Conclusions
This work highlights that trends in streamflow can be difficult to
detect, even using a range of methods. The analysis is sensitive to the
aggregation scale, the noise in the data and the length of the data
series. In this work, only very small downward trends in the streamflow
for 13 hydrological reference stations in Australia were observed
between 1970 – 2010, and only four stations indicated consistent
statistically significant trends across the different methods applied.
This is despite evidence of increasing trends in temperature in all
catchments, and declining rainfall in the southern part of the
continent. While the analyses identified trends in the 1970 – 2010
data, the Mann Kendall analysis under scaling hypothesis did not always
identify significant trends, meaning that we cannot yet draw a definite
conclusion on the long term decline in streamflow, but can be definite
about trends in the 41 years under investigation.
In the catchments where a statistical downward trend was detected, only
a small part of the trend could be directly explained by changes in
rainfall and temperature. As a result, extrapolation of models
calibrated on current datasets might bias future predictions,
specifically if the assumption is that the runoff response to rainfall
remains stationary.
Currently there is no clear biophysical explanation for the remaining
negative trends after accounting for rainfall and temperature changes
(the observed amplification). Given that the catchments were chosen
based on homogeneous landcover, this suggests that possible vegetation
effects impact the observed trends. An alternative explanation is that
timing and distribution of rainfall affects the rainfall runoff
response. Both these areas will require further research.