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Abstract. Numerous basin aquifers in arid and semi-arid regions of the world derive a significant portion of their recharge from adjacent mountains. Such recharge can effectively occur through either stream infiltration in the mountain-front zone (mountain-front recharge, MFR) or subsurface flow from the mountain (mountain-block recharge, MBR). While a thorough understanding of recharge mechanisms is critical for conceptualizing and managing groundwater systems, distinguishing between MFR and MBR is difficult. We present an approach that uses hydraulic head, chloride and electrical conductivity (EC) data to distinguish between MFR and MBR. These variables are inexpensive to measure, and may be readily available from hydrogeological databases in many cases. Hydraulic heads can provide information on groundwater flow directions and stream–aquifer interactions, while chloride concentrations and EC values can be used to distinguish between different water sources if these have a distinct signature. Such information can provide evidence for the occurrence or absence of MFR and MBR. This approach is tested through application to the Adelaide Plains basin, South Australia. The recharge mechanisms of this basin have long been debated, in part due to difficulties in understanding the hydraulic role of faults. Both hydraulic head and chloride (equivalently, EC) data consistently suggest that streams are gaining in the adjacent Mount Lofty Ranges and losing when entering the basin. Moreover, the data indicate that not only the Quaternary aquifers but also the deeper Tertiary aquifers are recharged through MFR and not MBR. It is expected that this finding will have a significant impact on the management of water resources in the region. This study demonstrates the relevance of using hydraulic head, chloride and EC data to distinguish between MFR and MBR.

[1] The vadose zone plays an important role in surface water-groundwater interaction and exerts strong influences on biogeochemical, ecological, and hyporheic processes. It is also the presence of an unsaturated zone that controls the state of connection between surface water and groundwater. Despite recent advances on how hydrogeological variables affect surface water-groundwater interactions, there is limited understanding of the hydroclimatic effects of precipitation and evapotranspiration. More specifically, there is a need for a physically based understanding on the changes that may occur in response to changes in vegetation. While it may seem qualitatively obvious that the presence of vegetation can cause an unsaturated zone to develop underneath a riverbed and alter the state of connection, it has so far not been demonstrated quantitatively. Also, the influence of variables such as root extinction depth, topography, and the influence of land clearance has so far not been explored. In this study, fully coupled, physically based 2-D transient homogeneous models were used to simulate the impact of land clearance and revegetation on the state of connection of a perennial river system. The simulations showed that the presence of vegetation can create an unsaturated zone between a river and an aquifer and affect the state of connection and that the removal of deep-rooted vegetation from a catchment may have a significant impact on the state of connection as well as the condition of the water resource.Citation: Banks, E. W., P. Brunner, and C. T. Simmons (2011), Vegetation controls on variably saturated processes between surface water and groundwater and their impact on the state of connection, Water Resour.