The hydrological function of a large chain-of-ponds: a wetland system with intermittent surface flows

Like many wetlands globally, the Mulwaree River chain-of-ponds system exists in two dichotomous states characterised by the presence or absence of surface flow connecting large, deep, permanently inundated ponds. We develop a conceptual model of hydrological function of this chain-of-ponds system combining surface and subsurface water levels, ²H and ¹⁸O stable isotopes and ²²²Rn as a groundwater tracer over a period of time that incorporated extended dry periods and large rainfall events. During high-flow or flood events, ponds are connected by flow along connecting channels and preferential flow paths. The water column is fully mixed to depths of up to 7 m. During high-flow, water level in the ponds can be greater than the water level in the surrounding floodplain aquifer, producing a hydraulic gradient away from the ponds, reflecting a losing wetland system. During no-flow periods, connecting channels and preferential flow paths are dry. A thermocline develops within the ponds and surface waters become enriched in ²H and ¹⁸O with evaporation losses. During periods of no-flow, increases in water level beyond atmospheric flux often occur during winter. Only small groundwater inflows enter the ponds from the floodplain aquifer. The hydrological function of this chain-of-ponds system is delicately balanced making it potentially sensitive to changes in climate that alter rainfall and evaporation rates, and any local-scale groundwater interference activities. Efforts to conserve and protect this system, and the aquatic ecosystems it supports, will be critical into the future.