Analysis of Annual and Seasonal Temperature Variability in The Karnali River Basin, Nepal
| Main Authors: | Arati Poudel, Ayush Raj Gyawali |
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| Format: | Article Journal |
| Bahasa: | eng |
| Terbitan: |
, 2019
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| Subjects: | |
| Online Access: |
https://zenodo.org/record/3243460 |
Daftar Isi:
- Nepal is a diverse country in terms of geography and climate situated between China and India. Ranging from 26°22ꞌ to 30°27ꞌ N in latitude and 80°04ꞌ to 88°12ꞌ E in longitude, it lies in the central part of the Himalaya region. Within a span of less than 200 km, a wide range of climatic condition, from subtropical to alpine, triumphs in Nepal mainly due to the rapid changes in altitude and aspect along the latitude. The seasons in Nepal are categorized as pre-monsoon (March-May), monsoon (June-September), post-monsoon (October-November) and winter (December- February). Pre-monsoon is hot, dry with the effect of westerly windy weather and generally localized precipitation while the monsoon is characterized by humid southeasterly monsoonal winds approaching from the Bay of Bengal and with widespread precipitation (Karki et al., 2017). Duringthe post-monsoon season(October–November) rainfall activity is greatly reduced, with November normally the driest month. Winter in Nepal (December–February) is generally dry andthe amount of precipitationis low i.e. dominated by westerly circulations (Kansakar et al., 2004; Shrestha et al., 2000). Nepal is placed in the top 4 countries to be affected by climate change by the Global Climate Risk Index, 2019. The effects of climate change are seen more noticeable in the higher elevational region as stressed by numerous literature (Pepin et al., 2015; Liu et al. 2009; Rangwala et al., 2009;Bhutiyani et al. 2017). Several studies in Nepal (e.g., Baidya et al., 2008; Shrestha et al., 1999; Shrestha et al., 2017)have shown the rise in temperature from 0.06° to 0.12°C year-1 in last few decades. The impact of even a small rise in temperature may result in greater complexities in a natural disaster-prone country like Nepal. Since the major river basins in Nepal i.e. Koshi, Gandaki and Karnali basin (Figure 1) have headwaters in the High Himalaya, studies show that the impact of rising temperature is being evident in these basins. Figure 1: Three major basins of Nepal (Karnali, Gandaki and Koshi) with major river channels Research on a vulnerability assessment framework of the IPCC explains that the Karnali River Basin (KRB) and its watersheds are more susceptible to climate change as compared to other major basins of Nepal (IWMI, 2012). Also, this region is not explored scientifically as compared to other regions of Nepal. Therefore, this research aims to analyze the annual and seasonal trend of maximum and minimum (Tmax and Tmin) temperature range as well as the diurnal temperature range to better understand the temperature variability occurring in KRB. Shrestha et al. (1999) Shrestha et al. (1999) Shrestha et al. (1999) They found that the average warming in annual temperature between 1977 and 1994 was 0.06°C year -1 Materials and Methods: Study Area: Figure 2: Map of the study area i.e. Karnali River Basin (KRB) with climatic stations (black triangle) used in the study Karnali river basin (KRB) lying between the mountain ranges of Dhaulagiri and Nanda Devi is one of the major basins in Nepal (Figure 2). The basin extends from latitudes 30.4° north to 28.2° east and covers an area of 45, 269 km2 with the outlet in Chisapani (Khatiwada, Panthi, Shrestha, & Nepal, 2016). In KRB, both Indian monsoons and westerlies play prominent roles to bring precipitation in this region (Shrestha, 2000). All together it comprises of 57 rivers and has six major watersheds as its tributaries of which all others originate in Nepal except Humla Karnali, which originates in China. Karnali River with a length 507 Km long is one of the longest river flowing inside Nepal, originating from south of Mansarovar and Rokas lakes located in China (Tibet). The summer monsoon which originates from the Indian Ocean (Shrestha & Paudyal, 1992) is the chiefsource of precipitation in Nepal. As the KRB lies in the western region of Nepal, the influence of summer monsoon is weaker than in the eastern regionof Nepal (Nayava, 1980). Data collection and analysis: Data were collected from Department of hydrology and meteorology of Nepal. The climate stations run by the Department of Hydrology and Meteorology are mostly below 3000m. Three stations in different regions with the best data quality with no missing years were selected. The details of the selected stations are presented in Table 1. After the data selection, the quality of the data was checked in Rclimdex software. The software checks and removes the outliers like Tmin> Tmax. The months with more than 10 days missing were not used for the analysis. To detect the trend in the maximum and minimum temperature, Man-Kendall