The role of climate variability and climate change in NSW water sharing arrangementsRichard Beecham (Water Modelling)

Shahadat Chowdhury (Water Modelling)

Mark Harris (Planning and Policy)

OVERVIEW OF TALK

Water resource plans in NSW Climate variability in developing the plans Climate variability versus climate change Future planning challenges

This presentation is relevant to large regulated rivers in NSW.

Unregulated rivers and aquifers have different challenges.

DEVELOPMENT OF WATER PLAN

Water Sharing Plans in NSW set the rules for sharing water between the environment and different consumptive users.

The plans are set for a ten year period. Plan is designed for climatic conditions that are likely to occur. Input: historical rainfall, evaporation, temperature and inflow

over 1895 to 2010 The impacts of various proposals are scrutinised using

numerical river system models.

TIME SERIES OF ANNUAL IRRIGATION DIVERSION

CURRENT PLAN’S RESPONSE TO CLIMATE STATE

PERIOD % of long term mean

Inflows Diversions

1893-2008 100 100

DRIEST DECADE 50 80

WETTEST DECADE 195 110

Simulated response of the Namoi Water Sharing Plan to climate variability

Ref: NamoWSP2010.sqq

CLIMATE VARIABILITY ISSUES

Do we understand climate variability well? Is 120 year data sequence long enough to reflect the extremes?

Statistical generation of longer time series may help simulate rare events.

Some limited palaeo studies indicated longer dry spells than the one found in the historical record.

Should the plan cater for extremes with say 500 year frequency?

Would that be a too risk averse plan?

FUTURE WATER PLAN CHALLENGES

More complex environmental or ecological targets. The need to consider climate change (key directive).

– National Water Initiative

– National Water Commission

– Commonwealth Water Act 2007

Significant uncertainty in quantifying the hydrological impact of climate change.

Demanding accreditation test – Eg. the Guide to the proposed (Murray Darling) Basin Plan. The

reduction of decadal water extraction at the same rate to any reduction of available water over that decade.

PLANNING IN STATIONARY CLIMATE

10 YR

VA

RIA

BIL

ITY

PLAN LIFE

Ann

ual R

ainf

all (

mm

)

Residual mass curve

NON STATIONARY CLIMATE STATE

10 YR

VA

RIA

BIL

ITY

PLAN LIFE

STATIONARY NON STATIONARY

YEARRA

INF

ALL

REFERENCE YEAR(2030 for Basin Plan)

MODELLING FUTURE CLIMATE

Options to estimate future climate time series Global Climate Model (GCM): 200 to 400 km grid Regional Climate Model (RCM): 10 km grid

– GCM to RCM: statistical or dynamical

Scaling of historical climate time series based on GCM– current practice

– computationally simple and provides long time series required for planning models

Scaling method

GCM/ RCM

ISSUES WITH GCM/RCM PROJECTION

Downscaling to be suitable for river system models, both in time and space scale. GCM to RCM– Computationally expensive.

Poor reproduction of historical rainfall including important low frequency events.

Large disagreements among different GCM’s rainfall projections.

Projections span from present to 2100. What should be the appropriate slice of time window to represent the climate variability during the life of the plan?

Selection of a short time window of GCM (10 year) means lower chance of experiencing long dry spells

SCALING METHOD

Poorly represent spatial differences of climate projections and hence do not represent altered spatial dependence expected due to climate change.

Do not alter historical long dry spell length, very important aspect of water planning! Hence underestimates extremes such as consecutive years of very low allocation.

d at e: 2 1 / 0 1 / 1 1 t i m e: 1 2 : 2 0 : 5 8 .6 0

W y an g al a D am S t o r ag e V o l u m e 2 0 3 0 C l i m at e C h an g e, M ed i an E m i ssi o n

0 1 /0 1 /1 8 9 6 t o 3 0 /0 6 /2 0 0 6

0

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

7 0 0

8 0 0

9 0 0

1 0 0 0

1 1 0 0

1 2 0 0

M

L*1

000

Y ear s

1900

1910

1920

1930

1940

1950

1960

1970

1980

1990

2000

C u r r en t C l i m at eC l i m at e C h an g e

Note the dry spells are of similar length!

THE WAY FORWARD

“Prediction is difficult, especially about the future” Niels Bohr “… it is premature to make definitive statements about the

levels of uncertainty in climate change impact ….”, Bates et al. (2010), Waterlines Report 28, NWC

Risk based approach of water planning in NSW: – plan formulated using current knowledge (past climate variability)

– test the robustness of the plan on plausible scenarios beyond the historical record

– include contingency measures and triggers for unprecedented events

– review plan at the 10 year planning cycle (use any new information).

The End

Author contacts:

Richard Beecham, richard.beecham@water.nsw.gov.au

Shahadat Chowdhury, shahadat.chowdhury@water.nsw.gov.au

Mark Harris, mark.harris@water.nsw.gov.au

SOME TYPICAL SIMULATION OUTCOMESIN THE PLANNING PHASE

What is the mean diversion over long term? : Important for comparison to 1993/94 MDB cap on diversion.

How often flows are available for environmental requirement (eg. end of system, wetlands) ? : Environmental outcome.

How often water allocation becomes low (say < 20%)? : Irrigation reliability.

What is the longest dry spell in terms of allocation? : Industry viability.

What is the lowest storage level? : Security of essential supply. … and many more

CLIMATE VARIABILITY VS CHANGE

Climate change can be defined as non stationary state of the climate.

Stationary: long term mean, spread (highs to lows), skew, persistence, seasonality, chance of rare events (extremes).

Our current modelling practice assumes stationarity, past long time series represents best estimation of future variability.

Climate Change : non stationary climate and hence past long time series may not be adequate to define future variability.

CURRENT PLAN’S RESPONSE TO CLIMATE STATE

AVERAGE MAJOR INFLOW

AVERAGE DIVERSION

AVERAGE END of SYSTEM FLOW

Full length

1893 to 2008

705 GL/a

(100%)

235 GL/a

(100%)

660 GL/a

(100%)

Driest decade

1936 to 1946

355 GL/a

(50%)

190 GL/a

(80%)

330 GL/a

(50%)

Wettest decade

1947 to 1957

1390 GL/a

(195%)

260 GL/a

(110%)

1590 GL/a

(240%)

Ref: NamoWSP2010.sqq

Simulated response of the Namoi Water Sharing Plan to climate variability

CURRENT PLAN’S RESPONSE TO CLIMATE STATE

AVERAGE MAJOR INFLOW

AVERAGE DIVERSION

AVERAGE END of SYSTEM FLOW

Full length

1898 to 2008

750 GL/a

(100%)

280 GL/a

(100%)

100 GL/a

(100%)

Driest decade

1901 to 1911

435 GL/a

(60%)

265 GL/a

(95%)

40 GL/a

(40%)

Wettest decade

1948 to 1958

1480 GL/a

(195%)

335 GL/a

(120%)

210 GL/a

(210%)

Simulated response of the Lachlan Water Sharing Plan to climate variability

Ref: LachW018.sqq

FUTURE PLAN’S RESPONSE TO CLIMATE STATE

AVAILABLE WATER

SURFACE WATER

EXTRACTION

MAJOR INFLOW *

CURRENT

IRRIGATION*

Full length 100% 100% 100% 100%

Driest decade A % A% or less ~ 50% ~ 90%

Wettest decade B % B % or less ~ 200% ~ 110-120%

Climate change

Full length

C % C % or less 90-95% 90-99%

The Guide to the Basin Plan proposed the accreditation tests shown in first two columns:

*case study: Lachlan & Namoi