Hydrology & Meteorology

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Browsing Hydrology & Meteorology by Academic Level "Ph.D."

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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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    Impact of Climate Change on Flow Regime in Himalayan Basins, Nepal
    (Faculty of Hydrology and Meteorology, 2014) Adhikari, Tirtha Raj
    This research studied the hydrological regime of three glacierized river basins in Khumbu, Langtang and Annapurna regions of Nepal using the Hydraologiska Byrans Vattenbalansavde (HBV), HVB-light 3.0 model. Future scenario of discharge is also studied using downscaled climate data derived from statistical downscaling method. General Circulation Models (GCMs) successfully simulate future climate variability and climate change on a global scale; however, poor spatial resolution constrains their application for impact studies at a regional or a local level. The dynamically downscaled precipitation and temperature data from Coupled Global Circulation Model 3 (CGCM3) was used for the climate projection, under A2 and A1B SRES scenarios. In addition, the observed historical temperature, precipitation and discharge data were collected from 14 different hydro-metrological locations for the implementation of this studies, which include watershed and hydro-meteorological characteristics, trends analysis and water balance computation. The simulated precipitation and temperature were corrected for bias before implementing in the HVB-light 3.0 conceptual rainfall-runoff model to predict the flow regime, in which Groups Algorithms Programming (GAP) optimization approach and then calibration were used to obtain several parameter sets which were finally reproduced as observed stream flow. Except in summer, the analysis showed that the increasing trends in annual as well as seasonal precipitations during the period 2001 - 2060 for both A2 and A1B scenarios over three basins under investigation. In these river basins, the model projected warmer days in every seasons of entire period from 2001 to 2060 for both A1B and A2 scenarios. These warming trends are higher in maximum than in minimum temperatures throughout the year, indicating increasing trend of daily temperature range due to recent global warming phenomenon. Furthermore, there are decreasing trends in summer discharge in Langtang Khola (Langtang region) which is increasing in Modi Khola (Annapurna region) as well as Dudh Koshi (Khumbu region) river basin. The flow regime is more pronounced during later parts of the future decades than during earlier parts in all basins. The annual water surplus of 1419 mm, 177 mm and 49 mm are observed in Annapurna, Langtang and Khumbu region, respectively.
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    Impacts of Climate Change on Hydrological Regimes in Southern Slopes of Central Himalayas
    (Institute of Science & Technology, T. U., 2023-09) Budhathoki, Bhumi Raj; Dr. Tirtha Raj Adhikari
    Ensuring the consistent accessibility of hydrological data highlights the sgnificance of careful planning, development and management of water resource projects. Within this framework, the effectiveness of diverse hydrological models, such as the Spatial Process in Hydrology (SPHY), Hydrologiske Byrån avdeling for Vattenbalans (HBV), and Hydrologic Engineering Centre Hydrologic Modelling System (HEC HMS) models, are assessed using a range of evaluation metrics. These metrics include the Nash Sutcliffe Efficiency (NSE), Coefficient Correlation (r), Coefficient of Determination (R2), Root Mean Square Error (RMSE), RMSE-observations Standard Deviation Ratio (RSR) and Volume Difference (P-Bias). All three hydrological models were successfully calibrated and validated, demonstrating NSE values exceeding 0.62 for daily and monthly discharge data, based on observations from Busti and Rasnalu stations within the Tamakoshi river basin. Among the various statistical parameters assessed, the coefficient of determination showed a relatively close fit. Similarly, continuous hydrological records for ungauged discharge predictions at locations like Benighat, are estimated using SPHY, HEC HMS, and HBV light models by leveraging data from donor catchments. These predictions span daily, monthly, and annual time frames, sourced from upstream gauged stations. It is significant to note that the accuracy of lowflow predictions surpasses that of high flow predictions, and discrepancies in flow predictions tend to emerge primarily during peak flow periods. Nevertheless, all three hydrological models exhibit similar flow patterns in simulating ungauged streamflow, with discharge at the ungauged receiver site exceeding that of the donor site during simulations. This comparative method proves to be a preferable alternative to individual methods for estimating ungauged discharge. Among these three hydrological models, HBV light model effectively simulates streamflow components using daily input parameters in the Tamakoshi river basin. The variations in streamflow are dependent on the temporal changes in its components. In this context, it is observed that rain runoff predominantly dominates streamflow, constituting 62% of the contribution, with baseflow runoff accounting for 20%, glacier melt runoff for 13%, and snow melt runoff for 5% at the Busti gauge station during the baseline period. It is important to note that there was a positive change in storage within the water balance during the baseline period. In order to evaluate climate change impacts, this study employs four General Circulation Models (GCMs) from the Coupled Model Intercomparison Projects (CMIP 6) Assessment Report 6, along with two scenarios for projecting temperature and precipitation in the Tamakoshi river basin. The findings of this study reveal a consistent trend among all GCMs, indicating a simultaneous increase in temperature and precipitation for both scenarios. Especially, the NorESM2 MM scenario under SSP245 stands as an exception, demonstrating a noteworthy decrease in