Impacts of Climate Change on Hydrological Regimes in Southern Slopes of Central Himalayas
Date
2023-09
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Institute of Science & Technology, T. U.
Abstract
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 ByrÄn 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.
Description
Keywords
SPHY, HEC HMS, Streamflow components, Water balance contribution, Climate change, Ungauged trans-boundary river, Tamakoshi river basin., HBV light