Hydrology & Meteorology

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Browsing Hydrology & Meteorology by Subject "Drought"

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    Drought Detecting and Monitoring over Terai and Mountain Region of Nepal
    (Institute of Science & Technology, 2023-07) Bagale, Damodar
    This study was conducted using 42 years rainfall data since 1977 to 2018 of 107 meteorological stations to examine monthly to decadal rainfall variability of 107 stations over the country were used. The western region has observed low rainfall in pre-monsoons, monsoon, and post-monsoon seasons but observed heavy rainfall in winter season in comparison with the central and eastern regions. The contribution of winter rainfall to annually varied from 0.68% in the year 2006 to 7.04 % in the year 1989. Similarly, the contribution of monsoon rainfall annually varied from 76 % in the year 1992 to 86 % in 1984.The decadal wise rainfall was decreased both in monsoon and winter seasons in the recent couple of decades. There was a strong correlation between the rainfall and Southern Oscillation Index (SOI) in the monsoon season and weak in winter. Generally, large negative/positive magnitudes of SOI on the Indian and Pacific Ocean influence weakening/strengthening monsoon rainfall in Nepal. During El Niño year’s average deficit rainfall was approximately 9 % below the average monsoon rainfall. However, the negative trends of annual rainfall dominated over the country. This study identified winter, summer and annual drought events using the Standard Precipitation Index (SPI). Monthly rainfall was used as an input variable to generate the SPI of 107 stations from 1977 to 2018. The SPI threshold was used to identify, categorize and monitor droughts over Nepal. For this, we investigated the frequency, duration, and severity of drought events. The SPI3, SPI4 and SPI12 month time scales were interpolated to illustrate the spatial patterns of major drought episodes and their severity. In winter large percentage of stations over the country showed a significant decreasing trend for SPI3 in comparison with the monsoon (SPI4) and annual (SPI12).The drought events in El Niño years and non-El Niño years were more strongly related between SPI and SOI than the average years. The relationship between SPI and the climate indices such as the SOI and ONI anomaly over the Niño 3.4 has suggested that one of the causes for summer droughts is El Niño. This study indicated that summer droughts occurred in both El Niño and non-El Niño years. Out of eight drought years, only four drought years were associated with El Niño episodes (1982,1992, 2009, and 2015), and the remaining four drought years (1977,1979, 2005, and 2006) were recorded in non-El Niño years. Similarly, winter and annual droughts evolved in El Niño and non-El Niño years. There is a strong correlation (0.53) between SPI4 and SOI in the monsoon season and a weak in SPI3 and SOI is - 0.31 in the winter at 95 percent confidence level. The regional analysis identified that there is strong correlation between rainfall and SOI for the western region than the central and eastern regions in the monsoon season. Similarly, the correlation coefficient between rainfall and SOI in winter is strong in the western region than in the central eastern regions. Generally, during drought years; SPI and SOI have a strong phase relation compared to average years. Droughts