precipitation. These assessments provide valuable insights into the complex interplay between climate change and the water balance components of the hydrological cycle in the specified region. Furthermore, water balance components are evaluated using an ensemble of two GCMs from CMIP 6, coupled with two Shared Socio-economic Pathways (SSPs) scenarios (SSP245 and SSP585). The assessment covers streamflow components on an annual, monthly, and seasonal basis, focusing on baseline, Near Future (NF), Middle Future (MF), and Far Future (FF) projections. It is worth noting that streamflow predominantly leads the Far Future (FF) projections. Among the streamflow components, precipitation, discharge, actual evapotranspiration, rain runoff, and glacier melt, results showed increase, while baseflow and snow melt runoff decreases by the end of the 21st century in comparison of baseline period under both scenarios, with SSP585 having a more pronounced effect. Particularly noteworthy is the substantial increase in glacier melt runoff attributable to climate change. Moreover, precipitation, governed by monsoons, emerges as the primary determinant of river discharge during the summer season in the Tamakoshi river basin. This study contributes to projecting future climate conditions and the evolving role of streamflow components in hydrological regimes. Additionally, it provides valuable insights for future reference in generating streamflow data at ungauged sites and adderessing deficiencies in data records. This, in turn, facilitates effective planning, management, and development of water resource projects, extending from gauged stations to the outlet at Benighat within the Tamakoshi river basin. The findings also aid in the assessment of suitable models for water resource projects and water-induced disaster management, especially in the context of climate change.
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    Impacts of climate change on river hydrology and energy economics in Budhigandaki river basin
    (Faculty of Hydrology and Metrology, 2022) Marahatta, Suresh
    Water management has become a challenging task due to the increasing population, rapid urbanization and industrialization. Availability of observed hydro-meteorological data plays a crucial role in water budgeting for the country like Nepal that relies heavily on hydroelectricity for its energy needs. Quantification of available water at the local scale and examining how it is impacted by climate change (CC) is extremely important from the water management perspective at the river basin level. Budhigandaki River Basin (BRB) of Nepal, was chosen for this study in assessing climate change impact on river hydrology utilizing well calibrated and validated Soil Water Assessment Tool (SWAT), consequent impact on hydroelectric energy generation. Extending this aspect further, the micro-economic assessment of the Budhigandaki hydroelectric project was also made in this study. This study assessed the interannual variability of hydroclimatic condition of the BRB using daily hydrological and meteorological data for the period from 1983 to 2012. To evaluate the impact of climate change on hydrological phenomenon in the study basin, future climate data under two Representative Concentration Pathways (RCP 4.5 and RCP 8.5) with four climatic conditions (cold-wet, warm-wet, warm-dry and cold-dry) for each RCP were considered. Digital elevation model (DEM) data, land use and land cover, and soil data of the basin are the spatial data required in the hydrological simulation that were utilized. The climate change impact on flow, hydroelectric energy generation and energy economics were evaluated by comparing these variables with the baseline. Historical data shows that there is a very high variability in daily, monthly, seasonal and interannual flow in the study basin. Future annual precipitation in BRB varies significantly and is projected from -9% to 23% for RCP 4.5 and -11% to 21% for RCP 8.5 scenarios compared to the baseline value (1530 mm). The mean annual temperature increases 1.7 o C for RCP 4.5 and 3.9 o C for RCP 8.5 by the end of this century. SWAT model was calibrated and validated at Arughat gauging station considering 30 years daily flow data. Model evaluation using four statistical parameters (NSE, PBIAS, RSR and KGE) showed that the developed model performed very well to simulate river flow. Additional validation of the model done at three supplementary points; two in the upstream and one in the downstream of calibration point (Arughat) also showed that vi the developed SWAT model for BRB is well calibrated. To compare the performance of flow simulation methods in the basin level, a new evaluation statistical index, the Global Performance Index (GPI), was introduced in this study. SWAT hydrological model preformed the best among the different methods considered for flow estimation as evaluated by GPI in BRB. Annual mean flows are projected to increase in the future scenarios; 10 to 31% in RCP 4.5 and 5 to 57% in RCP 8.5 scenarios with respect to the baseline flow of 240 m /s. The analysis of future extreme flow shows an increasing trend in case of annual maximum one-day flow and a decreasing trend in low flow case. These results indicate a need to alter the design of hydraulic structures and selection of storage project over runoff-river project for climate resilience. Future annual energy of the Budhigandaki Hydroelectric Project is expected to increase by 9 to 13% compared to the baseline value (3385 GWh) that is equivalent to annual revenue of 20 to 28 million USD. Results of this study show that storage hydroelectric projects with the provision of flexible operating rules are desirable. Financial policies related to hydroelectricity need to be revised with the changes in the future climatic conditions. The findings of this study are expected to be useful for hydrologists, economists and decision-makers to plan the use of available water judiciously in the future. Keywords: Climate change, river hydrology, hydroelectricity, energy economics, GPI, SWAT, Budhigandaki