## 3 Results and Discussion
### Drought Trend
#### 3.2.2 Seasonal Drought Analyses  
The three-month values of SPI up to March (for winter), June (for spring), and December (for autumn) were employed in seasonal study of drought. Indeed, the three month SPI value up to March is the result of a cumulative precipitation calculation in January, February, and March. The average SPI of the total points of the TRMM network for winter, spring, and autumn is shown in Figure 6.  
Figure 5: Mapping of drought severity in January 2009. Figure 6 shows that in winter, precipitation fluctuations in the study area were divided into two general sections. From 1998 to 2006, the precipitation exceeded the average, and in the year interval of 2008-2014, it was recurrently below the long-term average. Considering the values of 0.5 to \(-\)0.5 as the normal limits of SPI, the number of wet and dry periods of winter was, respectively, five and four. From the view point of severity, the wet periods are more intense than dry ones so that the frequency percentage of severe and very severe wet years is more than that of severe and very severe droughts. The wetness in winters of 1998 and 2005 had a good deal prevalence, respectively, with very severe and severe intensity in most areas of the study area. In spring and at the beginning of the study period, the precipitation situation was less than average, while during the middle period of the study it exceeded the average. In the last period, the precipitation fluctuations were more serious, and the dry and wet periods were intermittently observable. During this season, the frequency of dry periods totaled six, and that of wet periods amounted to five. As for duration, the droughts were more long-lasting than wetness. The longest duration of dry periods was three years, and that of wet periods was two years. Throughout autumn, the droughts happened irregularly and were distributed through the study period. The above-average conditions are easily discernible in the first decade of 2000. In this time of the year, the study area either got along in wet or normal-to-wet conditions. From 2007 on, the precipitation fluctuations became severe, and different periods repeated intermittently. The duration of wet periods was more than dry periods, so that from 2001 to 2004 the SPI were always positive. Meanwhile, the maximum duration of negative SPI values was two consecutive years. In autumn, despite the remarkable number of normal periods, the frequency of droughts was more than that of wet years. During this season, wet periods were longer and more intense. So that, the frequency of severe and moderate droughts was more than severe and moderate wetness. In fact, the severity of probable droughts in different zones of the study area was more than that of wetness notwithstanding the fact that their frequency was fewer. The mappings of SPI values were produced during the investigation, with the purpose of examining drought situation in winter, spring, and autumn all over the study area. Figure 7 illustrates the SPI mapping, for example, in winter of 2009, spring of 2008, and autumn of 2014. Figure 6: Average seasonal SPI. Figure 6: _Cont._  
In view of Figure 7a, the severity of drought in winter of 2009 increased southwards from the north. All over the northern part of the study area, the intensities of the droughts, except for Javanrud, were mild and generally medium. A greater extent of severe droughts corresponded to south-western, compared to south-eastern, areas of the study area. Figure 7b shows that spring of 2008 had the most severe drought during the study period with average SPI of \(-1.9\), 47% severe drought, and 47% very severe drought. According to this figure, the severity of drought in the spring of 2008 was decreased in the direction of the west to the east. In the west part of the study area, exclusive of the northernmost area of Paveh, and the south of Gilan-e-Gharb and Somar, the drought was extremely severe, and the SPI index fell below \(-2\).