The University of New South Wales, Amrit Kathuria, NSW Department of ... for the Barraba Post Office rainfall station (Commonwealth Bureau of Meteorology no.
Effect of rainfall on groundwater trends over the past century in fractured rocks of the New England Fold Belt in the Namoi catchment, NSW Aleksandra Rancic, NSW Department of Natural Resources, Ian Acworth, School of Civil and Environmental Engineering, The University of New South Wales, Amrit Kathuria, NSW Department of Primary Industries, Gabriel Salas, Bill Johnston, NSW Department of Natural Resources Before 1987, the groundwater levels in fractured rocks of the Namoi sector of the New England Fold Belt were monitored infrequently. Therefore, this study exploited data collected for the standing water level (SWL) at the time of bore construction. Although patchy, such records extend back to the beginning of the twentieth century. Based on these data, the median SWL was calculated for each year. Residual mass curves and a 21year moving average low-pass filter were applied both to the annual SWL data and annual rainfall data for the Barraba Post Office rainfall station (Commonwealth Bureau of Meteorology no. 54003). Change point analysis was used to detect abrupt changes in the mean. The analysis iteratively uses a combination of residual mass curve charts and bootstrapping. Rainfall change-point analysis for 1900 to 2003 showed two distinct periods, separated by an abrupt climate shift in 1947 that was characterised by an increase in average rainfall from 627 mm/yr to 730 mm/yr (P.=0.03). A sudden rise in groundwater began in 1949, with the median groundwater level rising from 23.2 m to 15.9 m below ground (P.=0.01). This finished in 1955, when the level stabilised at 11.7 m below ground (P.=0.05). Thus, there was a lag in the groundwater response to climatic change of two to eight years, and total rise in the watertable of 11.5 m. An alternative methodology, in the form of a simple long-term moving average produced similar result, but in a more transparent form. This showed two states of dynamic equilibrium of rainfall and groundwater, separated by a mid-century hydrological shift. A time lag of around 5 years between changes in Barraba rainfall and groundwater was observed. The abrupt hydrologic shift in the middle of the century was replicated, as well as the deepening of the groundwater levels associated with a gradual drying of the climate in the past few decades. The major clearing and associated change in land use in this area had finished by the end of the nineteenth century, and the short groundwater response suggests that any rise in groundwater levels triggered by clearing must have been completed before groundwater recording began. Groundwater levels in this system are currently showing a trend towards a fall, following a similar trend towards a decrease in rainfall.
