Hydrologist — Mount Barker & the Shire of Plantagenet

A shire that sits on a drainage divide

The Shire of Plantagenet covers about 4,790 square kilometres and does not belong to one catchment. The Kalgan River rises north-west of Kendenup and runs south to Oyster Harbour at Albany. The Hay River rises just west of Mount Barker and runs the other way, south-west to Wilson Inlet at Denmark. The Kent River headwaters sit near Tenterden at the shire’s northern edge and drain to Irwin Inlet. Which of those three a site sits above changes the receiving water, the downstream boundary condition and, in practice, which agency has an interest in the assessment.

The Kalgan is the dominant system. DWER puts its catchment at about 2,490 square kilometres, 77 per cent cleared as at 2018, with 12 gigalitres of flow and a median salinity of 6,300 mg/L in 2019. The longer salinity record, measured above the Stevens Farm gauge on 2,384 square kilometres, gives a mean of 4,700 mg/L across 1993 to 2002 against 62 per cent clearing in 1996 — moderately saline, on a long-term downward trend that reversed upward after 1990. Downstream the Kalgan discharges to Oyster Harbour, a 3,000 square kilometre catchment roughly two-thirds cleared, where the estuary’s seagrass ecosystem collapsed in the 1990s and has been in recovery since.

The Stirling and Porongurup ranges make the rainfall input harder than it looks. Bluff Knoll stands at 1,099 metres and the range runs more than 60 kilometres west to east — the only vertical obstacle to weather in any direction. The plains south of the range average about 575 millimetres a year and the plains north near Borden about 400 millimetres, while the peaks are estimated near 1,000 millimetres. That is a rainfall gradient approaching two and a half times across roughly twenty kilometres. Mount Barker itself averages 721 millimetres over a Bureau record beginning in 1886, sharply winter-dominant with a July mean of 102 millimetres. A design rainfall lifted from the townsite gauge and applied to a catchment reaching into the range will understate the input.

There is no DWER floodplain mapping anywhere in the Shire of Plantagenet — Mount Barker, Kendenup, Narrikup, Porongurup and Rocky Gully all sit outside the state’s twenty-six mapped study areas. Local Planning Scheme No. 5 nonetheless provides that a building shall not be constructed upon any land defined by the local government as being liable to flooding or inundation. In the absence of published mapping, establishing whether a particular site is liable to inundation is a site-specific hydrological question, and the shire’s Local Planning Strategy review beginning in 2026 will not change that for individual applications.

Two constraints frequently catch rural proposals here. Bolganup Creek, in the Porongurup Range, is a proclaimed surface water area under the Rights in Water and Irrigation Act 1914 — taking surface water there requires a licence. Outside it, most of the shire is unproclaimed for both surface water and groundwater, which means a farm dam may be built without a licence provided it does not store water on a watercourse or wetland and causes no sensible reduction in downstream flow. Establishing that second point is a yield and catchment calculation, not an opinion. Separately, only Mount Barker has reticulated sewerage; Kendenup, Narrikup and Porongurup do not, and Rocky Gully has neither potable water nor sewerage — so on-site effluent disposal, drainage and shallow water tables have to be resolved together rather than in sequence.