PROJECTION OF FUTURE DROUGHT CONDITIONS USING DROUGHT INDICES APPLIED TO GCM-SIMULATED WEATHER SERIES

Various drought indices are commonly used to assess drought conditions. In addition to the 1-month and 12-months Standardized Precipitation Index (SPI) and the Palmer Drought Severity Index (PDSI), the present contribution employs Z index, which is closely related to PDSI but does not account for the persistence - it rather characterizes the immediate (for a given week or month) conditions. While the SPI is based solely on precipitation data, the PDSI and Z indices are based on precipitation and temperature data and on the available water content of the soil. For climate change impact studies, we consider PDSI and Z to be better indicators as they account for the changes in both temperature and precipitation. The three indices are calculated by computer programs available from the National Drought Mitigation Center and Computer Science and Engineering, both located at the University of Nebraska-Lincoln. To allow assessment of the climate change impacts, we made the modification of the programs: the indices are calibrated using the present climate weather series and then applied to the future climate weather series (relative drought indices). In assessing impacts of climate change on the drought conditions, we analyze changes in the values of the indices and in the frequency of months belonging to drought spells, which are defined here as continuous periods in which the index: (a) does not exceed the selected upper threshold value, and (b) falls at least once below the lower threshold value. Threshold values of 0 and –1 are used to define the SPI-based drought spells, –1 and –3 are used to define PDSI-based and Z-based drought spells. The drought indices are derived from the grid-related GCM-simulated (whole globe) surface monthly weather series for 1961-2000 (present climate) and 2060-2099 (changed climate). To account for the inter-GCM uncertainties, multiple GCM simulations available from the IPCC database are used. Based on the results from a set of GCMs, the regions which exhibit significant trends towards drier or wetter climate are identified. The stress is put on Europe and North America.

Acknowledgement: The study is supported by the AMVIS-KONTAKT project (ME 844) and National Agency for the Agricultural Research (project QG60051).