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The PDSI measures the accumulated effect of monthly rainfall deficit/surplus relative to the monthly 'climatologically appropriate rainfall', defined as rainfall needed to maintain adequate soil water content for normal (water stress free) growth of plants in a region. This appropriate rainfall is a function of time and its monthly values are calculated from surface and soil water balance among evaporation, plant transpiration, runoff and available soil water for evaporation and transpiration (Palmer, 1965; Alley, 1984). In the soil water balance, evaporation and plant transpiration are major processes affecting water loss (Monteith, 1973, 1976; Abramopoulos et al., 1988). Because evaporation and transpiration rates are determined by the deficit of vapour pressure between soil/plant surface and surface air, and vapour pressure is a function of temperature, the appropriate rainfall is a function of air temperature. Thus, the equal dependence of the PDSI on monthly appropriate rainfall and actual monthly rainfall underscores a significant effect of air temperature on the PDSI. This temperature effect on the PDSI, however, has not been thoroughly investigated.

Numerical experiments have been used to evaluate the influences of temperature and precipitation anomalies on the PDSI and its related indices, e.g. Palmer Hydrological Drought Index (PHDI) (Guttman, 1991). Results of these experiments showed that precipitation anomalies tend to dominate the change of PDSI in the cold season when evaporation is minimal. Temperature effect on PDSI becomes important in the warm season. However, because the response of the PDSI often lags the anomalies of temperature and precipitation by a few months (e.g. Karl, 1986) and this lag relationship is not well understood, the questions of how temperature and precipitation affect the PDSI variation and how we may interpret the PDSI in terms of precipitation and temperature anomalies remain.

The increasing usage of the PDSI, particularly in climate research and monitoring, demands answers to the above questions and invites further understanding of the index. Separating and understanding effects of temperature and precipitation on the PDSI are essential for correct use of the index to monitor droughts and in accurately interpreting temperature and precipitation anomalies contributing to the PDSI.

In this study, we separate, using a theoretical analysis, temperature and precipitation effects on the PDSI. We use observational and experimental analyses to illustrate temperature and precipitation effects on the index. In Section 2, we will develop an analytic relationship between the PDSI and anomalies of precipitation and temperature. In Section 3, we will show variations of monthly PDSI corresponding to separate anomalies of monthly temperature and precipitation. Finally, in Section 4, we discuss the significance of these results.