Rainfall
This Analysis tool allows users to view different presentations of the the most recent dekad. The default map on This page displays dekadal (approximately 10-day) rainfall amounts over river basins. The default map shows rainfall totals for the most recently available dekad, but totals for previous dekads can be displayed as well. By clicking a location on the map, the user can generate four time series graphs that provide analyses of recent rainfall averaged over a river basin, with respect to that of recent years and the long-term mean.
Rainfall Anomaly
The rainfall Anomaly map displays the difference between the most recent dekadal rainfall and the long-term average (from 1981 to 2010). Positive (negative) values indicate dekadal rainfall that are above (below) the long-term mean or climatology.
Standard Precipitation Index (SPI)
The SPI map displays the standard rainfall index of the most recent dekadal rainfall (using 1981-2010 as base period). The SPI (McKee 1993) is the number of standard deviations that observed cumulative rainfall deviates from the climatological average. To compute the index, a long-term time series of rainfall accumulations over dekads are used to estimate an appropriate probability density function. The analyses shown here are based on the Pearson Type III distribution (i.e., 3-parameter gamma) as suggested by Guttman (1999). The associated cumulative probability distribution is then estimated and subsequently transformed to a normal distribution. The result is the SPI, which can be interpreted as a probability using the standard normal distrubtion (i.e., users can expect the SPI be within one standard deviation about 68% of the time, two standard deviations about 95% of the time, etc.) The analyses shown here utilize the FORTRAN code made available by Guttman (1999). Places where the dekadal climatology is less than 2 mm are masked out.
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Contact DGM with any technical questions or problems with this Maproom.