3.1. Trend analysis of SPI3
We were carried out MK trend test for winter SPI3 time scale over the country. The trends were identified Using MAKESENS Template (Salmi et al., 2002), depend on the Z value of individual stations; the increasing/decreasing trends were identified with reference the positive/negative value of Z.
Station wise significant trends were examined in different regions of Nepal for SPI3 time scale from 1977 to 2018. Overall 107 stations, 1 station showed significant increasing, and 51 stations showed significant decreasing trends in different regions over Nepal.
Figure 2 shows overall trends of SPI3 time scale over Nepal. Many of the stations of the country showed a decreasing trend for SPI3 time scales in winter. However, in the eastern region the significant decreasing trends were comparatively high than the central and western regions.
3.2. Temporal variability for SPI3 time scale
This study has used average monthly precipitation of 107 stations to generate the temporal variability for SPI3 time scale (Fig. 3). A season is defined as a winter drought season when the SPI3 time scale thresholds are < -1. The drought seasons are categories based on the intensity of SPI3 time scale values. They are tabulated in Table 1.
We used SPI3 time scale intensity for the study of the deficit/excess winter events with year to year variability. Intensity of the SPI3 time scale values categories the severity of the drought and flood years. The deficit years were observed 17 seasons, and excess years were 24 seasons. 3-month SPI of February captured the deficiency or excess of precipitation for detecting drought and flood in Nepal. Eight winter drought years were 2006, 2009, 1999, 2018, 2017, 2008, 2016, and 2001 with average SPI3 time scale values are − 2.41,-2.26,-1.85,-1.7,-1.38,-1.33,-1.21,-1.01 respectively. And 5 winter excess years were 1999, 1988, 1996, 2007 and 2003 with SPI3 average time scale values were 1.78, 1.59, 1.31, 1.17, and 1.05.
Interestingly, the recent decade (2008–2018) Nepal faced the frequently winter drought episodes in years 2008, 2009, 2016, 2017 and 2018 (Fig. 3). Winter drought 2006 was the worst drought and followed by the 2009 and surplus year 1999 followed by year 1988 respectively in the recent 4 decades climatologically history of Nepal. The fluctuation of SPI3 time scale values ranges from (-2.41) in the drought year 2006 and (1.78) in the excess year 1999.
3.3. Regional temporal variability for SPI3 time scale
We have used average monthly precipitation of 28, 47 and 32 stations to generate the western, central and eastern (WCE) regional temporal variability for SPI3 time scale (Fig. 4). The drought events observed in winter season over the WCE regions of Nepal had different intensities. Regional comparative study of droughts in Nepal showed the diverse characteristics of SPI dynamics. Individual drought year’s drought variability due to western disturbances is distinct nature and intensity of drought is crucial for the water resources planning and management.
The temporal variability of winter drought/flood events observed on the WCE regions of Nepal is tabulated on Table 2.
Table 2
The drought and flood years from 1977 to 2018
| Condition | Years | | | | | | | |
Western | Drought | 2009 | 1999 | 2006 | 2016 | 2008 | 2018 | 2017 | 2001 |
| SPI values | -2.1 | -2.0 | -1.8 | -1.7 | -1.6 | -1.5 | -1.5 | -1.1 |
| Flood | 2013 | 1989 | 2002 | 2003 | 2005 | 1998 | 2015 | |
| SPI values | 1.68 | 1.51 | 1.23 | 1.18 | 1.14 | 1.08 | 1.07 | |
Central | Drought | 2006 | 2009 | 2018 | 1999 | 2016 | 2008 | 2017 | |
| SPI values | -2.8 | -2.1 | -1.6 | -1.5 | -1.3 | -1.1 | -1.1 | |
| Flood | 1989 | 1998 | 1996 | 2007 | 2015 | | | |
| SPI values | 1.93 | 1.84 | 1.47 | 1.3 | 1.03 | | | |
Eastern | Drought | 2006 | 2009 | 1999 | 2018 | 2017 | 2010 | | |
| SPI values | -2.5 | -2.2 | -1.8 | -1.7 | -1.5 | -1.1 | | |
| Flood | 2007 | 1996 | 1998 | 1989 | 1980 | 1994 | 2003 | |
| SPI values | 1.65 | 1.35 | 1.34 | 1.32 | 1.26 | 1.23 | 1.04 | |
Out of eight winter drought episodes, four drought episodes (1999, 2001, 2008 and 2016) the western region affected more than the central and eastern regions. Similarly, in 2006 the central region of Nepal affected more than in the eastern and western regions. And three drought episodes (2009, 2017 and 2018) the eastern region affected more than the central and western region of Nepal. The western and central regions observed the consecutive drought in years 2008 and 2009 and in recent years 2016, 2017 and 2018.Similarly, in the eastern region consecutive drought years were 2009 and 2010 and in the recent years 2017 and 2018. The consecutive drought years are more hazardous on environmental issues causes the crops and water scarcity. So, during the drought events in the hill and mountainous region livelihood is more challenging than normal years.
