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

A ATM301, Lecture #3 (section 2.2) The Global Hydrologic Cycle Instructor: Aiguo Dai Date: 9/03/15 Thu 2

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

The Key Questions: 4 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 hrdrologic cycle?

The Global Hydrologic Cycle (From Trenberth et al., J. Hydrometeorol., 2007 ) 5

Summary of the Global Hydrologic Cycle 6 Definition: It refers to the movement of water on, above or below the surface of the Earth. Also known as the hydrological 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: 413,000km 3 /yr; land: 73,000km 3 /yr) - The atmosphere to the surface: precipitation (Ocean: 373,000km 3 /yr; land: 113,000km 3 /yr) - Atmospheric transport: Ocean to land (40,000km 3 /yr) - Land to ocean runoff: 40,000km 3 /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/m 2 of latent heat into the atmosphere. Water vapor and clouds affect atmospheric energy budgets.

Why Do We Care about the Hydrologic Cycle? 7 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:

9 From

Major Water Stocks

Major Water Stocks and Fluxes

13 An Estimate of the world water reservoirs. Source: MIT OpenCourseWare. Surface area (million km 2 ) Volume (million km 3 ) Volume (%) Equivalent depth (m) Residence time Oceans and seas 3611,370942,500~4,000 years Lakes and reservoirs < ~10 years Swamps<0.1< years River channels<0.1< ~2 weeks Soil moisture < weeks to 50 years Groundwater weeks to 100,000 years Icecaps and glaciers to 1,000 years Atmospheric water < ~10 days Biospheric water <0.1< ~1 week

14 Trenberth et al. (2009, BAMS) The Hydrologic Cycle is Linked to the Energy Cycle

Surface Water Balance P E R W Water Storage dW/dt = P- E - R 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. 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.

Main Water Cycle Fluxes: Precipitation Evaporation Runoff Main Water Cycle Fluxes: Precipitation Evaporation Runoff

Precipitation 17 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 5mm) to fall out of the cloud. -Snowflakes form when tiny super-cooled cloud droplets (d  10  m) freeze.

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? mm/day Precipitation Distribution GPCP = Global Precipitation Climatology Project

Annual Precipitation Over Land

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

Seasonal Precipitation Variations

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

Reversal of Monsoon Winds Wind convergence zone

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

Evapotranspiration over Land Wang and Dickinson (2012) 26 Evaporation

27 Turbulent Eddy Mixing From

Observations of E Eddy Covariance (EC) Method: It measures SH and LH fluxes from their covariance with vertical velocity using rapid response sensors at frequencies ≥10Hz. First used by Australian scientists in the 1950s, now used at 500+ FLUXNET sites. Error range: 5-20%.

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

Actual Evaporation Distribution 30

Evaporation (E) vs. Precipitation (P) 31

Long-term Mean E – P Difference Map Trenberth et al. (2007, JHM) 32

33 Zonal-mean Precipitation and Evapotranspiration

34 > < Annual P/PET Ratio

Key Differences between P and E 35 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.

Runoff 36 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.

37 Oki & Kanae, Sci., 2006 Global Discharge: ~38,000km 3 /yr = Global Potential renewable freshwater resource Potential Freshwater Resources

38

Runoff Ratio (R/P) Distribution 39

R/P vs. P/PET Ratio Comparison 40 > < R/P P/PET

Continental Water Balance

World’s 16 Largest Rivers

Distribution of Human-made Reservoirs

Key points of Today’s Lecture 47 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

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