Dealing with salinity in Wheatbelt Valleys - Department of Water

Hatton and Ruprecht
WATCHING THE RIVERS FLOW: HYDROLOGY OF THE WHEATBELT
Tom Hatton 1 and John Ruprecht 2
ABSTRACT
The surface hydrology of Western Australian wheatbelt catchments is characterised by low gradients, high potential
salt loads, and high variability in flows. It is also distinctive in the variability of the actual catchment area contributing
to flows at the outlet in any given year. The flood event of January 2000 is highlighted as an example of the
mercurial behaviour of these systems, in that the runoff originated from areas a great distance from the outlet,
linking many lakes systems on the way. The condition and trend of these catchments is described, in particular the
trends and impacts of nutrients, salt and flooding. While strategic revegetation and improved farm practice have the
potential to reduce sediment and nutrient yields, it is argued that only appropriate engineering can significantly
reduce the impacts of stream salinisation and flood risk.
INTRODUCTION
The rivers of the West Australian Wheatbelt are
distinctive from rivers elsewhere in a number of
significant ways, and pose unique challenges to the
people that live in that region and those that live
downstream. In this paper, we review the natural
characteristics of these systems, the changes they
have undergone since settlement, and forecast their
future condition. Against a background of the needs
and trends in regional sociology and economics, we
consider the options for river management open to
us.
Physical Setting
The region under consideration drains large portions
of the South West Drainage Division of Western
Australia (Mulcahy & Bettenay 1972), and comprises
the drainage basins of the Swan-Avon River, the
Moore-Hill Rivers/Yarra-Yarra/Ninghan system (also
known as the Monger System, sensu Beard 1999),
and the Blackwood River (Figure 1). Drainage Basins
with some similarities to these also include
Frankland-Gordon River, Kent River, upper Pallinup,
Gairdner, Fitzgerald, Lort and Young rivers.
By far, the largest portion of this region drains via the
Avon River into and through the Swan system. This
particular catchment is also significant in that its
health most directly impacts most West Australians,
in that it flows through the City of Perth. Note that
this catchment is roughly the size of Tasmania
(12 million ha). The bulk of this paper is devoted to
a consideration of wheatbelt valley hydrology in this
particular catchment, although examples are drawn
from other catchments where relevant.
Climate
1 CSIRO Land and Water, Private Bag No 5, PO WEMBLEY WA 6913
2 Water and Rivers Commission, Hyatt Centre, 3 Plain Street, EAST PERTH WA 6004
– 1 –
The first distinctive feature of these rivers is that
mean annual rainfall is highest at or near their outlets
to the sea (900–1,200 mm), and lowest in their
uppermost headwaters (less than 350 mm)
(Figure 2). The climate is typically Mediterranean,
with cool wet winters and hot dry summers.
However, summer rainfall does occur from northwest
cloudbands and less frequently from tropical
depressions. The summer events are typically the
most extreme rainfall events. The potential
evaporation is the reverse of the rainfall, with
800-1,200 mm mean annual evaporation near the
coast and over 2,000 mm in the most inland parts of
the catchment.