Natural Resources Research Articles

Document Type

Article

Publication Date

10-2-2025

Journal Title

Journal of The Royal Society of Western Australia

ISSN

0035-922X

Keywords

Groundwater Decline, Streamflow, Drying Climate, Non-stationarity, Forest Hydrology

Disciplines

Forest Management | Natural Resources and Conservation | Water Resource Management

Abstract

Previous analysis of hydrological data in the eucalypt dominated forests of Western Australia’s Darling Ranges show that reduced winter rainfall over the last 50 years has lowered groundwater levels and diminished streamflow by up to 70%. Consequently, climate-independent reverse osmosis seawater desalination and groundwater resources now supply Perth and nearby regional areas, as runoff from forested catchments is no longer sufficient or reliable. In the high rainfall (~1200 mm/yr) mixed forest areas dominated by Eucalyptus diversicolor (karri) and Eucalyptus marginata (jarrah) 200 km to the south of Perth (the ‘Southern Forest’), similar analysis of climate impacts on watertables has not been conducted. In this paper we collate over 48 years of groundwater data (1975/76–2024) along with climate and streamflow records to understand changes in the Southern Forest catchments. We assemble data from eleven research catchments that were intensively studied from 1975/1976 until mid-1990s, then abandoned. Some of these small watersheds were logged (1976–78 or 1982–83) and others left as unlogged controls. Both were monitored to study impacts on streamflow and water quality. We returned to these catchments after two decades of lapsed monitoring, comprising a period of reduced rainfall and forest regrowth, to determine how watertables contained in the deeply weathered regolith had responded. Analysis of climate data indicates that, like the Darling Ranges, annual rainfall reduced during the latter part of the 20th century. There have also been changes to seasonal rainfall and temperature patterns. Relative responses vary depending on location. Results from over 48 years of groundwater monitoring, comparing levels taken at the end of each dry season, show watertables in the deeply weathered regolith have fallen on average by 5.1 to 6.7 m ( < 0.16 m/yr) beneath the stream zone and hillslopes in the paired and comparison catchments. In all cases, watertables are now > 5 m below the stream zone (range: 5 to 13 m bgl), indicating little or no connection. Average falls were greater in the intermediate and higher rainfall catchments (paired catchments – 6.3 m (treated), 5.9 m (control); comparison catchments, 8.9 m (treated), 4.6 m (control)), with maximum falls of 10-12 m in some individual bores. In the low rainfall Yerraminnup catchments, falls were less (~2 m), however most bores are now dry. The timing and magnitude of reductions in watertables was variable between catchments, but in all cases had accelerated since 2000 and especially after 2010, when the driest years in the region were recorded. Reduced watertables at the end of the dry season equate to losses of 42% of the saturated zone storage under the stream zone and up to 64% storage under hillslopes. If rates of fall are sustained into the future, extensive areas of saturated regolith under hillslopes and stream zones would be depleted. Regionally, reduced rainfall and watertable decline has contributed to a 66% reduction in streamflow of the Donnelly River since monitoring commenced in 1952. From 2000, the Donnelly River has become intermittent, with dry periods in summer regularly exceeding 100 days. Ecologically important forests and riverine systems may become vulnerable to receded streamflow if these trends remain. In dry years, seasonal flows are dominated by water generated from upland cleared catchments, with first and last flows resulting from overflowing farm dams. CSIRO modelling, that accounts for the effects of non-stationarity, and uses current Global Climate Models, indicates a potential for additional reductions of streamflow of 32–45% by 2059, if forecasts of reduced rainfall remain classified as ‘almost certain’. This paper presents several metrics to help explain recent drying recorded in the Southern Forest. It suggests some drivers that explain the change in streamflow regime and outlines actions to improve data available to inform future decision making.

Download

This file is 6.9 MB. Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS
 

Digital Object Identifier (DOI)

https://doi.org/10.70880/001c.144793