Assessment of apparent non-stationarity in hydroclimatic series: A case study from

Western Australia

Bryson Bates (CSIRO, Australia) Richard Chandler (UCL, UK) Steve Charles & Eddy Campbell (CSIRO)

Climate Adaptation

11th IMSC Edinburgh 12/7/2010

Background

● Study region: ● southwest corner of Australian mainland

● Mediterranean-like climate – hot dry summers; cool wet winters

● ~ 80% rainfall occurs in winter half-year (May-October)

● low orography (max height 582 m above mean sea level)

● Integrated Water Supply System supplies water for 1.6 million in Perth & surrounding areas

● Previous research findings (IOCI): ● declines in number of wet days & extreme amounts; confined mainly to May-

July

● increase in MSLP in winter & decline in atmospheric moisture in winter & spring important in explaining declines

● reduction in intensity of cyclogenesis across southern Australia (esp. SWA)

● Infrastructure investment: $A921 million invested in source development 1996-2006; another $A1 billion after 2006

● Our approach: application of a suite of modern, model-based statistical approaches to build explicit representation of changes in surface water availability and associated climatic drivers

11th IMSC Edinburgh 12/7/2010

Data Flow Diagram

Trends or Change Points (LLR)

Trends & Explanatory Covariates (SLR & GLM)

Weather State ProbabilitySeries (NHMM)

At-Site Daily Rainfall

Atmospheric Predictors

Key Atmospheric Predictors (ANOVA)

Annual Inflow

Period of interest: 1958 to 2007

11th IMSC Edinburgh 12/7/2010

Transformed Annual Inflow Series

1920 1960 2000

040

080

0

Water Year

1920 1960 200010

100

1000

Water Year

Infl

ow

(G

l)

11th IMSC Edinburgh 12/7/2010

Significance Traces

2 6 10

0.00

0.01

0.02

0.03

6 8 10 12

0.1

0.2

0.3

Pro

bab

ilit

y v

alu

e

Smoothing parameter Smoothing parameter

LLR vs LR Discontinuity Test

11th IMSC Edinburgh 12/7/2010

Sub-region used for Rainfall Analysis

Dots indicate main IWSS dams

11th IMSC Edinburgh 12/7/2010

Results from Generalised Linear Modelling

Trend in log mean wet-day rainfall amounts per decade:

–0.0289 – (0.0222 x Lat1) – (0.0498 x Long1)

where Lat1 and Long1 first-order Legendre polynomial transformations of latitude & longitude

Interpretation: rainfall decline strongest in the north and east

Trend in daily rainfall occurrence:

Interpretation: decline strongest in the west at low altitudes

11th IMSC Edinburgh 12/7/2010

Changes in Atmospheric Circulation

M J J A S O

-10

12

a)

MSLP

M J J A S O

-0.5

00.

51

b)

PG

M J J A S O

01

23

c)

DTD

M J J A S O

-0.1

00.

10.

20.

3

d)

CV1

Month Month

Month Month

11th IMSC Edinburgh 12/7/2010

Rainfall Occurrence & Synoptic Patterns (NHMM)

11th IMSC Edinburgh 12/7/2010

Weather State Probability Series

'Dry' 'Mixed' 'Wet'

Little evidence of changes in persistence

May-June-July

a b c

Year Year Year

Pro

bab

ility

1960 1980 2000 1960 1980 2000 1960 1980 2000

0.00

0

.25

0.5

0

0.2

0

.4

0.

6

0.2

0

.4

0.6

11th IMSC Edinburgh 12/7/2010

Sensitivity of Weather State Probability Trends |

|

0.0 0.5 1.0 1.5 2.0

0.00

0.01

0.02

0.03

0.04

0.05

MJJ: P(S = 5)

HALF- NORMAL QUANTILES

● Factorial experiment

● fwd and bwd reordering of atmospheric predictor series for NHMM (intraseasonal structure preserved)

● reversal of sequence for dominant factor, reversal in sign of trend

● 24 factor combinations

● 3 NHMM simulations per combination

● compare differences in frequencies for 1st & last 10 years

● Linear model coefficients represent effects of factors individually & in combination

Influential predictors

11th IMSC Edinburgh 12/7/2010

Conclusions

● Little evidence of discontinuity in the mean of annual dam inflow series (smooth, nonlinear declining trend)

● Substantial regionally-varying declines in daily rainfall occurrence & wet-day amounts in vicinity of dam catchments

● Increasing/decreasing trends in probabilities of 'dry'/'wet' synoptic types for MJJ (bleak short-term prognosis) – little evidence of changes in persistence

● Increase in MSLP & DTD in MJJA; decrease in PG in MJJ – all important & all favour drier conditions

● Temporal orderings of MSLP, PG and DTD have demonstrable impact on trends in the weather states probability series – trends driven by individual factors & not their interactions

● Tangible benefit in using a multifaceted approach to the study of the nature and drivers of non-stationarity in hydrologic series