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1. Introduction
Drought is one of the main natural causes of agricultural, economic, and environmental damage (Burton et al. 1978; Wilhite and Glantz 1985; Wilhite 1993). Droughts are apparent after a long period without precipitation, but it is difficult to determine their onset, extent, and end. Thus, it is very difficult to objectively quantify their characteristics in terms of intensity, magnitude, duration, and spatial extent. For this reason, much effort has been devoted to developing techniques for drought analysis and monitoring. Among these, objective indices are the most widely used, but subjectivity in the definition of drought has made it very difficult to establish a unique and universal drought index (Heim 2002). A number of indices were developed during the twentieth century for drought quantification, monitoring, and analysis (Du Pisani et al. 1998; Heim 2002; Keyantash and Dracup 2002).
In recent years, there have been many attempts to develop new drought indices, or to improve existing ones (González and Valdés 2006; Keyantash and Dracup 2004; Wells et al. 2004; Tsakiris et al. 2007). Most studies related to drought analysis and monitoring systems have been conducted using either 1) the Palmer drought severity index (PDSI; Palmer 1965), based on a soil water balance equation, or 2) the standardized precipitation index (SPI; McKee et al. 1993), based on a precipitation probabilistic approach.
The PDSI was a landmark in the development of drought indices. It enables measurement of both wetness (positive value) and dryness (negative values), based on the supply and demand concept of the water balance equation, and thus it incorporates prior precipitation, moisture supply, runoff, and evaporation demand at the surface level. The calculation procedure has been explained in a number of studies (e.g., Karl 1983, 1986; Alley 1984). Nevertheless, the PDSI has several deficiencies (Alley 1984; Karl 1986; Soulé 1992; Akinremi et al. 1996; Weber and Nkemdirim 1998), including the strong influence of calibration period, its limited utility in areas other than that used for calibration, problems in spatial comparability, and subjectivity in relating drought conditions to the values of the index. Many of these problems were solved by development of the self-calibrated PDSI (sc-PDSI; Wells et al. 2004), which is spatially comparable and reports extreme wet and dry events at frequencies expected for rare...