Transcript Lamsal, K. - Third Pole Environment (TPE)

ESTIMATION OF SNOW AND ICE MELT CONTROBUTION IN
DISCHARGE IN GLACERISED LANGTANG KHOLA BASIN
Third Pole Environment Workshop, 26-28th October, Kathmandu
Krishna Lamsal and Rijan Bhakta Kayastha
[email protected], [email protected]
Introduction and Overview of the Problem
More than 3200 glaciers
Major rivers are glacier fed
Sixty-seven percent of glaciers are retreating at a startling rate in the
Himalayas and the major causal factor has been identified as climate
change (Ageta and Kadota, 1992; Yamada et al., 1996; Fushinmi, 2000).
 Glaciers ‘mother’ several rivers and streams with melt runoff. A
significant portion of the low flow contribution of Himalayan rivers
during the dry season is from snow and glaciers melt in the Himalayan
region.
 The Himalayan Rivers are expected to be very vulnerable to climate
change because snow and glacier meltwater make a substantial
contribution to their runoff (Singh, 1998).
 The magnitudes of snowmelt floods are determined by the volume of
snow, the rate at which the snow melts and the amount of rain that falls
during the melt period (IPCC, 1996b).
 In Nepal, there are 3,252 glaciers that cover 5,323 sq.km in area with
estimated ice reserve of 481 Km3. Koshi River Basin alone comprises
779 glaciers with an area of 1,409.84 sq.km and has an estimated ice
reserve of 152.06 cu.km (Mool et al., 2001).
 The contribution of snow and glacial melt to the major rivers in the
region ranges from less than 5% to more than 45% of the average flow
(Jianchu et al., 2008).
 The contribution of Nepalese rivers to the Ganges is about 41% to the
total runoff, and 71% of its lean season flow (Alfred, 1992).
 A comprehensive analysis of trends in river flow has not been performed yet in
Nepal. However a preliminary analysis of the river discharge i.e. trends in
large outlet rivers, southern rivers and snow fed rivers have been carried out.
 Gurung (1997) has revealed that there will be decrease in runoff in dry seasons
and increase in runoff in monsoon season under the doubled CO2-scenario
using the Canadian Climate Centre Model (CCCM) and Geophysical Fluid
Dynamics Laboratory (GFDL) models.
 The annual runoff of Kali Gandaki River in Nepal Himalayas increased by
about 1% annually for 1964-2000 (Shrestha, 2005).
 The contribution of snowmelt in stream flow is found as 17.5% in winter, 13%
in summer and 13% in annual flow in the average from 2002 to 2003
(Shilpakar et al.,2008). The study demonstrates that the impact of climate
change to stream flow is significant, which is in increasing trend resulting from
snow melt contribution.
Studies in Langtang Khola Basin
A conceptual runoff model called HYCY-MODEL
Fukushima et al. (1991)
Braun et al.(1993)
Rana et al. (1996)
Chaulagain (2007) focused on socio-economic dimension
Contribution of snow and Ice melt in stream flow?
This research is the study of Langtang Khola basin in view of the contribution
of snow melt in stream flow and impact of climate change on water resource in
view of runoff sensitivity
Objectives and Methodological Outline
The objective of the present study is the discharge estimation from the glacierized
Langtang Khola Basin.
Specific Objectives:
 To estimate the contribution of snow and ice melt in the discharge from
Langtang Khola.
 To determine the sensitivity of the river discharge in response to the
temperature change.
 To determine the change parameters of climate through the analysis of
hydrological and meteorological information of Langtang
Methodology contd………
Positive Degree Day Method
 This method is based on the assumption that the melting of snow or ice during that
period is proportional to the sum of the daily mean temperature above the melting
point during that period, and the sum is called the positive degree day (PDD) sum.
 The factor linking to this temperature sum is called the positive degree day factor
(k).
 The degree day factor involves a simplification of the complex processes that are
properly described by the energy balance of the glacier surface and overlaying
atmospheric boundary layer (Braithwaite and Olesen 1989). This is because the
factors that determine the melt processes are correlated with the temperatures.
