Investigations of surface water ground-water interactions in a water stressed semi-arid catchment

River and groundwater hydrographs and differences in river flow between gauging stations were analysed for more than 30 years of data from the Maules Creek Catchment, NSW. The analysis shows that groundwater abstraction for irrigation has had a measurable impact on the groundwater resource with up to 4 m of drawdown and subsequently on the baseflow in the nearby Namoi River. The data indicate that the investigated section of the river have been gaining in the past (for the periods where data were available) whereas since the mid-90ies it has been losing water, especially during periods of low flow in the river [1, 2]. A catchment scale groundwater model accounting for all water fluxes into and out of the catchment supports the analysis of the hydrographs and river flow data [3]. This change from gaining to losing conditions is caused mainly by the groundwater abstraction, as no direct link to climate variability (i.e. drought) could be shown. However, climate variability had an indirect effect since groundwater pumping for irrigation was elevated in drier years [3]. The overall agreement between the hydrograph analysis, the differential river flow analysis and the catchment modelling indicate that these methods are fairly robust for assessing the overall effect of ground water abstraction on surface water groundwater interactions. Environmental isotopes (stable isotopes of water, tritium, 13C and 14C) were used as tracers to determine the sources of groundwater, flow paths and groundwater flow and recharge rates. In a qualitative sense the environmental isotope data clearly supported the conclusions from the analysis of hydrographs and river flow data. For instance in the north of the catchment where the river has only recently made the switch from gaining to losing, old groundwater can still be found in the discharge zone. Further south in the catchment near the centre of the irrigation, modern river water was found to be recharging the aquifer below the river to depths of up to 100 m. Water exchange rates through the riverbed were obtained over a year at discrete points (6 locations over a 34 km stretch of the river) by tracing the daily heat variation [4, 5]. These flow rates were found to fluctuate greatly over both time and space. At any particular location increases in the loss rate (i.e. recharge) correlated with floods in the river whereas the during the recession period following the flood, conditions were often gaining (i.e. return flow of bank storage). The cumulated fluxes derived from the heat data over a flow event did not correlate with flux rates derived from the differential flow gauging indicating that larger variability in the river reach were not captured by the heat tracing at discrete locations or that the heat data did not represent flow processes through the banks. The times series of riverbed exchange rates derived from the heat data also revealed that the hydraulic conductivity of the riverbed is not constant over time (something that is often assumed in groundwater models). The hydraulic conductivity was found to increase during flood events due to scouring, whereas in periods of low flow the hydraulic conductivity was decreasing due to sedimentation and colmation.