Calibration of Internal Empirical Coefficients in the Palmer Drought Severity Index

Drought refers to the short-term or long-term period’s association with the lack of rainfall, increased temperature and reduced humid22ity. Perhaps one of the most widely recognized drought indexes is Palmer Drought Severity Index (PDSI) presented by Palmer in 1965 to quantify the intensity of drought. Since then, numerous researches have been conducted based on this index and some attempted to modify the index. The general aim of this study was to derive and calibrate the empirical coefficients named as duration factors in the Palmer drought severity index by the method introduced by Wells et al (2004) in a country scale. Study area The study area located between latitude of 25 to 40 degree in North and longitude of to 64 degrees in East. Average annual rainfall is estimated as 240 mm per year. Palmer moisture model PDSI is based on a lumped soil moisture model with specific supply and demand. Supply of moisture from precipitation is absorbed into the soil. technique (latitude and longitude and altitude digitization). Rainfall data was also spatially distributed using an IDW interpolation method for each month of each year for the period of 1975 to 2011 at a national scale. Palmer model was performed on a distributed raster scheme with 4 kilometer spatial resolution. In Figures 1 and 2, spatial distribution of duration factors q and p is shown in the dry and wet cases respectively. As you can see, the coefficients show different spatial variations in wet and dry conditions and the maps can be used to extract the Palmer Index in any region. For a more detailed study, relative frequency curves of duration factors p and q are derived for the both wet and dry periods. The results indicate that value of p in the dry period is more in comparison to its value in the wet period and for q the situation is reversed. This means that in drought period, the index at each step, rather than changes in precipitation and soil moisture is sensitive to the value of the index on the previous step, while in wet period, the situation is reversed. This means that the climate of the study area (all areas) typically has a tendency to dryness. Conclusion The regional empirical coefficients of the Palmer Drought Severity Index in the last 36 years, indicating the need for calibration of Palmer (SC-PDSI) in most parts of Iran. As general, the climate of the study area (all areas) typically has a tendency to dryness.