Long-term surface water trends and relationship with open water evaporation losses in the Namoi catchment, Australia

Economic pressures on natural resources have led to a modification of natural surface water patterns. This has affected the hydrologic cycle at a global scale with multiple environmental and socioeconomic implications. This study evaluates long-term trends in surface water occurrence and related climate variables in a large agricultural catchment of Australia to understand trends in evaporative water losses from open water bodies. Surface water detection was based on a supervised classification of Landsat images between 1988 and 2018 to obtain inundation and surface water frequencies across the catchment, which were compared with other surface water products. Climate trends were based on monthly SILO gridded datasets. Open water evaporation was estimated using the FAO56 methodology and compared with the Penman-Monteith-Leuning Evapotranspiration V2 (PML_V2) product. Evaporation trends were analysed using the Mann Kendall (MK) test and the Sen's slope (SS). Generally, open water frequencies showed significant negative trends, though these varied spatially. The number of dams, on the other hand, had an increasing trend. Temperatures are increasing in the catchment, while rainfall and relative humidity are decreasing, resulting in an overall positive trend for reference evapotranspiration (ETr) across 90% of the catchment. Even though ETr and evaporation per unit area of water show positive trends, lumped open water evaporation showed a negative trend, possibly associated with an average decrease in surface water frequencies. Annual evaporative losses averaged 201.9 GL, exceeding annual total account usage of surface water in the catchment. All the studied changes imply a loss of blue and green water in the catchment, and provide evidence of an overall intensification of the hydrologic cycle as predicted under climate change.