The hydrological function of upland swamps in eastern Australia: the role of geomorphic condition in regulating water storage and discharge

Temperate Highland Peat Swamps on Sandstone (THPSS) are a type of wetland found in low-order streams on the plateaus of eastern Australia. They are sediment and organic matter accumulation zones, which combined with a climate of high rainfall and low evaporation function as water storage systems. Changes to the geomorphic structure of these systems via incision and channelisation can have profound impacts on their hydrological function. The aim of this study was to develop an understanding of how changes to the geomorphic structure of these systems alter their hydrological function, measured as changes and variability in swamp water table levels and discharge. We monitored the water table levels and discharges of three intact and three channelised THPSS in the Blue Mountains between March 2015 and June 2016. We found that water levels in intact swamps were largely stable over the monitoring period. Water levels rose only in high rainfall events, returned quickly to antecedent levels after rain, and drawdown during dry periods was not significant. In contrast, the water table levels in channelised THPSS were highly variable. Water levels rose quickly after almost all rainfall events and declined significantly during dry periods. Discharge also showed marked differences with the channelised THPSS discharging 13 times more water than intact swamps, even during dry periods. Channelised THPSS also had flashier storm hydrographs than intact swamps. These results have profound implications for the capacity of these swamps to act as water storage reservoirs in the headwaters of catchments and for their ability to maintain base flow to downstream catchments during dry times. Changes to geomorphic structure and hydrological function also have important implications for a range of other swamp functions such as carbon storage, emission and exports, contaminant sorption, downstream water quality and biodiversity, as well as the overall fate of these swamps under a changing climate.