Runoff and Groundwater Responses to Climate Change in South West Australia
Authors: MCFARLANE DON , GEORGE RICHARD , RUPRECHT JOHN , CHARLES STEVE , HODGSON GEOFF 
Published: February 2025

A warming and drying climate in the South West of Australia since about 1975, and especially

from 2000, has reduced flooding and lessened salinisation risks, but also has reduced fresh water

supplies. As well as continuous trends, there have been abrupt changes in hydrological processes as

groundwater levels have receded from valleys.

In some cleared inland wheatbelt areas, saline groundwaters are still rising where the watertable

is deep, clearing has been recent and/or the reduction in rainfall has been limited. Over time,

groundwater level changes will better reflect the drying climate. The reduction in rainfall and

higher potential evaporation rates has dried catchments overall, and greatly reduced runoff and

major flooding, even in catchments where salinity is still expanding across valley floors. Major

flooding after rare storms may occur, but now is more likely in summer than winter. It is now

driven by infiltration-excess rather than saturation-excess runoff processes, depending on landscape

position, and rainfall amount and duration. Episodic events, such as occurred in 2017, may increase

salinity and flooding for a period despite the overall decreasing trend in such risks.

In the largely cleared Zone of Rejuvenated Drainage and vegetated Darling Range, groundwater

levels in cleared areas are close to reaching a new equilibrium, with the drier climate reducing

salinisation risks. In the largely forested Darling Range, groundwater levels are falling below

stream beds thereby substantially reducing runoff into dams in the western part of the zone where

groundwaters are fresh. As a result, Perth (population two million) has transitioned from being

almost entirely dependent on such runoff for its drinking water, to not having any usable runoff in

some years.

In the Perth Basin, groundwater levels are falling within sedimentary strata as the predominantly

perennial vegetation uses a high proportion of incoming rainfall. The less intense and more

intermittent rainfall is also increasing canopy interception and unsaturated-zone water losses.

Cleared areas with high watertables are least affected because a reduction in recharge may be

offset by less rejected runoff, lower drain flows and evaporation from vegetation tapping into

groundwater. This buffering will continue until groundwater levels fall beneath drain inverts and

plant rooting depths. The impact of the drying climate on groundwater levels has been masked

to date by increasing recharge due to clearing and urbanisation. Streams connected to strata in

the Perth and Collie basins usually gain fresher water from unconfined aquifers. With falling

groundwater levels drainages are transitioning from gaining- to losing-streams, with reducing

surface water flows and increasing risk of aquifer salinisation where the cross-cutting streams are

saline.

Climate projections indicate a continuing drying trend is likely with increased temperatures

and possibly a greater proportion of annual rainfall, and therefore flood risks, in summer. Water

yields in dams and aquifers will continue to decline if these projections are correct. However, risks

associated with too much water (salinisation, flooding, soil waterlogging) will probably continue to

abate unless the amount and/or intensity of rainfall increases in the future.