Runoff and groundwater responses to climate change in South West Australia

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 gainingto 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.