Combined Use of Multiple Drought Indices for Global Assessment of Dry Gets Drier and Wet Gets Wetter Paradigm

Global warming is expected to enhance the global hydrological cycle, leading dry regions to become drier and wet regions to become wetter (the DDWW paradigm). However, this hypothesis has been challenged by both observational and modeling studies. One major source of these disagreements originates from the choice of the drought indices used. Hydrological processes are complex, but drought indices are often based on a relatively simple calculation. A single index may, therefore, place undue emphasis on particular processes while ignoring others, with the result that it would not capture the holistic picture of hydrological changes and may even lead to an incorrect interpretation. Six common drought indices were calculated for the global vegetated land areas for the period from 1982 to 2012 and different indices tend to create apparently contradictory results for many regions. To overcome the single-index problem, the six drought indices were integrated into a composite map of global land moisture trends. By using this integrated approach, the majority (55%) of vegetated land areas experienced wetting or drying trends. For the regions with significant changes, supporting evidence was identified for the DDWW paradigm in one-fifth of the area. The opposite pattern to DDWW (dry areas becoming wetter and wet areas drier) occurred over 29% of the area. We also find an asymmetrical pattern with a larger proportion of wet areas getting wetter (12%) than dry areas getting drier (8%). The DDWW theory is more useful when the pure precipitation-driven drought metrics are considered but when evapotranspiration and soil conditions are integrated, the DDWW is not conclusive.