Surface water and alluvial groundwater connectivity at Mulloon Creek and the implications for Natural Sequence Farming

The effect of installing 22 instream weirs along a 2 km stretch of Mulloon Creek as part of a Natural Sequence Farming (NSF) pilot project was investigated. The outcomes of NSF are reportedly diverse: this study focussed specifically on floodplain aquifer rehydration. The alluvial groundwater response to NSF treatment was analysed by comparing floodplain piezometer measurements from two discrete monitoring periods. Using an ANOVA, the late period 0.37 m mean reduction in depth to floodplain water table was found to be significant (0.05). No monthly precipitation trends were detected by either the Mann-Kendall test or linear models over the monitoring periods, however the late period had higher rainfall intensity (1.8 vs 2.3 mm/day). Sedimentology conducted during installation and short-term monitoring of an additional piezometer network was used to determine alluvial groundwater flow dynamics at an untreated site located 2 km downstream of the pilot project site. A hydraulic gradient analysis technique was used to estimate and compare baseflow into the stream during each period and at each location. Groundwater flow across the Mulloon Creek floodplain is facilitated by semi-continuous coarse grained units transmitting water towards the gaining stream. Additional water is induced into the alluvial aquifer via the NSF process of step-diffusion. Often complex floodplain sedimentology results in groundwater flow patterns that are equally complex and difficult to predict purely from surface topography. Although the NSF treated site was improved, the degree of hydration in the untreated site appears to be higher. NSF is most effective in incised streams with disconnected and porous aquifers composed of hydraulically conductive floodplain material. This emphasizes the importance of site suitability and prior investigation before implementing river/floodplain restoration projects that emphasize the alteration of alluvial groundwater processes.