Projected changes in flood-frequency inBagmati river, Nepal
Binaya Kumar Mishra
Professor, School of Engineering
Pokhara University, Nepal

January 10, 2024

Climate change impact assessment• Climate change is one of the most critical global environmental issues,
specially for developing countries like Nepal• Nepal has high climatic variation (less than 300 mm to more than 5500
mm annual precipitation); about 80% of precipitation during JuneSeptember• Nepal is highly vulnerable to climate change and has alreadyexperienced changes in temperature and precipitation at a faster rate
than the global average• Understanding of changes in precipitation and river discharge in futureis highly important towards mitigating implications of climate change
on various sectors with focus on flood disaster2

Urban flood under climate change• Urban flooding is frequently reported as a result of anthropogenic
activities like climate change, unplanned growth and development infloodplains• Rainfall-runoff-inundation simulation is important in formulating both
structural and non-structural measures for managing flood risk andminimizing various damages• Climate change is considered one of main reasons behind increasing
flood events in Kathmandu valley• Change in precipitation extremes and peak river discharge wereanalysed in Bagmati river basin which covers Kathmandu valley,Nepal
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Precipitation projections• Dynamical Downscaling of Climate (Precipitation) Projection Data ofMRI-AGCM3.2 Output using Earth Simulator• NonHydrostaticRegional Climate Model (NHRCM)• Present climate (1980-2000); Future Climate (2080-2100)• Climate Change Scenario: SSP5-8.5 (Very high GHG emission)
Domain4Precipitation assessment

November – April: Dry months
May – October: Rainy months

197 meteorological stations with observation data for 1980-2000 period
5Monthly mean observation precipitation (Dry season)

January
March
NovemberFebruary

April

Present climate (1980 – 2000)

DecemberMonthly mean observation precipitation (Wet season)

May
July

June

AugustPresent climate (1980 – 2000)

September

October7

Comparison of monthly mean Observation and RCM precipitation
January

April
AugustMay

February

NovemberJune

Wet months

SeptemberDry months
March

December

JulyStations with more than 100% bias in monthly mean precipitation over 1980-2000

OctoberComparison of monthly mean Observation and RCM precipitation% change in monthly mean observation and RCM precipitation over 1980-2000• RCM precipitation was found relatively overestimated over Observation data
• January and February (both dry months) were found to have greater biases• Upper (high mountain) region stations were primarily found with higher biases

% change in future monthly mean precipitation with 1980-2000 and 2080-2100 periods% change in monthly mean precipitation in future

In general, wet months are projected with increasing precipitation and dry months
with decreasing precipitation in future.

Flood-Frequency Analysis

Rainfall-runoff modeling
Bagmati river basin upstream of Khokana;Catchment area is 593 km2

Khokana

District map of Nepal

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Observation and RCM daily precipitation over 1980-2000 period
Comparison for
Kathmandu Airport
station (#1030)• RCM precipitation values were found withgreater peaks.
• Precipitation with more than 100 mm were
found for 1- and 7-days for Observation andRCM series respectively.
• Highest precipitation was found to be 124.4 and198.5 for Observation and RCM series
respectively.

Observation and RCM daily precipitation over 1980-2000 period
Comparison forKhumaltar station(#1029) inside
Kathmandu valley• Again, RCM precipitation values were foundwith greater peaks.
• Precipitation with more than 100 mm were
found for 2- and 8-days for Observation andRCM series respectively.
• Highest precipitation was found to be 118 and265.9 for Observation and RCM series
respectively.

Change in RCM daily precipitation over 1980-2000 and 2080-2100 period
Comparison for Kathmandu Airport station (#1030)• Future RCM precipitation values were found with greater peaks.
• Precipitation with more than 100 mm were found for 7- and 16-days for present and future
RCM series respectively.
• Highest precipitation was found to be 198.5 and 245.9 for Present and future RCM series
respectively.

Change in RCM daily precipitation over 1980-2000 and 2080-2100 period
Comparison for Khumaltar station (#1029) inside Kathmandu valley• Future RCM precipitation values were found with greater peaks.
• Precipitation with more than 100 mm were found of 8- and 16-days for present and future
RCM series respectively.
• Highest precipitation was found to be 265.9 and 294.5 for Present and future RCM series
respectively.

HEC-HMS Basin Model

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Calibration (July – August, 2002)

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Validation (June – July, 1998)

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Validation (Sept. – Oct., 2011)19

Change in Peak river flow over 1980-2000 and 2080-2100 period

Current climate

Future climate• Greater peaks for future climate condition. Highest discharge for present and future climate
conditions was found as 1362 m3/s and 1818 m3/s respectively.

Present climate
Year Q (m3/s)
1981
1350.1
1982
479.1
1983
383.8
1984
716.6
1985
455.6
1986
541.8
1987
793.1
1988
415.4
19891362.1
1990
663.2
1991
379.6
1992
656.7
1993
460.4
1994
469.2
1995
317.2
1996
462.7
1997
713.0
1998
458.9
1999
397.7
2000
817.3

Future climate
Year Q (m3/s)
2080.0249.1
2081.01131.72082.01189.32083.0533.62084.01294.22085.0410.42086.0800.4
2087.0510.52088.0519.8
2089.0665.52090.0343.72091.0434.32092.0920.22093.01818.42094.01180.62095.01140.12096.0439.42097.0426.02098.0511.2
2099.0402.0

Flood-frequency analysis using Gumbel methodReturnperiod, Tyears
5
1025
50
100

3Discharge, m /s
Present climate Future climate
876
1077
1331
1520
1707

1120
1408
1771
2041
2308

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Conclusive remarks• Greater precipitation extremes were found for RCM precipitation output.
Thus, bias correction is important for better climate change impact
assessments.
• The increase in precipitation extremes for future climate condition
resulted larger flood risks in Kathmandu valley.

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