test was appliedwhich has been generally applied to estimate the monotonic trends and their significance in climatic time series (Mann, 1945; Kendall, 1975).The significance level is set at α = 0.05. Table 1: Details of the stations used in the study S.N. Station ID Latitude Longitude Elevation (m) Start year End year Total no. of years 1 218 29.26 80.94 563 1986 2015 30 2 303 29.27 82.18 2363 1986 2015 30 3 406 28.59 81.64 720 1986 2015 30 Results: Annual maximum and minimum temperature trend Figure 3: Temporal trend of stations averaged maximum and minimum temperature from the period 1986 to 2015 The stations averaged temporal trend of maximum and minimum temperature is shown in figure 3. The maximum temperature is increasing in high rate i.e. 0.038 °C year-1 as compared to minimum temperature i.e. 0.026 °C year-1. Both maximum and minimum temperature trend are statistically significant which signifies warming in the Karnali river basin. The results show that Tmax and Tminin the Karnali river basin is increasing significantly. The results are consistent with few other studies in Nepal like Shrestha et al. (1999), Khatiwada et al. (2016), Baidya et al. (2008) but differs from the global average (Alexander et al., 2006; Donat et al., 2013). The spatial distribution of thetrend ofTmax and Tminis presented in Figure 4 and 5. Star sign represents a statistically significant trend. The maximum temperature has a positive trend for all three stations and two of them are statistically significant. Whereas, minimum temperature has one negative trend which is not statistically significant and two statistically significant increasing trends. The maximum temperature trend ranges from 0.001 to 0.067 °C year-1 and minimum temperature trend ranges from -0.003 to 0.054 °C year-1. Figure 4: Spatial distribution of maximum temperature in KRB Figure 5: Spatial distribution of minimum temperature in KRB 3.2 Diurnal Temperature Range: Diurnal temperature range (DTR) is the difference between the Tmax and Tmin. The trend of DTR for the annual and seasonal time period is shown in Table 2. The annual DTR is increasing at the rate of 0.011 °C year-1but is not statistically significant. For seasonal, pre-monsoon and monsoon shows positive trend while post-monsoon and winter shows a negative trend but none of these are statistically significant. Due to the reason that the maximum temperature increase is higher than the minimum temperature increase, the diurnal temperature range has a positive trend. Table 2: Trend of Diurnal temperature range Time Period slope p-value Annual 0.011 0.376 Pre monsoon 0.03 0.272 Monsoon 0.012 0.339 Post monsoon -0.008 0.777 Winter -0.02 0.479 Seasonal Temperature Trend: The seasonal trend of stations averaged maximum temperature is shown in Figure 6. All four seasons (Pre-monsoon, monsoon, post-monsoon and winter) have positive trend but the statistically significant trend is seen in pre-monsoon and monsoon season. The highest increase is in pre-monsoon season with 0.063 °C year-1and the lowest increase is in winter with 0.016 °C year-1. Figure 6: Temporal trend of stations averaged seasonal maximum temperature The seasonal trend of minimum temperature is shown in Figure 7. All four seasons have a positive trend and the increase is statistically significant in pre-monsoon, monsoon and winter while statistically non-significant in post monsoon. The highest increase is in pre-monsoon which is similar to the result of maximum temperature and the lowest increase is in post monsoon. Figure 7: Temporal trend of stations averaged seasonal minimum temperature Summary and Conclusions: Annual and seasonal trend of maximum and minimum temperature and diurnal temperature range (DTR) were calculated and analyzed for Karnali river basin in Nepal for the climatic period of 30 years (1986-2015 A.D.). This study clearly shows that there is warming in the Karnali river basin. The seasonal trend also reveals that all seasons are experiencing a rise in temperature, especially in pre-monsoon and monsoon season. You et al. (2008) stated that warming in the mountainous region is possibly due to the increase in anthropogenic greenhouse gases emission which contributes to the change in cloud cover. The diurnal temperature range is increasing annually and during pre-monsoon and monsoon while decreasing in post-monsoon and winter. The temperature in the winter is not increasing as in other seasons possibly due to the decrease in winter temperature in the plain regions of Nepal due to fog episodes which reduce the Tmaxsignificantly (Manandhar, 2006; Baidya et al. 2008).The increase in Tmax(0.04°C/ year) is higher than Tmin (0.03°C/ year) which is a contrasting result from the surrounding Tibetan Plateau and global results. The effects of climate change can be seen in KRB and this might have significant impacts on social life and the environment. The detailed study of other climatic variables would contribute to comprehend the impact of climate change in detail in this region.