The Standard Precipitation Index (SPI) is based on precipitation alone. Its benefit is that it can be calculated for several time scales. The calculation of the SPI for the specified time period for any locations requires a long-term monthly precipitation data base with 30 years or more of data.

The question about the influence of the length and period of the data base arises naturally. If we have a trend in our precipitation time series, then the distribution depends on the time period and probably on the lenght of the database.

The Hungarian Meteorological Service was established in 1870. The same type of precipitation gauges are used since the beginning of the 20th century. A recent study (Climate of Europe, European Climate Assessment, 2002)shows significant decrease of the annual precipitation sum in some South European stations. The authors found significant trends for the annual precipitation amount, number of wet days, annual maximum 5-day precipitation (all -3 - -6 %/decade), and negative, but not significant at 5% trend for the number of days with precipitation >=20 mm at Budapest, capital of Hungary.

The country is relative flat (highest peak is about 1000 m) and small (93.000 km2). Therefore, we have a good data base to test the influence of time period on the SPI series for a network of precipitation gauges.

The significancy of the differences will be investigated by statistical tests.