Marine meteorology

27th October 2011

Evaporation and climate change

Marine Science DepartmentFisheries and Marine Science Faculty

Padjadjaran University

Outline

Evaporation

Stages

Processes

Climate change

Modelling evaporation

short term

Long term

Current observations

Summary

References

Boiling point of certain liquids

Liquid Boiling point Water 100°c

Mercury 357°C

Ethyl alcohol 79°C

Methyl alcohol 64°C

Glycerol 290°C

Turpentine 156°C

Tutorvista.com (2011)

Paul Durack/CSIRO (2010)

A) Sea surface salinity changes from 1950-2000. Red areas become saltier through

enhanced evaporation, and blue fresher through enhanced rainfall.

B) The mean annual water transport over the ocean. Blue is where more rainfall than

evaporation occurs over a year, whereas orange is the opposite.

Paul Durack/CSIRO (2010)

Evaporation Evaporation is the phenomenon where a substance is

converted from liquid or solid to a vapour.

Special cases of evaporation

Sublimation is were a substance goes from solid to vapour.

Transpiration is the vapourisation of water through stomata in living plants.

Units for evaporation

Depth of water (mm/day)

Mass of water (kg.m-2.s-1)

Energy consumed (Watt.m-2)

Number of molecules leaving a surface (number.m-2.s-1)

The Evaporation Process

Figure 1: Diagram of the evaporative process. Water molecules gain energy and move from the

liquid phase to the vapour phase.

Evaporation

There are different kinds of evaporation

Potential

Actual

Actual evaporation occurs in 3 stages

Stage 1

Stage 2

Stage 3

Potential Evaporation Evaporation that would occur given perfect unlimited water

supply.

Measured with a Class A pan

Calculated with Penman-Monteith equation

Used to Calculate Reference Crop Evapotranspiration.

Actual evaporation

The actual evaporation from a surface.

Limited by many factors

Water availability

Dryness of the air

Ventilation of wind

Can be measured in many ways:

Energy budgets

Lysimetry (lysimeter)

Salinity

Stages of Evaporation

Stage 1

Evaporation can occur freely at the surface.

Soil surface can freely

evaporate

Soil column is

wetted up to

field capacity.

Stages of Evaporation

Stage 2

Evaporation inhibited by soil surface

Soil surface had

dried. Water can

move through

capillaries to

evaporate at the

surface.

Saturated soil has

reduced.

Stages of Evaporation

Stage 3

Evaporation occurs within the soil column and must move as

a vapour

Soil column has

dried completely.

Water moves as

a vapour

through the

column of soil

Very little if any

saturated zone left

What affects Evaporation

Wind

Relative humidity

Temperature

Solar radiation

Energy

Water availability

Factors

Wind mixes the air adjacent to the surface.

Weak wind = not much mixing

Strong wind =lots of mixing

Relative Humidity limits the amount of vapour that can enter

an air parcel.

Temperature allows a body of water to gain energy.

High (low) temperature= more (less) evaporation

Factors

Energy into the system controls the amount of

evaporation as without energy the water molecules

cannot evaporate.

Water availability limits evaporation.

If there is no water then there is no evaporation!

Climate change

Are these parameters going to change with climate change?

Implications of this change?

So where do we go with this?

Modelling Evaporation

Why do we want to model evaporation?

Evaporation is the main part of the water cycle.

To understand evaporation is to understand the water budget of

the future.

Actual evaporation has been modelled

First was done in short term

Then done in the long term

Short term Actual Evaporation

Modelling

Eastham and Gregory (2000) used measurements from one

year to predict evaporation for the next year.

Developed from the regression analysis.

Separate equations for different phases of drying.

Successful from one year to the next

Eastham and Gregory (2000) Results

Figure 2: Correlation between evaporation measured beneath

canopies of wheat and lupin by micro-lysimetry (Ec) and

predicted using empirical models (Epred) for measurement in a)

1991 and b) 1992

a b

What does this mean?

In modelling evaporation from one year to the next. Why not

predict into the distant future?

Aydin et al 2008 did this.

Used Penman-Monteith and his own equation to derive a

predicted actual evaporation.

Modelled two periods

1994-2003

2070-2079

What was found for the future

Figure 3: Changes in annual reference evapotranspiration (ETr), and

potential (Ep) and actual (Ea) soil evaporation between present and

future climate in Adana (Aydin et al 2008)

Implications of assumptions made

Uses soil parameters to predict actual evaporation.

Matric potentials depend on temperature.

Used projected temperatures to get these.

Projected temperatures do not allow for changes in other

components of the atmosphere.

What does this mean?

Aydin et al (2008) suggests that the potential evaporation will

increase with climate change.

Roderick and Farquhar (2004) studied the potential

evaporation from Australia.

Roderick and Farquhar (2004) results

They found it has decreased.

Table 1: Number of sites showing statistically significant

changes (p>0.95) in annual pan evaporation and rainfall in

two reporting periods (Roderick and Farquhar, 2004).

Roderick and Farquhar (2004) results

Figure 4: Overall trends in annual pan evaporation rate Epan and annual

rainfall rate averaged over 30 sites from 1970-2002. (Standard error shown

in brackets, Epan trend is significant (p>0.95) but the rainfall trend is not

significant) (Roderick and Farquhar 2004)

Summary

Evaporation is a dynamic system. Depends on:

Wind

Temperature

Relative humidity

Energy

Water availability

These variables will change with climate change, but the

extent is not fully known.

Evaporation can be predicted in the short term.

Long term prediction of evaporation needs improvement.

references http://images.google.com/imgres?imgurl=http://www.tapintoquality.com/facts/glossary/evaporation.jpg&im

grefurl=http://www.tapintoquality.com/facts/glossary-d.html&usg=__vbzrZR7iIKjEhG3CSZRWV3bO-b0=&h=262&w=500&sz=42&hl=en&start=3&um=1&tbnid=2L3f8aH_DVeq5M:&tbnh=68&tbnw=130&prev=/images%3Fq%3Devaporation%26hl%3Den%26rls%3Dcom.microsoft:*%26sa%3DN%26um%3D1 evaporation image

Allen, R. G., L. S. Pereira, D. Raes, and M. Smith (1998). "Crop evapotranspiration - Guidelines for computing crop water requirements - FAO Irrigation and drainage paper 56." Available at: http://www.fao.org/docrep/X0490E/x0490e00.htm.

Aydin, M., T. Yano, F. Evrendilek and V. Uygur, 2008. Implications of climate change for evaporation from bare soils in a Mediterranean environment, Environ Monit Assess (2008) 140:123–130, DOI 10.1007/s10661-007-9854-4

Aydin, M., & Uygur, V. (2006). A model for estimating soil water potential of bare fields. In Proceedings of the 18th International Soil Meeting (ISM) on Soils Sustaining Life on Earth, Managing Soil and Technology, Şanliurfa, pp. 477–480

Eastham, J. and P. J. Gregory, 2000. Deriving empirical models of evaporation from soil beneath crops in a Mediterranean climate using microlysimetry, Aust. J. Agric. Res., 2000, 51, 1017–22

Intergovernmental panel for climate change, no date, ‘Working Group III Report "Mitigation of Climate Change"’, viewed 15/4/2009, http://www.ipcc.ch/ipccreports/ar4-wg3.htm

Roderick, M. L., and G. D. Farquhar, 2004. Changes in Australian Pan Evaporation from 1970 to 2002, Int. J. Climatol., Vol. 24, 1077–1090

Roderick ML, Farquhar GD. 2002. The cause of decreased pan evaporation over the past 50 years. Science 298: 1410–1411.