Nair et al. (2025) On the effects of climate downscaling for projecting hydrologic response of catchments in High Mountain Asia

Identification

• Journal: Frontiers in Water
• Year: 2025
• Date: 2025-11-27
• Authors: Anju Vijayan Nair, Diogo S. A. Araujo, E. D. Gutmann, Sungwook Wi, Jamyang Phuntshok, Karma Toeb, Mujtaba Hassan, Rijan Bhakta Kayastha, Efthymios I. Nikolopoulos
• DOI: 10.3389/frwa.2025.1611141

Research Groups

• Department of Civil and Environmental Engineering, Rutgers University, New Brunswick, NJ, United States
• National Center for Atmospheric Research, Boulder, CO, United States
• Department of Biological and Environmental Engineering, Cornell University, Ithaca, NY, United States
• National Center for Hydrology and Meteorology, Thimphu, Bhutan
• Department of Space Science, Institute of Space Technology, Islamabad, Pakistan
• Department of Environmental Science and Engineering, Kathmandu University, Dhulikhel, Nepal

Short Summary

This study investigates the impact of climate downscaling techniques on hydrological projections for glacierized catchments in High Mountain Asia (HMA). It finds that downscaling significantly reduces biases from original Global Climate Models (GCMs) and provides more realistic estimates of future water availability, streamflow components, and flood risks, highlighting the importance of downscaling for regional climate impact assessments.

Objective

• To investigate the effect of downscaling climate models on the hydrological projections of glacierized catchments in HMA, with a specific focus on streamflow projections, relative contribution of streamflow components, and peak flows over five study basins.
• To quantify the uncertainty of climate forcing due to GCM and downscaling technique in the HMA region.
• To determine the dependence of hydrological projections on climate models and downscaling techniques.
• To assess the impacts of climate forcing uncertainty on total streamflow, relative contribution of streamflow components, and extreme values of streamflow.

Study Configuration

• Spatial Scale: Five glacierized river basins across High Mountain Asia (Gilgit, Shigar, Marsyangdi, Budhigandaki, Punatsangchhu River Basins). Climate data downscaled to 5 km spatial resolution.
• Temporal Scale: Historic period (1990–2014), Mid-century (2041–2070), End-century (2071–2100) under SSP5-8.5 scenario. Daily scale for input data and simulations, analyzed seasonally and annually.

Methodology and Data

• Models used:
  • Hydrological Model for Distributed Systems (HYMOD_DS)
  • Statistical Downscaling Techniques:
    • Parametric Cumulative Distribution Function (CDF) matching
    • Generalized Analog Regression Downscaling (GARD)
• Data sources:
  • Global Climate Models (GCMs) (CMIP6, SSP5-8.5 scenario): CMCC-ESM2, MPI-ESM1.2-HR, MRI-ESM2.0, NorESM2-MM.
  • Reference Datasets (5 km resolution):
    • Precipitation (Pref): Blended IMERG, ERA5, CHIRPS (1990–2014).
    • Temperature (Tref): Downscaled ERA5 temperature.
  • Hydrological Model Inputs: Digital Elevation Model (SRTM), Land Use/Land Cover (GlobeLand30), Glacier Thickness (Farinotti et al., 2019).
  • Observed Streamflow Data: Water and Power Development Authority-Surface Water Hydrology Project (WAPDA-SWHP), Pakistan (for GRB and SRB); available for 2004–2010 (MRB), 1980–2015 (BRB), 2007–2022 (PRB).

Main Results

• Original GCMs exhibited significant biases: a wet bias in dominant precipitation seasons across all basins and a cold bias (−4.3 °C to −13.6 °C) over the glacier-dominated SRB.
• Both CDF matching and GARD effectively reduced biases from original GCMs, especially during high precipitation seasons. GARD performed particularly well for GRB winter precipitation (reducing bias from approximately 200% to 1.3% for MRI).
• Future precipitation is projected to increase over winter and spring for basins in Pakistan (GRB, SRB) and over summer and fall for basins in Nepal and Bhutan (MRB, BRB, PRB).
• Temperature is projected to increase across all basins and models in the future, with a higher rate towards the end-century than mid-century (e.g., GRB winter temperature increase of 4.6 °C by end-century).
• An increase in the rainfall-runoff component is anticipated across all basins in the future, while the contribution from snowmelt decreases over central and eastern basins.
• Overall water availability across the basins is projected to increase, along with extreme flows. Peak flows are expected to increase towards the end of the century, suggesting more intense snowmelt floods in western HMA and rainfall-runoff floods in central and eastern HMA.
• The choice of GCM has a more pronounced effect on the direction and magnitude of future climate trends than the downscaling method, though downscaling significantly corrects absolute magnitudes.
• Original GCMs without downscaling lead to overestimated peak flows, particularly in glacier-dominated SRB, implying unrealistic flood risks.

Contributions

• Provides a rigorous evaluation of the impacts and importance of downscaling climate forcing for assessing future projections of hydrologic response in glacierized catchments of High Mountain Asia (HMA), a region not extensively researched on this topic despite its complexity and water resource importance.
• Quantifies the uncertainty introduced by both GCMs and downscaling techniques on catchment climatology and subsequent hydrological projections (total streamflow, component contributions, and extreme flows).
• Demonstrates that while original GCMs may capture relative future climate trends, their significant biases necessitate downscaling for accurate absolute magnitudes and reliable hydrological impact assessments, especially for water resource management and flood risk planning.
• Compares two distinct statistical downscaling techniques (CDF matching and GARD) in a complex, data-scarce region, evaluating their relative benefits and limitations.

Funding

• National Aeronautics and Space Administration High Mountain Asia Program (grant #80NSSC20K1300).

Citation

@article{Nair2025effects,
  author = {Nair, Anju Vijayan and Araujo, Diogo S. A. and Gutmann, E. D. and Wi, Sungwook and Phuntshok, Jamyang and Toeb, Karma and Hassan, Mujtaba and Kayastha, Rijan Bhakta and Nikolopoulos, Efthymios I.},
  title = {On the effects of climate downscaling for projecting hydrologic response of catchments in High Mountain Asia},
  journal = {Frontiers in Water},
  year = {2025},
  doi = {10.3389/frwa.2025.1611141},
  url = {https://doi.org/10.3389/frwa.2025.1611141}
}