On the Use of Drought Indices for Drought Severity Assessment

This lecture provides a critical analysis of drought indices, emphasizing their role in evaluating drought severity while addressing the challenges associated with their application. It highlights the inherent complexity of drought assessment, given the multifaceted nature of drought phenomena, the various types of drought, and the intricate mechanisms underlying their development. A central focus is the distinction between drought and aridity, as well as between drought metrics and indices—concepts that are frequently misunderstood or conflated.

Particular attention is given to atmospheric drought indices, especially those incorporating atmospheric evaporative demand (AED). These indices are crucial for assessing water stress but have faced criticism for certain limitations. One notable issue is the "index-impact gap," where atmospheric drought indices often indicate more severe droughts than those reflected in hydrological and ecological metrics derived from Earth System Models (ESMs), particularly in future climate scenarios. Atmospheric indices do not directly account for soil moisture or vegetation dynamics. Nonetheless, AED reflects atmospheric conditions rather than direct water reservoirs and fluxes, making AED-based indices valuable for understanding atmospheric drivers of drought. This value is reinforced by AED's critical role in intensifying drought through increased evaporation, heightened plant water stress, and reduced photosynthesis.

The lecture further focuses into the uncertainties inherent in ESM projections of ecological and hydrological variables, such as soil moisture and runoff. These uncertainties arise because ESMs often underestimate drought severity due to challenges in simulating complex hydrological and physiological processes. The difficulties stem from limitations in modelling plant physiology, water cycles, and ecosystem responses, compounded by biases in key variables such as evapotranspiration. While ESM outputs are valuable for drought assessments, relying exclusively on them risks producing misleading conclusions.

This issue connects with the role of rising atmospheric CO₂ concentrations, a factor commonly incorporated into ESM simulations, which adds another layer of complexity. Elevated CO₂ levels can enhance plant water-use efficiency and photosynthesis but also introduce uncertainties regarding their impacts on evapotranspiration and soil moisture. These dynamics generate complex feedbacks with AED and other variables, further complicating drought severity assessments, particularly in future ESM simulations.

To address these challenges, the lecture advocates for an integrated approach that combines atmospheric drought indices with hydrological and ecological metrics. Such an approach ensures that the intensifying role of AED under global warming is neither overlooked nor overstated, thereby improving the accuracy of drought assessments, especially in the context of future climate scenarios.