Changing land use in an uncertain climate: impacts on surface water and groundwater in the Goulburn River NSW

Abstract

As a key contribution to the United Nations (UN) sustainable development goals, the UN high-level panel on water (HLPW) in 2018 identified as a priority the prevention of degradation and pollution of rivers, lakes and aquifers, and the restoration and maintenance of acceptable environmental conditions and water quality at the local, national and regional scale. The HLPW report recognised water crises are exacerbated by climate change. Three of the ten objectives of the Australian National Water Initiative were: transparent, statutory-based water planning; statutory provision for environmental and other public benefit outcomes and improved environmental management practices; and recognition of the connectivity between surface and groundwater resources with connected systems managed as a single resource. This work has applied the HLPW's Foundation for Action to a river water quality issue, salinity, at the local scale, where natural processes are added to or altered by land-use change and climate variability. The focus is to better inform the planning process in a surface-groundwater connected river subject to variable climate. While the 7,800 km2 catchment is of local concern, it has national economic and environmental significance. The Goulburn is the largest tributary of the Hunter River, a highly connected surface-groundwater system with diverse hydrogeology, land use and vegetation, multiple regulations and a spatially and temporally varying climate. Land-use across the Goulburn catchment, once dominated by agriculture and national parks, has been changing with expansion of coal mining. Salinity is a major issue in the Hunter and the Hunter River Salinity Trading Scheme (HRSTS) was introduced to control mine discharge. The Goulburn is not part of HRSTS. The aim of this work is to assess the impacts of changing land use on trends in stream flow, catchment yield, surface and groundwater interaction, and stream salinity in the Goulburn River system, and to separate them from the impacts of climate change, natural climate variability and regulatory changes. Analysis of seasonal and annual rainfall data since 1920 found the only significant trend in rainfall was in warm season, October to March, rainfall. The trend was +10 mm/decade. Climate change model projections suggest cooler season rainfall will decrease. Significant decreases and increases in rainfall trends over 30-year periods were found with a frequency similar to the Pacific Decadal Oscillation. Reliable pan evaporation (Epan) data since 1970 has no significant annual or seasonal trends despite published reports of general declines in Epan globally. Previously recognised decadal flood and drought dominated periods were evident in stream discharge. A change in response of the catchment to recent droughts was found, including a loss of low flows over the past 30 years. Catchment water losses have increased by about 10 mm/year since the 1970s. Current water allocation licences are equivalent to about one third to one half of mean catchment specific discharge. Vegetation changes in response to regulation and altered farm practice may have contributed as well as mine dewatering. Stream discharge elasticity to rainfall shows an additional non-rainfall source of water in the upper catchment in the last 20 years. Discharge of mine make water has occurred over this period. Salinity profiles of the river during dry periods shows an array of sources of salinity. These were associated with tributaries incising into geologies with higher salinity aquifers, such as the Permian Coal Measures and Jurassic shales. Importantly, tributaries draining lower salinity geologies such as the Tertiary volcanics or Triassic sandstones mitigate salinity in the Goulburn. The inverse relationship between stream salinity and stream discharge leads to an important conclusion. Extracting more fresh water from catchments will increase the salinity of Goulburn discharged into the Hunter. .conts. Show more