Palmer Drought Severity Index (PDSI) and Palmer Z-Index
Description
One of the first and most highly used drought indices is the Palmer drought severity index (PDSI; Palmer 1965), which is based on a simplified soil water balance and is a measure of the departure of average soil moisture conditions. Instead of the typical parameterization of PDSI using Thornthwaite temperature only based potential ET, we utilize more physically based Penman-Monteith reference ET, which is a function of solar radiation, temperature, humidity and windspeed. A PDSI value between -.5 and 0.5 represents near normal soil moisture conditions, with positive/negative values representing wet/dry conditions. The magnitude of PDSI gives an indication as to the severity of the departure from normal conditions. PDSI> 4 represents very wet conditions, while PDSI<-4 represents an extreme drought.
PDSI and Z are calculated using a modified version of the Palmer formula, which uses reference evapotranspiration and precipitation from gridMET, and a static soil water holding capacity layer (top 1500mm) from STATSGO. The Z-index is calculated using non-overlapping 6-pentad (30-day) blocks of precipitation and potential ET. To compute the pentad PDSI, the authors apply modifications to the standard PDSI equations similar to Rhee et al. 2007. First, the Z-index is divided by 6 given the differences between a ~30-day month and a 5-day pentad. Second, the coefficient of the drought severity equation is modified as X(i)=0.9828*X(i-1)+Z/3; and modified effective wetness/dryness accordingly to scale for differences between the timescales. The baseline period for PDSI and Z calculations is 1979-2018. Given the non-standard nature of the calculations, the pentad PDSI and Z-index are experimental.
This PDSI uses a new methodology for sub-monthly PDSI, based on the methodology outlined in Rhee, J., and G. J. Carbone. 2007. "A Comparison of Weekly Monitoring Methods of the Palmer Drought Index." Journal of Climate, 20(24): 6033-6044.
References