Drought is defined as a period of insufficient rainfall for normal plant growth, which begins when soil moisture is so diminished that vegetation roots cannot absorb enough water to replace that lost by transpiration.
Palmer Drought Severity Index (PDSI): attempts to measure the duration and intensity of the long-term drought-inducing circulation patterns. Long-term drought is cumulative, so the intensity of drought during the current month is dependent on the current weather patterns plus the cumulative patterns of previous months. The PDSI is updated weekly by the NOAA Climate Prediction Center. It is based
on rainfall, temperature and historic data, and is computed based on a complex
formula devised by W.C. Palmer in 1965. Although the Palmer is the main
drought index used by the U.S. government, it is slow to detect fast-emerging
Palmer Hydrological Drought Index (PHDI): measures hydrological impacts of drought (e.g., reservoir levels, groundwater levels, etc.) which take longer to develop and longer to recover. This long-term drought index was developed to quantify these hydrological effects, and it responds more slowly to changing conditions than the PDSI. (Details)
Crop Moisture Index (CMI): The Crop Moisture Index (CMI) uses a meteorological approach to monitor week-to-week crop conditions. It was developed by Palmer (1968) from procedures within the calculation of the PDSI. (Details)