Editing IPCC:AR6/WGI/Chapter-11 (section)

==== 11.6.1.5 Atmospheric-based Drought Indices ====

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Given the difficulties of drought quantification and data constraints, atmospheric-based drought indices combining both precipitation and AED have been developed, as they can be derived from meteorological data that is available in most regions (with few exceptions). These demand/supply indices are not intended to be metrics of soil moisture, streamflow or vegetation water stress. Because of their reliance on precipitation and AED, they are mostly related to the actual water balance in humid regions, in which ET is not limited by soil moisture and tends towards AED. In water-limited regions and in dry periods everywhere, they constitute an upper bound for overall water-balance deficits (e.g., of surface waters) but are also related to conditions conducive to vegetation stress, particularly under soil moisture limitation ( [[#11.6.1.2|Section 11.6.1.2]] ).

Although there are many atmospheric-based drought indices, two are assessed in this chapter: the Palmer Drought Severity Index (PDSI) and the Standardized Precipitation Evapotranspiration Index (SPEI). The PDSI has been widely used to monitor and quantify drought severity ( [[#Dai--2018|Dai et al., 2018]] ), but is affected by some constraints (SREX Chapter 3; [[#Mukherjee--2018a|Mukherjee et al., 2018a]] ). Although the calculation of the PDSI is based on a soil water budget, the PDSI is essentially a climate drought index that mostly responds to the precipitation and the AED ( [[#van%20der%20Schrier--2013|van der Schrier et al., 2013]] ; [[#Vicente-Serrano--2015|Vicente-Serrano et al., 2015]] ; [[#Dai--2018|Dai et al., 2018]] ). The SPEI also combines precipitation and AED, being equally sensitive to these two variables ( [[#Vicente-Serrano--2015|Vicente-Serrano et al., 2015]] ). The SPEI is more sensitive to AED than the PDSI ( [[#Cook--2014a|Cook et al., 2014a]] ; [[#Vicente-Serrano--2015|Vicente-Serrano et al., 2015]] ), although under humid and normal precipitation conditions, the effects of AED on the SPEI are small ( [[#Tomas-Burguera--2020|Tomas-Burguera et al., 2020]] ). Given the limitations associated with temperature-based AED estimates ( [[#11.6.1.2|Section 11.6.1.2]] ), only studies using the Penman-Monteith-based SPEI and PDSI (hereafter SPEI-PM and PDSI-PM) are considered in this assessment and in the regional tables in [[#11.9|Section 11.9]] .

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