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

11 th 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 ; 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

11 th IMSC Edinburgh 12/7/2010 Data Flow Diagram Trends or Change Points (LLR) Trends & Explanatory Covariates (SLR & GLM) Weather State Probability Series (NHMM) At-Site Daily Rainfall Atmospheric Predictors Key Atmospheric Predictors (ANOVA) Annual Inflow Period of interest: 1958 to 2007

11 th IMSC Edinburgh 12/7/2010 Transformed Annual Inflow Series

11 th IMSC Edinburgh 12/7/2010 Significance Traces

11 th IMSC Edinburgh 12/7/2010 Sub-region used for Rainfall Analysis Dots indicate main IWSS dams

11 th IMSC Edinburgh 12/7/2010 Results from Generalised Linear Modelling Trend in log mean wet-day rainfall amounts per decade: – – ( x Lat1) – ( 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

11 th IMSC Edinburgh 12/7/2010 Changes in Atmospheric Circulation

11 th IMSC Edinburgh 12/7/2010 Rainfall Occurrence & Synoptic Patterns (NHMM)

11 th IMSC Edinburgh 12/7/2010 Weather State Probability Series 'Dry''Mixed''Wet' Little evidence of changes in persistence May-June-July

11 th IMSC Edinburgh 12/7/2010 Sensitivity of Weather State Probability Trends |  | ●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 ●2 4 factor combinations ●3 NHMM simulations per combination ●compare differences in frequencies for 1 st & last 10 years ●Linear model coefficients represent effects of factors individually & in combination Influential predictors

11 th 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