Changes in Blue/Green Water Partitioning Under Severe Drought

Much attention has been given to the disproportionate streamflow deficits (relative to rainfall deficits) experienced by many catchments during the Millennium Drought (1998–2009) in southeastern Australia, along with lack of post-drought streamflow recovery in some cases. However, mechanisms behind the coupled hydrologic and ecosystem dynamics are poorly understood. We applied a process-based ecohydrologic model (RHESSys) in a Melbourne water supply catchment to examine changes in ecohydrologic behavior during and after the drought. Our simulations suggested that average transpiration (green water) was maintained under drought despite a substantial (12%) decrease in average rainfall, meaning that the entire rainfall deficit translated to reduced streamflow (blue water). Altered spatial patterns of vegetation behavior across the terrain helped the ecosystem maintain this unexpectedly high green water use. Decreased transpiration upland was compensated by increases in the riparian zone, which was less water limited and therefore able to meet higher water demand during drought. In the post-drought period, we found greater transpiration and reduced subsurface water storage relative to pre-drought, suggesting a longer-term persistence in altered water partitioning. The post-drought outcome was attributed to a combination of warmer climate and the persisting effects of the drought on nutrient availability. Given the importance of shifting ecohydrologic patterns across space, our results raise concerns for applying lumped conceptual hydrologic models under nonstationary or extreme conditions. Additionally, the processes we identified have important implications for water supply in Australia's second largest city under projected drying.