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Table 1: SPI and Corresponding Percentile Rank and Character of Index. (Recreated <br />from McKee, et al. 1993) <br />SPI <br />Percentile <br />Rank <br />Character <br />-3 <br />0.14% <br />Extremely D <br />-2.5 <br />0.62% <br />Extremely D <br />-2 <br />2.28% <br />Extremely D <br />-1.5 <br />6.68% <br />Severely D <br />-1 <br />15.87% <br />Moderately D <br />-0.5 <br />30.85% <br />Near Normal <br />0 <br />50.00% <br />Near Normal <br />0.5 <br />69.15% <br />Near Normal <br />1 <br />84.13% <br />Moderately Wet <br />1.5 <br />93.32% <br />Very Wet <br />2 <br />97.72% <br />Extremely Wet <br />2.5 <br />99.38% <br />Extremely Wet <br />3 <br />99.86% <br />Extremely Wet <br />The cumulative probability of precipitation events for any given month or time scale (i.e. <br />3, 6, 9, 12, 24, 48 months) can be calculated. The time scale used will affect the <br />frequency of drought, the duration and the magnitude of precipitation deficit (McKee, et <br />al. 1999). The longer the time scale analyzed, the fewer dry and wet periods will be <br />identified, however the duration of those dry and wet periods are longer than if shorter <br />time scales are analyzed (McKee, et al. 1999). <br />Colorado Modified Palmer Drought Severity Index (CMPDSI) <br />In 1965, Wayne C. Palmer developed the Palmer Drought Severity Index (PDSI) in the <br />paper "Meteorological Drought" (Palmer, 1965). This method uses a simple water <br />balance approach to identify prolonged unusually wet or dry periods. The data <br />requirements include: precipitation and temperature. Temperature is used to estimate <br />evapotranspiration (Palmer and Havens, 1958). Assumptions also must be made <br />regarding soil available water capacity (awc) in order to calculate runoff and soil <br />moisture recharge to complete the water accounting procedure. For each area, <br />climatically dependant coefficients must be determined. Table 2 provides the coefficients <br />used and their description. These coefficients are used to determine the long term normal <br />for a specific time of year. Departures from these normals are then used to identify wet <br />or dry periods. <br />Table 2: PDSI coefficients and their description. <br />Coefficient <br />Description <br />Coefficient of Evapotranspiration <br />Average ET /Potential ET <br />Coefficient of Recharge <br />Average Recharge/Potential Recharge <br />Coefficient of Runoff <br />Average Runoff /Potential Runoff <br />Coefficient of Moisture Depletion <br />Average Depletion/Potential Depletion <br />