Drought forecasting using the Standardized Precipitation Index

University of Catania, Catania, Sicily, Italy
Water Resources Management (Impact Factor: 2.46). 05/2007; 21(5):801-819. DOI: 10.1007/s11269-006-9062-y

ABSTRACT Unlike other natural disasters, drought events evolve slowly in time and their impacts generally span a long period of time.
Such features do make possible a more effective drought mitigation of the most adverse effects, provided a timely monitoring
of an incoming drought is available.

Among the several proposed drought monitoring indices, the Standardized Precipitation Index (SPI) has found widespread application
for describing and comparing droughts among different time periods and regions with different climatic conditions. However,
limited efforts have been made to analyze the role of the SPI for drought forecasting.

The aim of the paper is to provide two methodologies for the seasonal forecasting of SPI, under the hypothesis of uncorrelated
and normally distributed monthly precipitation aggregated at various time scales k. In the first methodology, the auto-covariance matrix of SPI values is analytically derived, as a function of the statistics
of the underlying monthly precipitation process, in order to compute the transition probabilities from a current drought condition
to another in the future. The proposed analytical approach appears particularly valuable from a practical stand point in light
of the difficulties of applying a frequency approach due to the limited number of transitions generally observed even on relatively
long SPI records. Also, an analysis of the applicability of a Markov chain model has revealed the inadequacy of such an approach,
since it leads to significant errors in the transition probability as shown in the paper. In the second methodology, SPI forecasts
at a generic time horizon M are analytically determined, in terms of conditional expectation, as a function of past values of monthly precipitation.
Forecasting accuracy is estimated through an expression of the Mean Square Error, which allows one to derive confidence intervals
of prediction. Validation of the derived expressions is carried out by comparing theoretical forecasts and observed SPI values
by means of a moving window technique. Results seem to confirm the reliability of the proposed methodologies, which therefore
can find useful application within a drought monitoring system.

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    ABSTRACT: Drought is one of the major natural disasters in the Loess Plateau of China. We aimed to reveal drought occurrence regularity, relations between different magnitudes of drought and precipitation intensity changes. Based on Chinese climate divisions and digital elevation model graphs of the Loess Plateau region, our study area was divided into five subregions by means of cluster analysis of the region’s meteorological stations using a self-organizing map neural network. Utilizing daily precipitation data from 72 stations for the 1961–2012 period, and using the number of consecutive days without precipitation as the drought level evaluation, the frequency and differing magnitudes of drought occurrences in each season were analyzed. In addition, the first precipitation intensity after a drought period was analyzed using a Pearson type III frequency curve. Precipitation frequency changes following drought occurrences with different magnitudes in each subregion were analyzed, and the relationship between rainfall intensity and different drought magnitudes was observed, as was the relationship between rainfall intensity and the number of drought occurrences. The results of our study indicate that, first, changes in drought levels in the region within and between years are not obvious, but that changes are significant between decades. Moreover, drought frequency in subregions I, IV and V of our study was higher than in areas II and III. Second, as the drought level increased, the probability of heavy rainstorms and extraordinary rainstorms in the Loess Plateau region became lower, and the frequency of spatial changes at different precipitation intensities varied. The frequency of different precipitation intensities in subregions I, II, IV and V initially showed a decreasing trend, but, with continued development of drought and when extraordinary drought occurred, moderate and heavy rain became an increasing trend, with the frequency of rainstorms and heavy rainstorms decreasing at first, then becoming the same. The probability of moderate and heavy rain in subregion III showed a decreasing trend; that of rainstorms, heavy rainstorms and extraordinary rainstorms initially decreased then became the same. Third, while the frequency of drought occurrences in the Loess Plateau region for the 1961–2012 period showed an increasing trend overall, the extent of the increase differed among the different subregions.
    Natural Hazards 05/2015; 77(1). DOI:10.1007/s11069-015-1594-0 · 1.96 Impact Factor
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    Water Resources Management 01/2014; 28(4):1045-1060. DOI:10. 1007/s11 · 2.46 Impact Factor

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