Groundwater maps help to protect floodplain forests

Posted on January 5, 2013

Declines in the frequency and duration of floods can cause the extensive dieback of floodplain forests.  For this reason, environmental managers need to understand the relationships between water availability and living tree cover.  In Australia, the Murray-Darling river basin extends over 14 % of the country.  Over the last 20 years, some 70% of forests of the river red gum, Eucalyptus camaldulensis, the dominant tree in the lower Murray, have died back in response to infrequent flooding and water extraction for human use.  Because reduced flooding increases the reliance of floodplain trees on groundwater, groundwater conditions could be good predictors of forest dieback.  To test this idea, historical data on groundwater depth and salinity from a 100,000 ha area of floodplain were compared with forest cover information from ground surveys, remote sensing and vegetation modelling.  It emerged that current groundwater depth and salinity are strong predictors of tree condition , but there are also strong regional differences in red gum responses.  In the upper Murray, groundwater is fresh (<15 mS /cm2) and dieback increases as groundwater retreats below the root zone, but in the lower Murray groundwater is more saline (>30 mS /cm2)  and red gum condition improves as the groundwater level falls.  These findings show that changes in groundwater conditions can be used as an early warning system for dieback and so help to ensure that management action is timely.  Near-surface saline groundwater can be lowered temporarily by pumping, and more permanently through perennial plantings and reducing the level of water in dams, but deep-lying fresh groundwater can be raised only by the addition of water.  Regardless of groundwater conditions, however, regular rainfall and flooding are essential for the long-term sustainability of these red gum forests.

Reference:   Cunningham, S.C. et al.  2011.  Groundwater change forecasts widespread forest dieback across an extensive floodplain system.  Freshwater Biology 56, 1494–1508.