Drought Indexes (Indices) An indicator of drought, or below-normal precipitation conditions. Drought indexes, based on raw data inputs, are most typically represented as numeric values and are useful for planning and decision- making. The drought index may be useful not only in comparing precipitation over different time periods, but in some cases the indexes can also provide comparative analysis between different geographic and climatologic areas. The drought index is intended to provide an indication of a period’s precipitation conditions relative to an average or standard value, frequently referred to as “normal” or “near normal” conditions. A number of drought indexes have been developed over time for specific conditions. While no one drought index is inherently superior to the rest under all conditions and circumstances, some are better suited than others for particular uses. For example, some drought indexes may be better when working with relatively uniform topography, while others may specifically take snowpack conditions into account and therefore be better suited for mountainous regions with complex regional microclimates. Some of the most widely used drought indexes include: Percent of Normal – One of the simplest measurements of rainfall for a location, a percent of “normal” figure is calculated by dividing the actual period precipitation by the normal precipitation, which is typically based on a historical average of a 30-year mean or average value. This figure is then multiplied by 100 percent to provide the drought index value. Normal precipitation for a specific location is always considered to be 100 percent, therefore, drought index numbers below 100 percent indicated various degrees of drought conditions. The percent of normal index, while easily understood and effective for assessing a single region or season, can be misunderstood as the concept of normal is a mathematical construct and does not necessarily correspond with what one should expect precipitation to be in any given year (i.e., the normal value may be merely an average of extreme conditions). Standardized Precipitation Index (SPI) – A precipitation index based on the probability of precipitation for any time duration. The SPI was specifically designed to quantify the precipitation deficit for multiple periods of time reflecting the impact of drought on the availability of different water resources. Soil moisture conditions respond to precipitation anomalies on a relatively short scale, while groundwater, stream flow and reservoir storage levels reflect the longer-term precipitation anomalies. For this reason, the SPI is calculated on both a short-term and long-term (up to four years) basis. Positive SPI values indicate greater than median (middle-most) precipitation, while negative values indicate less than median precipitation. Because the index is normalized, wetter and drier climates can be determined in the same way. A drought event occurs any time the SPI is continuously negative and reaches an intensity where the SPI is -1.0 or less. SPI indexes range as follows: 2.0 or greater – extremely wet 1.5 to 1.99 – very wet 1.0 to 1.49 – moderately wet -0.99 to 0.99 – near normal -1.0 to -1.49 – moderately dry -1.5 to -1.99 – severely dry -2.0 or less – extremely dry Palmer Drought Severity Index (PDSI) – The PDSI was the first comprehensive drought index developed in the United States. It is a meteorological drought index based on a balance between moisture supply and demand and responds to weather conditions that have been abnormally dry or abnormally wet. The index is calculated based on precipitation and temperature data, as well as the local Available Water Content (AWC) of the soil, i.e., soil moisture. From the index’s inputs, all the basic terms of the water balance equation can be determined, including evapotranspiration, soil recharge, runoff, and moisture loss from the surface layer. The index, however, is less well suited for mountainous regions or areas of frequent climatic extremes. Human impacts on the water balance, such as irrigation or drainage, are not considered. The index roughly ranges from extremes of -6 to + 6, with negative values denoting dry spells and positive values indicating wet periods. There are also a few values in the magnitude of -7 or +7. Ideally, the PDSI is designed so that a -4.0 in South Carolina has the same meaning in terms of the moisture departure from a climatological normal as a -4.0 in Idaho. The following shows the more typical PDSI classifications: 4.00 or more – Extremely wet 3.00 to 3.99 – Very wet 2.00 to 2.99 – Moderately wet 1.00 to 1.99 – Slightly wet 0.50 to 0.99 – Incipient wet spell 0.49 to -0.49 – Near normal -0.50 to -0.99 – Incipient dry spell -1.00 to -1.99 – Mild drought -2.00 to -2.99 – Moderate drought -3.00 to -3.99 – Severe drought -4.00 or less – Extreme drought Crop Moisture Index (CMI) – The CMI, as a derivative of the Palmer Drought Severity Index (PDSI), uses a meteorological method to monitor week-to-week crop conditions. Differing from the PDSI, the CMI was designed to evaluate short-term moisture conditions across major crop-producing regions. It is based on the mean temperature and total precipitation for each week within a “climate division”, as well as the CMI value from the previous week. The CMI is specifically designed to monitor short-term moisture conditions affecting a developing crop and is not a good long-term drought monitoring tool. The CMI was designed for areas in which mountain snowpack is a key element of water supply conditions. The index is based on snowpack, streamflow, precipitation and reservoir storage and is calculated uniquely for each water basin, which tends to limit interbasin and inter-regional index comparisons. Surface Water Supply Index (SWSI) – Originally developed to complement the Palmer Drought Severity Index (PDSI), the SWSI, unlike the PDSI which is basically a soil moisture index, was designed to be an indicator of surface water conditions in which mountain snowpack is a major component. The intent of the SWSI is to incorporate both hydrological and climatological features into a single index value resembling the PDSI. Four inputs are required for the SWSI: snowpack, streamflow, precipitation and reservoir storage. Because the SWSI calculations are unique to each watershed or defined region, it is difficult to compare SWSI values among these geographic areas. Reclamation Drought Index (RDI) – The RDI was developed as a tool for defining drought severity and duration, and for predicting the onset and end of periods of drought. The impetus for the development of the RDI came from the Reclamation States Drought Assistance Act of 1988, which allowed states to seek federal drought relief assistance. The RDI is calculated at a river basin level and incorporates the supply components of precipitation, snowpack, streamflow and reservoir storage levels. The RDI differs from the Surface Water Supply Index (SWSI) in the incorporation of a temperature component. The RDI is adaptable to a particular region and readily accounts for both climate and water supply factors. RDI classifications include: 4.0 or more – extremely wet 1.5 to 4.0 – moderately wet 0 to 1.5 – normal to mild wetness 0 to -1.5 – normal to mild drought -1.5 to -4.0 – moderate drought -4.0 or less – extreme drought Deciles – The deciles drought index system is based on a relative frequency distribution of long-term precipitation divided into tenths of the overall range of distribution. Each tenth division or precipitation category is termed a “decile” and ranges from the lowest (drought) ten percentile of precipitation levels to the highest (wet) ten percentile. By definition, the fifth decile is the median (middle-most) and is the precipitation amount not exceeded by 50 percent of the occurrences over the period of record. One disadvantage of the deciles drought index system is that a long climatological record is required for accurate deciles classifications. As a drought index, the deciles are grouped into five classifications as follows: deciles 1-2 (lowest 20%) – much below normal deciles 3-4 (next lowest 20%) – below normal deciles 5-6 (middle 20%) – near normal deciles 7-8 (next highest 20%) – above normal deciles 9-10 (highest 20%) – much above normal