3.4. Spatial overview of winter drought episodes
Interestingly, there were severities, extreme, severe, moderate, and mild of winter drought over different regions of Nepal in each drought episodes {Fig. 5 (a-h)}. Each event had different characteristics over Nepal. There were severities of extreme, severe, moderate, and mild drought in winter over Nepal during worst years in 2006 and 2009 {Fig. 5(c and e)}. The worst drought episodes affected more extreme drought comparatively in the western and central regions than eastern region. The severities of drought affected locations are clearly depicted through spatial interpolation process which helps to identify the severities of extreme, severe, moderate, and mild drought over Nepal. Worst winter drought years 2006 and 2009 affected the larger areas of Nepal. The drought events affected particularly western and central regions of Nepal with extreme and severe drought. In winter seasons drought in 2006 and 2009; percent proportional weightage was affected by extreme, severe, moderate and mild drought conditions in different regions over Nepal are presented in Table 3. Around 96% of stations are negative SPI3 time scale values in worst winter drought years (2006, 2009) so large locations of Nepal affect the precipitation deficit in worst drought years. Comparisons between two worst winter drought years, in 2009 the western and central region affected more extreme drought than in 2006. The severities of drought are simply identified, and understandings are shown in Fig. 5(c, e). So, large locations of Nepal observed the deficits precipitation from the climatologically mean precipitation. Drinking water and winter agricultural practices point of view, these episodes were one of the most damaging period over the study period. This study showed most of the western and central part of Nepal recorded the extreme and severe drought and most of eastern part of Nepal recorded the moderate drought in 2006 and 2009 with some exceptions.
Recent winter drought events in 2017 and 2018 interpolated through spatial process which help to identify the severities of drought over Nepal which indicates that the drought events affected with severities of drought in the different region of the nation. In drought year 2017 the western and eastern part of Nepal affected by drought more than the central parts of Nepal. Particularly, most of the central part of Nepal affected by mild drought (near normal); but the western and eastern parts of Nepal affected by severe and moderate drought in 2017. Similarly, drought in year 2018 far western terai and middle mountain region of central and most part of eastern region affected by severe and moderate drought and most of the high mountain region of the central part of Nepal affected by mild drought (near normal) in 2018. Recent winter drought years of 2017 and 2018 affected drought severities (extreme, severe, moderate and mild) in different locations of Nepal clearly depicted in Fig. 5(g, h). Drought events recorded frequently is harmful for human beings as environmental aspects. Percent of the stations were affected by extreme, severe, moderate, and mild drought severities over country are tabulated in Table 5. Around 93% of stations are negative SPI3 values in years 2017 and 2018. So, large parts of Nepal face the winter precipitation deficits. Drinking water, irrigation and agricultural and point of view, this event was also crucial and damaging of the study period (1977–2018).
The drought years in 1999 the western part of Nepal affected most of locations by severities of drought (extreme, severe and moderate drought). Eastern parts of Nepal affected by severe and moderate drought. In 2001 some locations of the western region of Nepal affected by moderate drought. Large locations of the central and eastern region of Nepal affected under mild drought. In 2008 the western part of Nepal affected by drought severities more than central and eastern parts. In this episode the central and eastern part is affected by mild drought. In 2016 the far western part of Nepal affected by drought but the eastern part of Nepal is normal and central parts of Nepal near Pokhara and Annapurna regions affected by drought. The individual drought year’s severities extreme, severe, moderate and mild drought based on the proportional weightage of stations expressed in percent in different years is shown in Table 3. In these particular years the different regions of Nepal have different rainfall dynamics.
Interestingly, droughts have been observed frequently since 2001 in Nepal. Drought events in the recent decades were frequently in years 2008, 2009, 2010, 2016, 2017 and 2018 in Nepal. There were different severities of drought, extreme, severe, moderate, and mild drought during major eight winter drought episodes in Nepal {Fig. 5 (a-h)}. The proportional weightage of severities of the drought episodes were tabulated in Table 3.
Table 3
Winter drought severities based on stations proportion expressed in percent in different years over Nepal
Rank | Year SPI3 | Ave SPI3 | Extreme | Severe | Moderate | Mild |
1 | 2006 | -2.41 | 17.59 | 38.38 | 33.33 | 9.25 |
2 | 2009 | -2.26 | 4.63 | 41.67 | 40.74 | 17.59 |
3 | 1999 | -1.85 | 3.70 | 25.93 | 41.67 | 26.85 |
4 | 2018 | -1.7 | 1 | 15 | 52 | 23 |
5 | 2017 | -1.38 | 0.97 | 11.65 | 39.81 | 42.72 |
6 | 2008 | -1.33 | 3.70 | 15.74 | 28.70 | 43.52 |
7 | 2016 | -1.31 | 1.91 | 14.29 | 27.62 | 28.70 |
8 | 2001 | -1.01 | 0 | 6.48 | 28.70 | 56.48 |
The magnitude and spatial severity of drought events were investigated from SPI3 time scale. There are severities of drought during recent decade in drought episodes. During the 42 years worst winter drought years were 2006 and 2009. These droughts severities extremes, severe, moderate and mild drought shows the drought dynamics over the study areas. Spatial extent of severities of SPI3 time scale values are interpolated over Nepal in Fig. 5(c and e) for worst winter drought years. Similarly, the severities of the recent drought events (2017 and 2018) are depicted in Fig. 5(g and h). During study period observed eight drought episodes out of those five drought episodes are in the recent decade (2008–2018). So we conclude winter drought events in Nepal are increasing generally in the recent years. With the overview of these SPI3 dynamics over Nepal could help to know the drought characteristics of typical drought episodes. The proportional weightage of winter droughts severities for extreme, severe, moderate drought are 4, 21, 37 and 33% during study period 1977–2018. The proportional weightage of winter droughts severities covered about 95% locations of Nepal with the negative SPI3 values.