Drought Termination and Amelioration
Drought in the U.S.
The incidence of drought in the United States has varied greatly over the past century. From the dust bowl years of the 1930's to the major droughts of 1988 and 2000, much of the U.S. has suffered from the effects of drought during the past century. While annual and seasonal precipitation totals have generally increased in the United States since 1900, severe drought episodes continue to occur.
The nation's most devastating drought occurred in the 1930's during what many refer to as the 'Dust Bowl' years. The drought affected almost the entire Plains and covered more than 60% of the US during its peak in July 1934. It brought devastating economic impacts to many and caused the migration of millions of people from the Plains to other parts of the country, many to the Western US. Although the nation has not since experienced a drought as severe as the drought of the 1930's, subsequent droughts (e.g. those of the 1950's, 1988 and 2000) have also had serious economic and societal impacts.
Although a variety of weather related phenomena have the potential to cause great economic and personal losses in the US, drought has historically had the greatest impact on the largest number of people. Since 1980, 178 weather/climate-related disasters have each caused at least 1 Billion dollars in economic losses. Of these 178 disasters, the greatest losses have been attributed to drought. Economic losses exceeded 40 Billion dollars in the droughts of 1980 and 1988, and the combination of drought and heat-related deaths totaled more than 5000 in each event. The drought of 2000 resulted in losses of 4 Billion dollars and 140 deaths.
Although not as widespread as the droughts of the 1930's and 1950's, persistent above normal temperatures and below normal precipitation across much of the Western and Southern US in 1999 and 2000 brought drought to these regions by the summer of 2000. More than 1/3 of the country suffered from severe to extreme drought by August leading to heavy agricultural losses, water rationing for many, and one of the worst wildfire seasons in the last 50 years. While some parts of the nation have received drought-ending precipitation since that time, parts of the nation continue to suffer from severe precipitation deficits through the spring season of 2001.
The wide variety of disciplines affected by drought, its diverse geographical and temporal distribution, and the many scales drought operates on make it difficult to develop both a definition to describe drought and an index to measure it. Common to all types of drought is the fact that they originate from a deficiency of precipitation resulting from an unusual weather pattern. If the weather pattern lasts a short time (e.g., a few weeks or a couple months), the drought is considered short-term. But if the weather or atmospheric circulation pattern becomes entrenched and the precipitation deficits last for several months to several years, the drought is considered to be a long-term drought.
Many quantitative measures of drought have been developed in the United States, depending on the discipline affected, the region being considered, and the particular application. The most frequently used indicators of drought are those developed by Wayne Palmer in the 1960's. These include the Palmer Drought Severity Index (PDSI), the Palmer Hydrological Drought Index (PHDI), the Palmer Z Index and the Crop Moisture Index (CMI). These indices have been used in countless research studies as well as in operational drought monitoring during the past 35 years. The Palmer drought index has proven to provide one of the best indications of drought for much of the United States. It is superior to other drought indices in many respects because it accounts not only for precipitation totals, but also for temperature, evapotranspiration, soil runoff and soil recharge.
The Z Index measures short-term drought on a monthly scale while the CMI measures short-term agricultural drought on a weekly scale. The PDSI measures drought duration and intensity of long-term drought-inducing circulation patterns and responds fairly quickly as meteorological patterns often quickly change from one regime to another. However, a reflection of the long-term effects of drought on systems affected by long-term precipitation deficits is measured by the PHDI, a measure of the hydrological impacts of drought. These impacts, such as reservoir levels, groundwater levels, etc., take longer to develop and it takes longer to recover from them. It is from this index, the PHDI, that we calculate the precipitation amounts and probabilities of ending or ameliorating drought.
The Amount of Precipitation Needed to End a Drought (Based on the PHDI)
Because of the far-reaching societal and economic impacts of drought, there is considerable interest in determining how much precipitation is required to end a drought as well as the probability that a region may receive the necessary amount of precipitation. Ending a hydrological drought requires that the moisture needs associated with recharge, demand and runoff have been brought back to normal or above normal.
Many factors affect the quantity of precipitation required to end or ameliorate (reduce the severity of) a drought. Knowledge of the severity of the drought, as defined by the Palmer Hydrological Drought Index (PHDI), is the essential starting point for determining the needed precipitation. The typical conditions that a region experiences during each month and season of the year (i.e., that region's climatology) is also essential. Given a drought of equal magnitude in a dry and wet climate, the wetter region requires more precipitation to end the drought.
The season in which the precipitation falls can also greatly influence the quantity of precipitation required to end a drought. During a typically moist month (such as those experienced in the winter and spring along the West Coast) more precipitation may be required to end a drought than during the typically dry months of the summer. Because soil moisture conditions are generally lower in the dry months, the precipitation needed to bring soil conditions back to normal may be less than that required to return soil moisture conditions to normal during a generally wetter season. Nevertheless, regardless of a region's climate, over a sufficiently long period of time, near-normal precipitation is often sufficient for ending a drought with moisture conditions gradually returning to normal.
However, the quantity of precipitation needed to end a drought says nothing about the probability that a region will actually receive that amount of precipitation. A region, such as the West Coast, that does not typically experience excessively heavy precipitation during the summer season, may be less likely to receive a quantity sufficient for ending a drought than a region which has a record of experiencing extreme precipitation events during the same season. The months which have the greatest probability of receiving substantially more precipitation than normal would be those with precipitation distributions with the largest positive skew (that is, those subject to more extreme precipitation events), not necessarily those months that normally receive the greatest amount of precipitation.
The technical details associated with the calculation of precipitation totals needed to end or ameliorate drought and the probability of receiving the required precipitation can be found in "Drought Termination and Amelioration: It's Climatological Probability" by Tom Karl et al. 1987.
Maps of Precipitation Totals and Probability
More than 2000 maps of the contiguous U.S. are provided which show the precipitation totals needed to end or ameliorate drought from periods of 1 month to 6 months based on PHDI values from -2 to -6. These data were calculated for each month of the year and include precipitation values for each of the 344 contiguous U.S. climate divisions. The end of a drought is defined by a PHDI value of -0.5 while drought amelioration is achieved when a PHDI value of -2.0 is reached. Maps showing the probability of receiving the necessary amount of precipitation are also provided.
Maps of precipitation needed to end or ameliorate a drought for those divisions currently experiencing drought are also available. Values are provided in all divisions with a monthly PHDI less than -2.0. Precipitation needed over periods from 1 month to 6 months is included as well as the associated probability of receiving that quantity of precipitation. These maps are replaced on a monthly basis with the values reflecting conditions in the previous month.