A drought index that uses temperature and precipitation values from the climate division dataset of the National Climatic Data Center along with other components of the water balance equation to measure the departure of soil moisture supply from normal. The output of the PDSI consists of positive (wet) or negative (dry) values centred on 0 (normal) with values above +4.0 or below −4.0 generally considered extreme (Goodrich (2007) Geog. Compass 1, 4). See Hessel and Graumlich (2002) J. Biogeog. 29, 7 on tree-ring reconstructions of this index. Yan et al. (2013) Mathematical Problems in Engineering present a modified PDSI.