have been recorded more frequently in Nepal since 2000.The areas of Nepal affected by extreme, severe and moderate drought in winter were 4, 21 and 37 percent. Likewise, the areas of Nepal affected by average extreme, severe and moderate drought both in summer and annual events are 7, 9, and 18 percentages and 7, 11, and 17 percentages respectively. The drought-hazardous zones are highest in the western and northwest parts in comparison with the central and eastern regions on both SPI4 and SPI12 time scales. About 47 and 30 percent of areas of Nepal were found to be under high and very high drought hazardous zones of the total area based on SPI4 and SPI12 time scales. यो शोधकार्य नेपालका एक सय सात वटा मौसमी केन्द्रहरूको मासिक तथा वार्षीक वर्षाको परिवर्तनशीलता तथा परिणात्मक अनुसन्धान अन्वेषण गर्नका लागि गत ४२ वर्ष (सन् १९७७–२०१८) को तथ्यङ्क प्रयोग गरी गरीएको हो । पश्चिम क्षेत्रमा प्रि–मनसुन, मनसुन र मनसुन पश्चातको मौसममा कमवर्षा हुने गरेको छ । तर मध्य र पूर्वी क्षेत्रको तुलनामा त्याँहा हिउँदमा भारी वर्षा हुने गरेको पाइयो । हिउँदे वर्षा सन् २००६ मा ०.६८ प्रतिशत र सन् १९८९ मा ७.०४ प्रतिशतसम्म परेको देखियो । त्यसैगरी मनसुन वर्षाको योगदान १९९२ मा ७६ प्रतिशत र सन् १९८४ मा ८६ प्रतिशत सम्म वार्षिक भिन्नता पाइयो । पछिल्ला चार दशकहरुमा वर्षे मनसुन र जाडो मौसममा हिउदे वर्षा घटेको अनुसन्धान बाट देखिएको छ । मनसुनी वर्षा र साउदन ओसिलेसन इन्डेक्स (SOI) विचको सम्बन्ध वर्षा याममा बलियो र जाडोमा कमजोर पाइयो । सामान्यतया हिन्द र प्रशान्त महासागरमा SOI को नकारात्मक र सकारात्मक परिणामले नेपालमा मनसुन वर्षालाई कमजोर र सशक्त बनाउन प्रभाव पार्दछ । एलनिनो वर्षको समयावधीमा (कम बर्षाको अवधिमा) औसत मनसुन वर्षा भन्दा लगभग ९ प्रतिशत कम वर्षा परेको अनुसन्धानले देखायो । यद्यपि वर्षाको घढ्दो क्रम देशमा बढिरहेको छ । यस अध्ययनले जाडो, गर्मी तथा वार्षिक खडेरी घटनाहरू मानक वर्षा सूचकांक (SPI) प्रयोग गरी पहिचान गरेको छ । सन् १९७७ देखि २०१८ सम्म एक सय सात वटा मौसमी केन्द्रहरुको SPI निकाल्नको लागि मासिक वर्षालाई उपायोग गरिएको थियो । SPI थ्रेसहोल्डलाई नेपालमा खडेरी पहिचान गर्न, वर्गीकरण गर्न र निरन्तर निगरानी गर्न प्रयोग गरिएको थियो । यसका लागि खडेरीका घटनाहरुको आवृति, अवधि र गम्भीरताको अनुसन्धान गरियो । हिउँदमा (SPI3), वर्षामा (SPI4) र वार्षिक रूपमा परिमाण (SPI12), विभिन्न अवधिहरुमा, प्रमुख खडेरी एपिसोडहरु र तिनीहरुको वार्षीक मनसुनी प्रभाव को तुलनामा हिउदमा उल्लेखनीयरुपमा घट्ने प्रवृत्ति देखायो । एलनिनो वर्ष र गैर एलनिनो वर्षहरुमा खडेरीका घटनाहरु SPI र वर्षा बिचमा बढी जोडदार रूपमा सम्बन्धित बडेको पाइयो । औसत वर्ष भन्दा SPI, निनो (३.४) क्षेत्रमा SOI र ONI जस्ता जलवायु सुचकाङ्कहरु बिचको सम्बन्धले ग्रीष्म कालीन खडेरीको समयको कारण एलनिनो हो भनी कीटान गरिएको छ । यस अध्ययनले ग्रीष्मकालीन खडेरी एलनिनो (१९८२, १९९२, २००९ र २०१५) वर्षहरुमा र आठ खडेरी वर्षहरु मध्ये केवल चार खडेरी वर्षहरु एलनिनो एपिसोडहरुसंग सम्बन्धित थिए र बाकि खडेरी वर्षहरु (१९७७, १९७९, २००५ र २००६) समेत पाइएको थियो । त्यस्तै खडेरीका घटनाहरु हिउद, ग्रीष्म कालीन र वार्षिक खडेरी एलनिनो र गैर–एलनिनो वर्षहरुमा विकसित भयको पाइयो । मनसुन याममा SPI र SOI बिच बलियो सम्बन्ध र हिउँदमा केही कमजोर सम्बन्ध रहेको (९५ प्रतिशत) सार्थक स्तरमा देखियो । क्षेत्रीय विश्लेषणगर्दा मनसुन समयमा मध्य र पूर्वी क्षेत्रको तुलनामा पश्चिमी क्षेत्रको वर्षा र SOI बिच कमजोर सम्बन्ध रहेको पाइयो । त्यसैगरी, हिउँदे वर्षा र SOI बिचको सम्बन्ध गणांक मध्य पूर्वी क्षेत्रहरु भन्दा पश्चिमी क्षेत्रमा बलियो देखियो । सामान्यतया खडेरी वर्षहरुमाः SPI र SOI बिच औसत वर्षको तुलनामा बलियो चरण सम्बन्ध अध्यनले पुष्टी गरेको छ । नेपालमा सन् २००० यता खडेरी धेरै पटक रेकर्ड गरिएको छ । नेपालको हिउँदमा चरम, गम्भीर र मध्यम खडेरीबाट प्रभावित क्षेत्रहरु क्रमश ४, २१ र ३७ प्रतिशत पाइयो । त्यसैगरी ग्रीष्म र वार्षीक समयावधीमा औसत चरम, गम्भीर र मध्यम खडेरीबाट प्रभाभित क्षेत्रहरु क्रमश ७, ९ र १८ प्रतिशत र ७, ११ र १७ प्रतिशत छन् । त्यसैगरि मध्य र पूर्वी क्षेत्रहरुको तुलनामा पश्चिम र उत्तर–पश्चिमी भागहरुमा खडेरीको आँकडा उच्च र अति उच्च भएको पाइयो । दुवै क्षेत्रहरुमा वर्षाका परीमाणहरु भने सबैभन्दा बढी भएको अध्यनले देखायो । नेपालका करिब ४७ र ३० प्रतिशत क्षेत्रहरु SPI4 र SPI12 टाइम स्केलमा उच्च र अति उच्च खडेरीको जोखिमयुक्त क्षेत्रहरु अन्तर्गत रहेको पाइयो ।
