Présentations Colloques

    Oral Presentation
    Session 8.06: Verification of conceptual patterns and expected natural effects of regional groundwater flow by interpretation of relevant field observations
    Cresswell Richard
    When Groundwater Input Results in Reduced Pool Persistence- Isolated Waterholes in an Arid Environment
    The presence of groundwater inflows to rivers and creeks is generally presumed to result in increased and prolonged flow in ephemeral systems, with groundwater inputs maintaining baseflow during dry spells, particularly where seasonal potential evaporation significantly exceeds precipitation.**In a dryland river environment in western Queensland, Australia, however, where river flow is intermittent and where surface water during dry times is limited to isolated in-channel pools, termed waterholes, we found a different pattern of interaction between groundwater and surface water. Two waterholes, of 15 we investigated, showed evidence of groundwater input during periods of high flow. These waterholes, however, also proved to have the greatest rates of water loss during extended periods without surface flow. **Analysing a time sequence of stable isotopes in this system confirmed that waterholes that received groundwater-augmented flow also drained faster than those that did not, while those with no groundwater input exhibited a purely evaporative reduction in water volumes during periods of no flow.**We surmise, therefore, that a rise in shallow groundwater tables during periods of rainfall resulted in preferential input to two waterholes only (possibly related to faulting). These waterholes also exhibit hard bedrock associated with local igneous intrusions. Waterholes that did not receive groundwater input exhibit fine-sediment river beds that remain sealed by fine-grained clays, inhibiting downwards leakage. As the river recessed and waterholes became isolated, those that were augmented by groundwater drained faster than those that did not receive groundwater input and the combined drainage and evaporation in groundwater-fed pools exceeded the evaporation only in surface water fed waterholes.**The use of baseflow indices to indicate stream resilience of ephemeral systems, therefore, should be used with caution in arid areas with variable and shallow water tables.


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