Perth Laboratory – Public Seminar Series 2009

Streamflow Decline in South-west Western Australia: Groundwater and Surface Water Threshold Response in a Changing Climate

Dr Kevin Petrone
CSIRO Land and Water

Thursday 23 July 2009 at 3.30pm, CSIRO Auditorium

Abstract

A 10-20% reduction in rainfall in the catchments of the Darling Plateau of south-west Western Australia (SWWA) over the last thirty years has resulted in a 50 to 70% decline in streamflow to Perth dams. This decline is most pronounced in small headwater catchments, and streams that were once perennial have recently ceased to flow in the winter months. The cause of the amplified streamflow decline is the subject of a major project funded by the WA Water Foundation (formerly WA Premier’s Water Foundation). Here we examine a 20 year runoff and rainfall record for 14 catchments in the high rainfall zone. We found that 13 of 14 catchments showed a significant negative trend in runoff. While rainfall decline is well documented over the last 50 years in SWWA (BoM 2009), we did not observe significant rainfall decline since the late 1980s. This observation suggests that other catchment factors may be driving recent non-linearity between rainfall and runoff.

To examine the specific hydrologic mechanism for hydrologic response, we examined a minimally disturbed catchment with a continuous record of rainfall, runoff and groundwater depth. For this catchment, we observed a significant decrease in annual runoff (270 to 102 mm), runoff ratio (0.48 to 0.18), and average depth to the watertable (2.1 to 4.3m) over the 1989 to 2008 period. Further, negative relationships between depth to groundwater and runoff ratio suggest that changes catchment storage and the mechanism for streamflow generation may have changed appreciably in small headwater catchments with important implications for water yield. Simple hydrologic modelling (IHACRES) supports this assertion, with residual error between modelled and observed runoff increasing steadily over the last decade. We hypothesize that a reduction in the saturated area adjacent to the stream has created a threshold response that has accentuated the runoff decline in a changing climate in SWWA.

About the speaker

Kevin Petrone is a research scientist with CSIRO Land and Water in Perth. He received his Bachelor of Arts in Biology from Hampshire College, Amherst in Massachusetts, USA in 1993 and was awarded a doctorate from the University of Alaska, Fairbanks, Alaska, USA in 2005. Before joining CSIRO, he was a Carl Tryggers Postdoctoral Fellow at the Swedish University of Agricultural Sciences. His work has focused on climate change in pristine catchments in Alaska and Sweden, where he examined the effects of permafrost, fire, wetlands and snowmelt processes on carbon, nitrogen and solute flux from boreal forests.

More recently Dr Petrone’s work has focused on disturbed catchments in Western Australia and Tasmania with varying degrees of forest, agriculture and urban-dominated land-use. He is currently examining how different land uses influence water, energy and nutrient transport in natural and modified catchments. This research, being delivered through CSIRO’s Water for a Healthy Country Flagship, is guiding the restoration of Australian catchments in the face of changing climate and hydrologic conditions.

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