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    Remote sensing and ground based analysis of drought in Karnali River Basin (KRB), Nepal
    (Department of Hydrology and Meteorology, 2023) Acharya, Prabin
    Drought, a frequent and impactful climatic extreme, has significant impact on the context of topographically varied Karnali River Basin (KRB) in Nepal. This study executed a comprehensive assessment of drought dynamics by employing two key indices based on Vegetation Condition Index (VCI) derived from Moderate Resolution Imaging Spectroradiometer (MODIS) data, and single parameter based Standard Precipitation Index (SPI) from ground based station data, with a focus on spatial and temporal variations. Remote sensing and Ground based indices were utilized across all seasons within a year, while also establishing correlations between vegetation dynamics and climatic parameter (precipitation). MK-Trend test have been performed to analysis SPI trend inside the basin. Drought indices derived by climatic data were correlated with Southern Oscillation Index (SOI) to evaluate relationship with indices and seasons. This analysis revealed that drought has occurred more than half of the study period, underscoring its recurrent nature inside the KRB spatially. In particular, the winter and monsoon SPI drought patterns exhibited an alarming increasing rate, which can be surmounted by concerning decrease in precipitation by 4 mm per year. Concurrently, the VCI time series demonstrated an upward trajectory of drought occurrences during the pre-monsoon and winter seasons inside the basin emphasizing increasing winter drought in the lower altitude and summer monsoon drought in the whole basin accessing 50% area experienced vegetation failure temporally. Further investigation revealed that the manifestation of drought is intricately linked not only to local conditions but also to larger climatic oscillations, notably the SOI. Negative value examined between drought events and SOI index in the monsoon season, with enhancing El-Nino phase. The correlation between VCI and precipitation, particularly when precipitation leads VCI by one month, proved to be a robust and noteworthy observation, emphasizing the predictive utility of this relationship. Notably, during the pre-monsoon season, the correlation nearly doubled when a one-month lag in VCI-precipitation is considered. Lower elevations face heightened pre-monsoon and winter drought, while higher altitudes encountered intensified monsoon-related droughts. This results from shifting atmospheric circulation patterns, uneven convection, and increased temperatures, leading to excessive evaporation and soil moisture loss within the year. These findings hold crucial implications for water resource man-agement, agriculture, and overall resilience strategies in the face of changing climatic conditions inside the KRB. Keywords: Drought, SPI, VCI, SOI, Nepal, KRB.