1. A Satellite Perspective on Jökulhlaup in Greenland
location of potential jökulhlaup lakes
and
calculation of water discharges using remote sensing techniques
This presentation is about jökulhlaup in Greenland and have to locate them and calculate the released water volumes only by remote sensing.
Morten Larsen
M.Sc. Water and Environment
Asiaq – Greenland Survey
INUTEK Nytårskur 2012

2. Table of contents Introduction Study area
Location of potential jökulhlaups lakes
Water volume assessment
Verification
The presentation contains five points:
A short introduction to jökulhlaup and our project
A little about our study area
Our method to locate potential jökulhlaup
Our method to assess water volumes from jökulhlaups
A verification of our method
INUTEK Nytårskur 2012

3. 1. Introduction What is a jökulhlaup?
                                                
1. Introduction
What is a jökulhlaup?
A jökulhlaup is a very intense flood where a water reservoir releases a large amount of water abruptly.
The water reservoir is filled by water over time and when the level reaches a critical point the reservoir is emptied.
First; what is a jökulhlaup?
A jökulhlaup is a very intense flood where a water reservoir releases a large amount of water abruptly.
The water reservoir is filled by water over time and when the level reaches a critical point the reservoir is emptied.
The water reservoirs can be under or along the glacier margin.
The lakes we work with in this project are along the glacier margin and dammed by it.
The magnitude and frequency of the jökulhlaup can vary over time.
Here is an example of a jökulhlaup lake in A.P. Olsens land in North East Greenland. The left picture is from August 11th 2009 and the right picture is from August the 12th Jökulhlaups from the lake was first time observed in The lakes empties almost every year and emptied again in Marts 2011.
GeoBasis August 11th, 2009
GeoBasis August 12th, 2009
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4. 1. Introduction The aim of the work:
                                                
1. Introduction
The aim of the work:
To develop a remote sensing method for locating potential jökulhlaup lakes
To develop a remote sensing method for assessing the water volume during a jökulhlaup.
To build up knowledge about remote sensing in Greenland
Why developing the methods?
Jökulhlaups influences the ecosystem
Jökulhlaups are a risk for human activity in downstream areas
Jökulhlaups affects the design and construction of hydro power plants
Jökulhlaup frequency and magnitude can be used in climate change research
The aim of the work:
To develop a remote sensing method for locating potential jökulhlaup lakes
To develop a remote sensing method for assessing the water volume during a jökulhlaup.
To build up knowledge about remote sensing in Greenland
The reason for building the methods only on remote sensing is that they can be used everywhere in Greenland.
Why developing the methods?
Jökulhlaups influences the ecosystem
Jökulhlaups are a risk for human activity in downstream areas
Jökulhlaups affects the design and construction of hydro power plants
Jökulhlaup frequency and magnitude can be used in climate change research
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5. Table of contents Introduction Study area
Location of potential jökulhlaups lakes
Water volume assessment
Verification
The Study area
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6. 2. Study area Study area 16.000 km2 Lake North Hydrological station
A study area of km2 in west Greenland was chosen for developing the method for locating potential jökulhlaup lakes.
The area contains at least 2 known jökulhlaup lakes. In project they were named Lake North and Lake South. Downstream the lakes Lake ISTA is sited and in the outlet a hydrological station is placed. Lake ISTA is a potential reservoir for hydro power.
Hydrological station
Maps by: KMS
Lake South
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7. Table of contents Introduction Study area
Location of potential jökulhlaups lakes
Water volume assessment
Verification
The method for locating potential jökulhlaup lakes
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8. 3. Location of potential jökulhlaups
Mapping lakes
Mapping glaciers
Mapping surface anomalies
Spatial location analysis
Potential jökulhlaup lakes
In order to locate potential jökulhlaup lakes a four-task approach was used.
In the three first tasks lakes, glaciers and surface anomalies were mapped.
In the 4th task the mapped layers were used in a spatial location analysis and potential jökulhlaup lakes were selected
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9. 3. Location of potential jökulhlaups
1st task - Mapping lakes
Tasseled cap transformation
Surface classified as water if the brightness index > 0.4 and the wetness index > 0
Multi-year observation: Time series of LANDSAT scenes from late summers
If surface is classified as water in at least 5 out of 6 years it is mapped as water
1st task - Mapping lakes:
For mapping the lakes the Tasseled cap transformation of Landsat scenes was used. The result of the Tasseled Cap transformation is 6 new bands:
Brightness
Greeness
Wetness
And three other bands
From the tasseled cap transformation we classified the surface as water if the brightness index > 0.4 and the wetness index > 0
The classification of lakes by the Tasseled Cap transformation was used in multi- year observation of Landsat scenes from late summers 2003 – 2009
The surface was classified as water if it was mapped as water in at least 5 out of the 6 years
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10. 3. Location of potential jökulhlaups
2nd task - Mapping glaciers
New image produced as average of the Tasseled cap parameters in the LANDSAT time series
Unsupervised classification with 50 spectral classes
Spectral classes thematically coded into glacier ice or non-glacier ice
The 2nd task was to map glaciers.
