water droplets freeze. The storm’s updraft (a rising air plume) blows the hailstones into the upper part of the clouds. As the updraft dissipates, the hailstones fall back to the lower part of the clouds. This process can repeat many times until the hailstones become large enough (d>5mm) to fall out of the cloud.
Snowflakes form when tiny super-cooled cloud droplets (d10m) freeze.

17. Precipitation Distribution
mm/day
GPCP = Global Precipitation Climatology Project
Questions:
1. How do we produce such a precipitation map?
2. What causes the large variations over the oceans?
3. Why deserts are located in the subtropics?

18. Annual Precipitation Over Land

19. Annual Precipitation Over the U.S.
Orographic P
~40” at Albany
10-15”
15-20”
30-35”
35-40”
5-10”
40-50”
50-60”
1inch=25.4mm
60-70”

20. Seasonal Precipitation Variations

21. Monsoon Climate
Monsoon: seasonal reversal in atmospheric circulation and rainfall due to differential heating over land and ocean.

22. Reversal of Monsoon Winds
Wind convergence zone
or the ITCZ

24. Movie: Earth’s Water Cycle
(~6min., made in 2012)

25. Evapotranspiration over Land
Evaporation
Transpiration is largest contribution to total E over areas covered with vegetation.
Wang and Dickinson (2012)

26. From http://www.instrumentalia.com.ar/pdf/Invernadero.pdf
Turbulent Eddy Mixing
From

27. Annual Potential Evapotranspiration
(mm/yr Source: UNEP)
PET represents atmospheric demand for moisture.
It is the ET without water limitation.

28. Actual Evaporation Distribution

29. Evaporation (E) vs. Precipitation (P)

30. Long-term Mean E – P Difference Map
Trenberth et al. (2007, JHM)

31. Zonal-mean Precipitation and Evapotranspiration

32. Annual P/PET Ratio (a measure of aridity)
>
<

33. Key Differences between P and E
Precipitation occurs only in a fraction of the time, while evaporation occurs all the time, albeit largest around noon;
E P
t t
Precipitation rates vary greatly in space, while evaporation rates are more uniform, especially over the oceans ;
Precipitation occurs at rates usually much larger than evaporation rates;
Evaporation cools the surface, while precipitation heats the atmosphere; and
Evaporation is controlled mainly by surface heating and wind speed, while precipitation is determined by atmospheric circulation and water vapor content.

34. Runoff
Surface runoff: the water flow on earth’s surface. It occurs only when the soil is saturated or can not absorb water from rain or snow/ice melt fast enough. The flow follows the terrain into streams.
Subsurface runoff (or return flow): the lateral water flow below the earth’s surface, usually following the terrain under gravity.

35. Potential Freshwater Resources
Oki & Kanae, Sci., 2006
Global Discharge: ~38,000km3/yr
= Global Potential renewable freshwater resource

36. Runoff Ratio (R/P) Distribution

37. R/P vs. P/PET Ratio Comparison
>
<
P/PET

38. Continental Water Balance

39. World’s 16 Largest Rivers
Amazon discharge = total of No.2-10 rivers combined!

42. Distribution of Human-made Reservoirs

43. Key points of Today’s Lecture
The Global Hydrologic Cycle:
- Main reservoirs and residence times:
Oceans (1000yrs), atmospheric water vapor (10days), soil moisture and groundwater (days-decades), the cryosphere (snowpack, ice- cap, glaciers: seasons-1000yrs)
- Main fluxes:
Evaporation, precipitation, continental discharge,
ocean-land moisture transport
- Main physical processes:
Precipitation, evaporation, runoff

44. Next Topic:
Precipitation