Variation trend of extreme drought during

Extreme drought always brings about severer natural disasters, and threatens much the agricultural production and human's daily life. Lots of statistical results show the rapid increase of the damage caused by extreme drought. Thus it is necessary to study the frequency and variation trend of extreme drought. Here, the variation trend of extreme drought over China during 1951-2004 was analyzed (Ma and Fu, 2003).

The method is to calculate the month-to-month Palmer Drought Severity Index (PDSI) of each station in 54 years, if PDSI<-3.0, this month is considered as an extreme drought month and counts for one. The number of extreme drought month every year is the extreme drought frequency. For the contrast analysis, the frequency distribution of monthly PDSI<0 is shown as well, to better study the evolution characteristics of extreme drought through temporal variation study of frequency.

The variation trend of frequency for monthly PDSI<0 (drought) and monthly PDSI<-3.0 (extreme drought) in the latest 54 years presents the increasing frequency of monthly PDSI<0 in eastern China and the areas to north of 30°N, indicating that the drought occurrence frequency is increasing yearly. The significant increasing regions are located in the central part of North China, the eastern part of Northwest China, the central and the southern parts of Northeast China, nearly corresponding to the regions of obviously decreasing precipitation. The similar spatial variation trend of the occurrence frequency between drought and extreme drought was found. The occurrence frequency of extreme drought in the area to east of 100°E and north of 30°N shows the increasing trend, but the scope of significant trend is slightly smaller. The extreme drought frequency decreases in the western part of Northwest China and the areas to the south of Yangtze River, and the most prominent occurs in the western part of Northwest China.

The analysis results show that the frequency of drought (monthly PDSI<0) is increasing in the eastern part of Northwest China, the central part of North China and Northeast China, and the frequency of extreme drought also shows an increasing trend. In the late 1990s, the number of extreme drought in the eastern part of Northwest China and Northeast China increased sharply by two times or more, especially in Northeast China, the average was 6 times/year during 2001 — 2004. The increase number in the central part of North China occurred in the 1980s, earlier than that of the eastern part of Northwest China and Northeast China. The extreme drought frequency in the western part of Northwest China varies little and stabilizes at about 1. The analysis above shows the warming beginning around the 1980s and the obvious increase after the mid 1980s, which may be one of the important reasons for the significant drying trend in northern China during the latest 20 years.

The analysis indicates a rapid increase of extreme drought frequency in northern China and the areas to east of 100°E during the latest 54 years. Despite the different starting time in the eastern part of Northwest China, the central part of North China and Northeast China, a sharp increase in latest 10 years is the common characteristic in the three subregions, which is closely related to the significant warming in recent years.

Continue reading here: Global aspects of aridity trend Palmer Drought Severity Index for

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