For mapping the glaciers a new image was produced as average of the Tasseled cap parameters in the Landsat time series. The image was produced to:
separate glacier ice from other types of ice and snow as these areas differs from year to year while the glaciers are steady
separate debris covered ice from land as the debris covered areas also differs from year to year
An unsupervised classification with 50 spectral classes was used on the new image
Spectral classes thematically coded into glacier ice or non-glacier ice
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11. 3. Location of potential jökulhlaups
3rd task - Mapping surface anomalies
The NDVI anomaly is the difference between NDVI for a given year and mean NDVI for all years in the LANDSAT time series.
The 3rd task in locating potential jökulhlaup lakes was to map surface anomalies or changes in the surface.
As the water surface area in most jökulhlaup lakes changes by the filling or when they are emptied. The surface anomalies are used to locate potential jökulhlaup lakes.
The surface anomalies were mapped by anomalies in the Normalized Difference Vegetation Index. The anomalies were found as the difference between the NDVI for a Landsat scene a mean NDVI for the Landsat timseries.
In the figure surface anomalies for two know jökulhlaup lakes and for two reference lakes are seen. As it is seen are the NDVI anomalies for the jökulhlaup lakes higher than for the reference lakes.
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12. 3. Location of potential jökulhlaups
4th task – Spatial location analysis
Area selection threshold
Number of potential jökulhlaups
No restriction
336 m2 73 m2 20 m2 5
Glacier ice
Surface anomaly
The 4th task was a spatial location analysis.
By definition jökulhlaup lakes borders the glacier margin. Therefore, to locate jökulhlaups the mapped water surfaces, the mapped glacier margin and the mapped surface anomalies were in a GIS used to select lakes with surface anomalies that borders the ice margin. These lakes were found as potential jökulhlaup lakes.
With no restriction more than 300 lakes were found in the study area. To filter out smaller lakes a surface anomaly threshold of square meters were used. Then twenty potential jökulhlaup lakes were selected.
Lake
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13. 3. Location of potential jökulhlaups
Results
Discussion
All in all the method showed good results
Ice covered lakes were difficult to classify; Two known jökulhlaups were not selected due to this problem
Further development in discriminating between glacier and lake ice is needed
Only jökulhlaup with great changes in the surface area are mapped
The twenty selected lakes are seen in this map. The two known jökulhlaup lakes; Lake North and Lake South were selected. Lake North is the lake with largest surface anomaly in the study area while Lake South is the lake with third largest surface anomaly.
All in all the method the showed good results.
Ice covered lakes were difficult to classify; 2 known jökulhlaups were not selected due to this problem
Further development in discriminating between glacier and lake ice is needed
Only jökulhlaup with great changes surface in the surface area are mapped
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14. Table of contents Introduction Study area
Location of potential jökulhlaups lakes
Water volume assessment
Verification
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15. 4. Water volume assessment
Developing a method only based on satellite images
Water volume difference:
The method:
Extract a DEM from ASTER stereo images at lowest possible water level
Find a relation between the surface area and the volume in the lake
Estimate water surface areas from a LANDSAT time series
Calculate water volumes in the lake from the relation
water volume (before jökulhlaup)
water volume (after jökulhlaup)
= water volume (jökulhlaup)
For estimating the water volumes from jökulhlaups we wanted to develop a method based only on satellite images.
The idea of the method was to estimate the water volume from a jökulhlaup as the difference between the water volume in a lake just before and just after a jökulhlaup.
This could be done by subtracting two elevations model. One from just before the jökulhlaup and right after the jökulhlaup.
Our first idea was to extract the DEMs from stereo images from the ASTER satellite. But no images were available at the right times. Therefore another approach was needed.
The method:
Extract a DEM from ASTER stereo images at lowest possible water level
Find a relation between the surface area and the volume in the lake
Estimate water surface areas from a LANDSAT time series
Calculate water volumes in the lake from the relation
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16. 4. Water volume assessment
DEM extracted for Lake North August 8th, 2004
Relation between the water surface area and the volume
The DEM was extracted from a ASTER stereo images from August the 8th 2004.
As it is seen was the lake not empty. Actually the images is almost 2 years after the last jökulhlaup. The jökulhlaup frequency from Lake North is about 9 years.
