Understanding the hydrological aspects of the Himalayan Rivers is important for water resource planning and development. The transboundary Gandaki River originates from high altitude areas of Nepal (and few tributaries from China) and flows downstream to the Indo-Gangetic Plain before meeting with the mighty Ganges.

1. 2017-05-23
1
International Centre for Integrated Mountain Development
Kathmandu, Nepal
Water balance assessment of
the Gandaki river basin by
applying the J2000
hydrological model
Santosh Nepal, Saurav Pradhananga and
Ajay Bajracharya
3rd International Conference on “Mountain Hydrology and Meteorology for the Sustainable
Development”, Kathmandu, Nepal, 10-11 April 2017
Significance of hydrological
modelling
Environmental
changes (land use)
Climate change
(precip, temp)
Water availability and
demand
Scenarios analysis
hydrological system
dynamics
Role of snow, glacier
ground water etc.
Spatial runoff
components
Understanding
Sustainable water
resources
management
Decision making
Precip
Temp
RH, WS, SH..
Measured
Hydrological models
Precip
Temp
RH, WS, SH..
Modelled (eg.WRF)
Flood forecasting

2. 2017-05-23
2
Motivation
• Importance of the Gandaki river basin
– Hydropower development (about 350 MW)
– Future potential for hydropower (~10% so far)
– Irrigation potential (1.1M ha in India & 0.4 M ha in Nepal)
• Understanding present hydrology of the Gandaki
river basin
– Water balance assessment
– Snow and glacier melt contribution
– Spatial distribution of different variables (precipitation, ET etc)
• We used process based J2000 hydrological model and
compared the model discharge in different percentiles
Gandaki River basin

3. 2017-05-23
3
Gandaki River basin
Main Features
• High gradient and steep
topography (450-8,000 m)
• Sub-tropical to alpine climate
• Total area: 32,000 km2
Station network
• 21 meteorological and 4
climate stations
• Discharge station (Devghat)
Land cover
• Agriculture: 40
• Forest areas : 22%
• Bareland : 15
• Glacier : 7%
• Rest: 15%
J2000 hydrological model:
Process based distributed model
• Glacier melt approach
– Enhanced degree day factor
• Distributed output for each HRU
• Open source (test data Dudh K)
• Implemented in JAMS
framework
• Successfully implemented in
Nepal Himalayan (Koshi,
Gandaki, Marsyandi, Karnali)
• Micro-meso to macro scales in
the range of 18 km2 to 88,000
km2
Krause, 2001
Nepal et al. 2014 www.jams.uni-jena.de

4. 2017-05-23
4
Topography
Land-use, land- cover
Soil-types
GIS
Overlay
HRU 1
HRU 4
HRU 2 HRU 3
HRU 5
HRU 6
Reach 1 Reach 3
6059 HRUs
Reach 2
Distribution of modelling entities
Hydrological Response Unit (HRUs)
Modelling strategy
• Calibration (1999-2004) and validation period (2005-2009)
– 1998 as warming period
• Optimization of parameter
– Trail and error method based on parameter settings from previous
studies
• Nepal et al. 2014: Dudh Koshi (HydroProc)
• Nepal et al. 2016: Parameter transferability in Dudh Koshi and Tamor
• Larger Koshi (unpublished)
• Comparing results with objective functions
– Different percentile values (Q90, Q90-75, Q75-50, Q50)
• Distributed water balance assessment

5. 2017-05-23
5
Percentile
distribution
of
hydrograph
Floods
High flows
Mid range flows
Low flows
Floods
High flows
Mid range flows
Low flows
2001
Modelling results:
Calibration period (1999-2004)
Discharge(m3/sec)
15% increase in precipitation during the monsoon period (JASO)

6. 2017-05-23
6
Modelling results:
Validation period (2005-2009)
Efficiency criteria 1999-2004 2004-2009
Nash-Sutcliffe 0.94 0.95
R2 0.96 0.95
RMSE 434 360
Nepal et al. 2013, Hydro. Proce.
Discharge(m3/sec)
Comparison
of
hydrographs
for different
percentile
values
Floods (>Q90) High flow (Q75-Q90)
Mid range flow (Q60-75) Low flows (<Q50)
ObsQ (m3/sec)
SimQ(m3/sec)
ObsQ (m3/sec)
SimQ(m3/sec)
ObsQ (m3/sec)
SimQ(m3/sec)
ObsQ (m3/sec)
SimQ(m3/sec)
R2: 0.55
R2: 0.83
R2: 0.5
R2: 0.63

7. 2017-05-23
7
Precipitation (2126 mm)
21 33 44
81 192 330
542 501 297
75 6 6
Potential Evapotranspiration (867 mm)
41 51 92
108 109 88
80 77 70
64 48 39

8. 2017-05-23
8
Actual Evapotranspiration (611 mm)
20 24 40
54 82 78
77 75 66
50 27 18
Runoff (1515mm)
52%
14% 34%

9. 2017-05-23
9
Hydrological system analysis
(1986-1997)
Glacier melt contribution: 13%
• Snowmelt: 5%
• Glacier ice: 8%
Snowmelt (outside glacier): 9%
Runoff components
Overland flow (RD1) : 52%
Interflow (RD2+RG1) : 14%
Baseflow (RG2) : 34%
Overland flow dominates the
hydrograph
Water balance of the
Gandaki river basin
Icemelt
122 mm
Evapotranspiration
692 mm (33%)
Surface runoff
789 mm (52%)
Interflow
209 mm (14%)
Groundwater
518 mm (34%)
Precipitation
2126 mm

10. 2017-05-23
10
Conclusions
• High spatial variability in precipitation distribution
• The J2000 hydrological model is able to represent
hydrological dynamics for different percentiles values
– Higher confidence in low flows than high flows
• The overland flow dominates the hydrograph and singificant
contribution from baseflow
• Glacier contributes 13% of the total discharge, although pre-
monsoon contribution is significant (7-28%)
• Future plan
– Climate chagne scenarios using RCP 4.5 and 8.5 and impact on
hydrological regime and extreme
Thank You!