2.1.5.3 Overview of controls on surface water – groundwater connectivity based on previous investigations in the Namoi river basin

There is a degree of subjectivity in categorising a stream reach as connected or disconnected and a flux as gaining, variably gaining-losing or losing due to the dynamic variations in both groundwater and stream stage elevations that change temporally and spatially in response to a range of natural and human factors, some of which are listed below.

2.1.5.3.1 Natural controls

Hydrologic fluxes between surface water and groundwater systems are partly controlled by:

2.1.5.3.2 Human controls

Human factors will have a measurable control on groundwater and stream stage elevations, with hydraulic gradients continually responding to these factors. Human factors include:

The shallow aquifers of the Upper Namoi and Peel rivers have been identified by Barrett (2012) as highly connected to the adjacent streams, with more than 70% of the groundwater extraction volumes estimated to be derived from streamflow (Broadstock, 2009). In these areas groundwater recharge and resource availability is highly dependent on surface water flows (Green et al., 2011), and groundwater extraction bores are commonly located within a few kilometres of connected aquifer–stream systems.

Groundwater extraction from aquifers in hydraulic connection with a stream may result in a reversal of flux direction, with the direction of flow dependent on the difference between the groundwater elevation and stream stage. Gaining streams may become variably gaining-losing streams as groundwater elevation and stream stage relationships fluctuate under the influence of groundwater extractions. A lowered watertable, relative to stream height, results in the loss of streamflows to the underlying groundwater system (induced recharge). Eventually, a gaining stream may become a losing reach if the watertable continues to be lowered and there are insufficient volumes of groundwater recharge to compensate for the volumes of groundwater extracted. A time lag exists between the commencement of groundwater extraction and the impact on streamflow. As the distance between an extraction bore and stream increases, so does the lag in the timing between the start of pumping and the impact on streamflow (CSIRO and SKM, 2012b). A time lag also exists between the onset of groundwater extractions upgradient, and the resultant impact on throughflow down gradient, at the end of a river basin.

The impact of groundwater extractions on the adjacent stream was demonstrated by Ivkovic et al. (2009), who used a lumped parameter model to simulate the influence of groundwater extractions in the Coxs creek basin using 15 years of streamflow data. The model indicated that groundwater extractions from 1988 to 2003 had the effect of reducing baseflow discharges by approximately 82% of the volume of groundwater extracted (for rates up to 9000 ML/year), although the actual yearly reductions might be more or less depending on the particular climatic period. The remaining 18% of the total volume of groundwater extraction was assumed to affect the available volumes of subsurface throughflow, ultimately impacting on downstream flows. At extraction rates above 9000 ML/year, the model findings indicated that the stream would transition to a disconnected stream reach. Similar findings have been made for the Lower Namoi river basin. CSIRO (2007) reported that the Lower Namoi River has changed from a substantial gaining river prior to irrigated agricultural development to now become a largely losing river. They estimated that the current level of groundwater extraction in the Namoi river basin (as of 2007) would eventually reduce mean streamflow by a total of 99 GL/year.