The relation between water surface area and the volume in the lake was derived by a simple script.
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17. 4. Water volume assessment
The water surface area time series
LANDSAT scenes from
The water surface areas was estimated from a LANDSAT time series from 1999 – 2010.
The graph shows the increasing surface area, the jökulhlaup and a new filling period.
The upper image shows the lake just before it was emptied in 2002.
The lower image shows the lake after it was emptied.
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18. 4. Water volume assessment
Water volumes
1,98 km3
From the water surface area time series and the area volume relation the water volumes in the lake was estimated.
In the left graph the volumes are seen as a function of time.
In the right graph the volumes are seen as a function of accumulated positive degree days. The volumes correlated very good with linear regression lines. From the regression line the volume right after the jökulhlaup was estimated.
Hereby the water volume from the jökulhlaup was estimated to 1.85 cubic kilometers.
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19. Table of contents Introduction Study area
Location of potential jökulhlaups lakes
Water volume assessment
Verification
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20. 5. Verification Hydrological station Lake North Lake South Lake North
As mentioned earlier a hydrological station is placed downstream the jökulhlaup lakes. Data from this station was used in the verification of the method for the water volume assessment.
The hydrological station was established in the 1975 and data has been collected since 1976.
Collected water levels are seen from 2002 to 2008 in the graph. In the period three jökulhlaups has occurred. Two from Lake South and one from lake North. For these jökulhlaups volume assessments have been carried out with our method and has been verified.
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21. 5. Verification ISTA water balance equation:
Qjökulhlaup * dt + Qinflow * dt = QISTA * dt + dVOLISTA
where
Qjökulhlaup is inflow from jökulhlaup [m3/s]
Qinflow is inflow from other sources [m3/s]
QISTA is discharge out of Lake ISTA [m3/s]
t is time [s]
VOLISTA is volume of water stored
in Lake ISTA [m3]
Qinflow
Qjökulhlaup
VOLISTA
For the verification a simple water balance equation for Lake ISTA was derived, where:
Where
Qjökulhlaup is inflow from jökulhlaup to Lake ISTA.
Qinflow is inflow to Lake ISTA from other sources.
QISTA is discharge out of Lake ISTA.
dt is the change in time [s]
dVOLISTA is the change in volume of water stored in Lake ISTA.
Qinflow
QISTA
Qinflow
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22. 5. Verification Estimating Qinflow :
Qjökulhlaup * dt + Qinflow * dt = QISTA * dt + dVOLISTA
Qinflow is mainly from the glacier
Qinflow
Qinflow
The inflow from the catchment area into Lake ISTA is the most uncertain part of the equation.
Qinflow is mainly from the glacier and was estimated from a simple relation between postive degree days and inflow
Qinflow
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23. 5. Verification Estimating the water volume:
Qjökulhlaup * dt + Qinflow * dt = QISTA * dt + dVOLISTA
VOLISTA
The water volume in Lake ISTA was estimated from the measured water level and a relation between the level and the volume in Lake ISTA.
The relation was derived from a DEM.
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24. 5. Verification Estimating QISTA:
Qjökulhlaup * dt + Qinflow * dt = QISTA * dt + dVOLISTA
The discharge out of Lake ISTA was estimated from stage-volume relation for Lake ISTA a the measured water level at the hydrological station.
As the water level in Lake ISTA during a jökulhlaup is about 15 m higher than the normal water level an extrapolation of the stage-volume relation was needed.
The extrapolation was established based on the recession curve for the water level in Lake ISTA after a jökulhlaup and the level-volume relation for Lake ISTA.
QISTA
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25. 5. Verification Results Discussion
The method showed reliable results for the two test lakes
Challenge to obtain ASTER stereo images immediately after a jökulhlaup
Challenge to derived reliable surface areas from LANDSAT due to ice, shadows and clouds
The method is less accurate for lakes with a steep bathymetry
Lake North
Lake South
September
2002
August
2003
2007
Water balance
QISTA·dt [m3]
1,25·109
0,06·109
0,05·109
VOLISTA[m3]
0,64·109
0,24·109
0,15·109
Qinflow ·dt [m3]
0,03·109
0,02·109
Qjökulhlaup·dt [m3]
1,85·109
0,28·109
0,18·109 RS
Vjökulhlaup[m3]
1,98·109
0,19·109
Difference
7 %
0,4 %
2 %
The water volume from three jökulhlaups has been estimated in the work.
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26. The project was funded by:
The Commission for Scientific Research in Greenland
The project was carried out by:
Eva Mätzler
Asiaq
Bo Naamansen
Morten Larsen
Christian Tøttrup
GRAS
Dorthe Petersen
Kisser Thorsøe
INUTEK Nytårskur 2012