|One of the most robust drought indices is so called Standardized Precipitation Index (SPI). The SPI can be calculated at various time scales which reflect the impact of the drought on the availability of water resources. The SPI calculation is based on the distribution of precipitation over long time periods (30 years (1961-1990) was used). The long term precipitation record is fit to a probability distribution, which is then normalised so that the mean (average) SPI for any place and time period is zero.
|| SPI values above zero indicate wetter periods and values less than 0 indicate drier periods.
Please select year, month and time scale:
| Another way to define drought are percentiles. A percentile is the value of a variable below which a certain percent of observations fall. Long term precipitation record is sort by rank by month; 50 years period (1951-2000) was used. The 5th (10th, 15th etc.) percentile is the value below which 5 (10, 15 etc.) percent of the observations may be found. The 25th percentile is also known as the first quartile; the 50th percentile as the median.
|| Percentile values above 50 indicate wetter periods and values less than 50 indicate drier periods.
Please select data, year and month:
For making maps monthly precipitation data from GPCC VASClimO were used. This data are land-only in latitude/longitude format in resolution 0.5 degree x 0.5 degree and were also used for calculating monthly percentiles (5, 10, 15, 20 and 25 - with R).
Data were imported into GRASS GIS and interpolated to 50x better resolution (0.01 degree) (using regularized spline with tension and smoothing). For reprojecting maps into Lambert Conformal Conic Projection (lcc) nearest interpolation method was used.