Time- and depth-resolved mechanistic assessment of water stress in Australian ecosystems under the CMIP6 scenarios

This work provides a comprehensive analysis of soil water dynamics in Australia for the climate projections of the Coupled Model Intercomparison Project 6 (CMIP6). We modelled the historical soil water dynamics from 1981 to 2018 at various depths within and below the root zone using the BRTSim computational solver to generate the "current conditions". We then investigated how the CMIP6 scenario can affect water accessibility by plants, and hence their potential impact on croplands and native ecosystems. We found that surface soil moisture can decline by 7% across Australia between 2020 and 2050, with the 2030 decade projected to experience the greatest soil water loss. Above-average precipitation during the 2040s will still lead to 2% soil moisture decline relative to current conditions, with about 1 million km² projected to recover from this deficit later on. Seasonally, our results inferred drier summers and winters with 13% and 5% loss in soil water, respectively. Shrublands and savannas were the most affected native ecosystems with a moisture decline between 16% and 7% within the root zone, respectively. More importantly, 36% to 52% of croplands were found to undergo a 7% decline in soil moisture within the root zone, which was spatially and temporally heterogeneous across crop types. Within the crop calendar, wheat-growing regions were affected by soil moisture deficiencies from sowing to harvest in almost the entire time frame of our assessment.