 In this study, the classical degree day method is used to estimate snow and ice melt,
but the PDDs are calculated from monthly mean air temperature using the concept
of normal distribution (Braithwaite 1985).
Methodology contd………
Input data
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Monthly mean Temperature
Monthly Precipitation
Debris properties
Thermal resistance
Critical thermal resistance
Degree-day factors:
Snow
7.0
10.5
Ice (mm d-1 oC-1)
8.0 (4000-5000 m)
9.5 (> 5000 m)
Methodology contd………
 The energy available at the top of debris layer is used to melt ice under the debris
and assumed that no energy loss while transferring energy from the top surface to
debris-ice interface and all energy is used for melting ice.
 Calculated value of kd/kb is 0.54. Two values of kd/kb are used.
kd/kb value
0.50
0.58
Altitude band
4125m - 4375m asl
>4375 m asl
Melting from ice/snow from glacier and rocky areas
= Monthly PDD × Positive Degree Day factor for snow or ice ablation.
Methodology contd………
Precipitation was assumed as a function of altitude (Seko, 1987) as fallows
PZ
= PBH
when Z<4000m
= PBH[1+0.0003 (Z - 4000)]
when Z= 4000m to 5000m
=1.3 PBH
when Z> 5000m
Methodology contd………
100
Snowfall (%)….
80
60
40
20
0
0
1
2
3
4
Monthly mean air temperature (oC)
Calculated monthly snowfall amount in precipitation versus monthly air
temperature on glacier A0X10 (after Kayastha et al., 2000)
5
Methodology contd………
Discharge
= Melting from bare ice/snow + Snowmelt above debris & rock+ Ice melting
under debris +Rainfall
The snow and ice melt are calculated in every 250 m altitude bands by dividing the drainage basin
of 500 m as shown in Fukushima et al., 1987
Map of the Langtang Khola Catchment with Gauging Station (cyan) and Climate Station Kyangjing (green).
Main Physical Characteristics of the Investigated Basin
Name of the basin
Langtang Khola
Name of the area
Langtang Valley
Mountain range
Himalayas
Elevation range of the basin (m a.s.l.) 3840-7234
Latitude/Longitude
28008’-28023’N/85035’-85048’E
Area (km2)
333
Glaciers and permanent snow(%)
38
Dominant vegetation type
No vegetation
Forest(%)
-
 Runoff Modeling of Langtang Khola @ Kyangjing
Result and Discussion contd……..
Snow/Ice melt Contribution in Discharge
20
18
Rain
16
Snow and Ice
Discharge
Discharge(m3/s)
14
12
10
8
6
4
2
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sept
Oct
Nov
Dec
Result and Discussion contd……..
 Considerable contribution
 12.32% contribution in Winter Season
 10.00% in Pre Monsoon Season
 9.56% in Post Monsoon Season
 13.59% in the average annual stream flow
 Continuous fed by the snow and ice melt
 Contribution more in the low flow season
Result and Discussion contd……..
Sensitivity Analysis
S. No.
Change Scenarios
% change in
Annual runoff
1
T+2, P+10%
+1.02%
2
T+4, P+10%
+3.4%
3
T+5, P+20%
+7.2%
4
T+2, P-10%
-2.85%
5
T0, P-10%
-2.6%
6
T-1, P-10%
-0.8%
The subscriptions7P and T denote the changes in temperature
the plus and
T-2, P-20%(°C) and precipitation (%) and
-4.7%
minus sign indicates the increment and decrement in them.
Result and Discussion contd……..
Time series analysis of Hydrological and
Meteorological data
Annual mean temperature, total precipitation, discharge in the Langtang Station.
CONCLUSIONS
 Snow/ice melt has a significant contribution in the river
discharge of glacier fed Langtang Khola, with
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12.32% contribution winter season,
10% in Pre monsoon season,
9.56% in Post monsoon season and
13.59% in the average annual stream flow.
Thank you