Hydrology & Meteorology

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https://hdl.handle.net/20.500.14540/130

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Analysis of Heat Wave over Nepal
(Department of Hydrology and Meteorology, 2022) Bhattarai, Hira Prasad
Climate change imposed negative impact on socio-economic sectors of the mountainous region like Nepal. The 21 st century has brought many records breaking warm year. Such warm periods brought many health-related problems, heat wave is one among them. Daily maximum temperature data of 46 stations of DHM is used to calculate the heat wave (HW)patterns over the country. To better understand the regional pattern of HW all the stations are further classified into 6 different regions: East Terai, Central Terai, Western Terai, Eastern Mountain, Central Mountain and Western Mountain based on the 3 large river basins (Koshi, Gandaki and Karnali). The percentile-based method is used for the calculation of the HW. The 90 th percentile values are used as the threshold and HW is declared after the daily maximum temperature on exceeding the threshold and the normal temperature of the station by 4° C for at least 3 consecutive days. Based on the criteria, HW’s different characteristics like the total number of events, total days of HW, longest event, average days in HW, Maximum deviation of temperature in HW, the onset date of HW and the withdrawal date of HW are calculated using MATLAB and MS-Excel. The western region records the highest frequency and total heat wave days. The central and western mountain region shows the highest maximum temperature deviation than the Terai regions. Although the HW events are more severe in the mountain region but terai region shows consistent severe HW in these regions. The average days of heat wave event throughout the country is 2.6 days. Also, the monsoon event controls the heat wave events. The ElNino phenomena shows no significant relation with the heat wave but the peak of the heat wave is found at ElNino+1 year. The criteria used in this study is very easy to calculate and can be used for the national decision-making process.
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.
The Impact of Climate Change on Runoff Generation in Langtang Basin Nepal
(Department of Hydrology and Meteorology, 2014) Adhikari, Gopi
Climate change has potential impacts on economy, ecology, and environment of Himalayas.Climate change studies in Himalayan regions have focused mainly on glacier melting and retreating, Glacial Lake Outburst Flood (GLOF) etc. Changing temperature has direct impacts on glaciers and snow that affects the snow melt and river discharge.Hence, this research has been carried out to understand the impact of climate change on runoff generation of Langtang basin.This runoff is important for planners and designers in the aspect of irrigation, hydropower,and drinking water supply and so on.So, the runoff estimation study is essential.This study is carried out using monthly Thornthwaite water balance model.Thornthwaite monthly water balance model is one of the popular model developed by USGS for the runoff estimation and can be applied to estimate the runoff of snow and glacier bound catchment. This study also focuses on the runoff estimation of past, present and future scenario at Langtang region of Nepal by using the Thornthwaite model.The outputs of the analysis on temperature trend revealed a fasterwarming trend in Langtang area (i.e. 0.084 ºC/year).The mean annual soil moisture storage is increasing pattern (i.e.0.71mm/year).The precipitation and runoff are also observed increasing (i.e.10.59mm/year and0.8mm/year).The coefficient of determination of calibration and validation are 0.926and 0.996 that implies that the model is well validated and calibrated as well.The increase/decrease in temperature and runoff has proportional relationship and increase/decrease in rainfall and runoff has also proportional relationship.The projected runoff by the model is slightly decreasing from 2001 to 2060 this result shows that the chances offlood in summer and possible droughtin winter may further enhanced in the future.The main outputs of this study help to implement appropriate strategy for water resources management and hydropower development and provide a strong message on the scenario of the Global impact of warming in the Himalayan region.
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.
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
Observed Trends and Spatial Distribution in Daily Precipitation Indices of Extremes over the Narayani Basin, Nepal
(Department of Hydrology and Meteorology, 2020) Lamichhane, Dipendra
Climate change is one of the biggest environmental challenge that plays out through changing intensity, duration, and frequency of extreme events. To fulfill the research gap in understanding and quantifying the recent changes in precipitation extremes over the Narayani river basin of central Nepal, the long-term daily precipitation data from 1980 to 2018 were run in ClimPACT2 an R software package to calculate ET-SCI extreme precipitation indices. In this study physically relevant 14indices obtained from 23 stations were examined for their spatial and temporal variation. Before the calculation of indices data quality and homogeneity test was performed. The results suggest that the variations of extreme indices throughout the study area are quite different from that of seasonal and annual patterns to some extent. The monsoonal precipitation was mostly concentrated in the central part of the basin within the Middle Mountain region (Lumle and its surroundings). Especially the lowlands (Terai and Siwaliks) and including some parts of middle mountains the precipitation intensity-based indices like as, percentile indices (R95p) and absolute indices (RX1day, RX3day, RX5day) were in the increasing trends, but the frequency of precipitation like threshold indices (R1mm, R10mm, R20mm) along with the duration of precipitation seemed to be decreasing. This implies that the lowlands regions bringing about rainfall related hazards like floods and soil degradation with inundation and may cause possible impact on agriculture and livelihood due to intense rainfall and prolongation of dry spells with the weakening of rainfall duration (days/year). However, the light to moderate precipitation and associated days over the high altitude and that could be the possible cause of landslides. This study also highlights the suggestion that there may be a possible impact on agriculture facilities, food security, and water scarcity in the eastern part of the basin due to the significant decreasing trend of annual total wet days precipitation (PRCPTOT)

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