Presentation of Four Centennial-long Global Gridded Datasets of the Standardized Precipitation Index

Drought is a natural phenomenon that poses significant problems around the world, and places huge demands on rural and urban water resources as well as enormous burdens on agricultural and energy production. It is also common in terms of geography, climate and political boundaries and may be considered as a normal, recurrent feature of the climate, although a common misconception is that it constitutes an extraordinary event. In general, drought is defined as the water scarceness due to insufficient precipitation, high evapotranspiration and over-exploitation of water resources or a combination of these factors. Despite its complex nature, there is overall agreement (Barua et al 2009) that precipitation is the primary factor controlling the formation and persistence of drought conditions. Drought Indices (DIs) have been commonly used to define drought conditions. In general, a DI is a function of several hydro-meteorological variables (e.g. rainfall, temperature, streamflow, snowmelt). They can be integrated in decision support systems as a drought management tool to trigger drought relief programs. Moreover, it has been used to quantify deficits in water resources and as a drought monitoring tool. Researchers, however, are confronted with the ambiguity of drought definitions and DIs, which has never been resolved to the satisfaction of all professionals. In attempt to overcome this issue, an Inter-Regional Workshop on Indices and Early Warning Systems for Drought was held at the University of Nebraska-Lincoln from 8 to 11 December, 2009. It was jointly sponsored by the School of Natural Resources of the University of Nebraska, the U.S. National Drought Mitigation Center, the World Meteorological Organization (WMO), the U.S. National Oceanic and Atmospheric Administration (NOAA), the U.S. Department of Agriculture (USDA), and the United Nations Convention to Combat Desertification (UNCCD). The workshop reviewed the drought indices currently in use in various regions of the world to explain meteorological, agricultural and hydrological droughts, assessed the capacity for collecting information on the impacts of drought, reviewed the current and emerging technologies for drought monitoring and discussed the need for consensus standard indices for describing different types of droughts. The outcome of the workshop was the Lincoln declaration (Hayes et al 2011), in which the Standardized Precipitation Index (SPI) was proposed to be used for characterizing meteorological droughts. Moreover, NMHSs around the world were encouraged to use the SPI to characterize meteorological droughts in addition to the indices currently in use. The free availability of digital maps for the monthly precipitation sums in the recent decades, either from objective analysis or from reanalysis, has encouraged the authors to compute the SPI for the frequently used time windows of 1, 3, 6, and 12 months (noted traditionally as SPI-1, SPI-3, SPI-6 and SPI-12) from four sources of data and for the full time length of each dataset. Consequently, following our strong conviction that the free exchange of data and software services are a basis of effective scientific collaboration, we will provide these results free of charge. Main aim of this study is to present these datasets rather than perform comprehensive drought climatology for a selected region and time spans. Thus, the examples presented in this paper should be considered as a small illustration of the wide variety of potential applications at any possible geographical location, depending on the particular interest of each end-user. The paper is organized as follows: Section 2 provides a description of some theoretical aspects of the SPI, its strengths, limitations and application for objective drought assessment. Section 3 contains a concise description of the used precipitation datasets. The calculations performed, validation of the output and description of one problem, which arose during the computational process, are presented in Section 4. In Section 5 some illustrative examples and qualitative comparisons are provided. The short summary and conclusion are placed in Section 6.