Long-term droughts are quantified by The Palmer Drought Severity Index (PDSI), calculated by NOAA from a combination of precipitation, temperature, and soil moisture data. It represents the accumulation or deficit of water over a long-term period, about 9 months. Soil moisture data is calibrated to a homogeneous climate zone. PDSI has been widely used to recognize long-term agricultural drought and hydrological drought, and to identify the abnormality of a particular drought in a region.
Short-term droughts are calculated by the Palmer Z-Index (or short-term Palmer Index). The Z-Index corresponds to monthly drought conditions with no memory to previous monthly deficits or surpluses. This is beneficial because it is possible to have a short-term moist period in the midst of a long-term drought or vice versa. The Z-index corresponds to meteorological drought and short-term agricultural droughts.
Because the Z-index is not affected by moisture conditions in the previous month, its values can vary dramatically from month to month. On the other hand, the PDSI varies more slowly because antecedent conditions account for two-thirds of its value, although heavy rain events can dramatically change PDSI values.