MODELLING THE INFLUENCE OF CLIMATE CHANGE ON TONLE SAP WETLAND
Tonle Sap wetland and the influence of climate change
Model of Tonle Sap
Tonle Sap is the largest lake in South-East Asia and is a
wetland of international importance and is recognised by the Ramsar convention.
Like most wetlands its area varies significantly through the year, from 2000 km2
at its lowest to ten times that figure at its largest. The bed of lake is close to sea level and its
maximum level is normally only 10 m above sea level. The channel from the lake
to the Mekong can flow in either direction. When levels in the lake are higher
than those in the Mekong water flows out of the lake toward the Mekong
(generally from October to April) and for the rest of the year it flows in the
opposite direction.
The following map shows three significant locations for level and/or flow measurement. Levels in the lake are recorded at Kampong Loung.
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Levels
and flows in the Mekong are measured at Kampong Cham and in the channel
connecting the lake to the Mekong at Prek Kdam.
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The next chart shows the level at Kampong Cham and in Tonle Sap Lake. Two features are worth noting:
- there is approximate synchronicity in the timing of the two sets of levels but with peaks in the Mekong generally being a bit earlier than those in Tonle Sap
- the range of levels in the Mekong, about 15 m, is higher than in the Lake, about 7 m.
As the water levels are recorded relative to local data it is not possible to know from this graph the relative levels between the two measuring locations.
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| Figure 4 - Water levels in Tonle Sap Lake and the Mekong at Kampong Cham |
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The above data sets, levels in the lake and the Mekong and
flow via the Tonle Sap channel give us many of the important elements for a
model of Tonle Sap. However there are a
number of other important factors. These are:
- Flow into the lake from surrounding rivers
- The relationship between depth, volume and area of the lake
- Precipitation on the lake
- Evaporation from the lake
There are four usable records of flow into the
lake. There are shown on the following map and comprise he flows records at
Battombong (Stung Sangker), Kampong Kdey (Stung Chikriang), Kampong Chen (Stung
Staung) and Pursat (Stung Sen). There are other level records but they do not
have an accurate rating curve linking levels and flows.
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The curves on the chart were fitted using Excel. In the case
of the flood area the relationship is:
Area = 30.061(Level)2
+ 1094.1(Level) + 716.69
Where the surface area of the lake is in square kilometres
and level is in metres.
The equivalent relationship for volume is:
Volume = 0.914(Level)1.883
...where the volume of the lake is in cubic kilometres.
The final elements for a model of the lake, rainfall and
evaporation were based on average values taken from climate stations around the
lake. For each daily time step the volume of rainfall and evaporation were
based on the amount in millimetres multiplied by the area of the lake. The
model also included a further loss mechanism. As Tonle Sap contracts in size
during the October to April period water evaporates from the exposed soil
which, given there is little rain in this period, becomes very dry. When, later, Tonle Sap again expands the
water flows from the lake over land which has been dry for, in some cases,
several months. This water then sinks into the voids in the soil. The model
applied this loss cumulatively. If the area of Tonle Sap was expanding the loss
was equivalent to the total evaporation during the period of expansion at that
point in time. The maximum loss by this mechanism was 200 mm. Once the lake
started contracting then the loss from this component was set to zero.
The basic formula for the lake model was:
Volume[t+1] = Volume [t] + Inflows –
Outflows
The model operated on a daily time step and was developed as
an Excel file.
The inflows were: flow from local rivers, flow via the Tonle
Sap channel and rainfall on the lake. During calibration it was found the
estimate of local inflows based on the above method (adjusted in proportion to
the drainage area) overestimated the inflow by a factor of two. A preliminary
analysis suggested two reasons for this. One is that the area used refers to
the area of the Tonle Sap ecosystem which might be larger than the drainage
area upstream of the level measuring point. The second is that the gauging
stations receives water from the upland parts of the drainage basin and
therefore exaggerates the average runoff. Since the contribution from the
surrounding rivers is small compared to the contribution from the Mekong
through the Tonle Sap channel this parameter is not of great importance to the
overall accuracy of the model.
The flow via the Tonle Sap channel was based on the
following equation:
Flow = a * (Mekong level – Tonle Sap level – b)c
If the flows were toward to the lake then this formula was
used as above. If it was toward the Mekong then it adjusted by a further factor
d.
The values of the four parameters a, b, c and d were obtained
by using the ‘Solver’ add-in of Excel. ‘Solver’ adjusts each of the four
parameters to see how they change the accuracy of the model. In this case the
accuracy of the model is defined as the sum of the squares of the errors in the
estimation of the levels in Tonle Sap Lake.
The outcome of Solver optimisation process is that the
formula became:
Flow = 1126 * (Mekong level – Tonle Sap level – 3.98)1.32
The value of ‘d’, relating to the direction of flow, was 0.59. In reality this parameter is compensating
for some hydraulic factors not included in this model. A full solution of the
equations would take account of the inertia of the water in the Tonle Sap
channel; in simple terms when the relative levels in the lake and the Mekong
change they first have to stop the river flowing in one direction before they
can increase its flow in the opposite direction.
The values of parameter ‘b’, 3.98 m, which allows for the
difference in the datum at Kampong Cham and at Prek Kdam is compatible with Figure
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of water levels at the two sites above.
Another factor not
included this model is the time delay between changes in relative water levels.
To have included a hydraulic model would have required information on the
channel shape and dimensions and a whole project on its own.
The next chart shows the simulated and observed flow of the
Tonle Sap channel.
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At first sight this does not look encouraging. In particular
the peak inflows are not well represented.
However examination of the current meter gaugings carried
out in 2008 to 2010 suggests a reason. The following chart is for the ‘out’
period only; that for flows in the other direction is very similar. It shows
that for mid-range levels there is a reasonably consistent relationship between
flows and levels but at low levels and high levels, when the flow direction is
changing, the relationship is unclear. In the case of the following chart
levels from 6 to 8 metres can be associated with either increasing flow, as in
2009, or falling flow, as in 2010. The flow associated at that level can also
vary from 1,000 m3/s to almost 20,000 m3/s.
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This suggests that the calculation of actual flow values at Prek Kdam might not be consistent and that to simulate them might be as much a case of simulating peculiarities of the flow calculation as of representing the underlying flow patterns.
It should also be noted that the simulation of flows in the Tonle Sap channel is not an end in itself. The overall objective is to simulate water levels in Tonle Sap Lake. The following chart shows that very simulation.
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As can be seen the simulation is generally accurate. Many of
the peaks of water level are slightly underestimated but otherwise it is good.
The correlation between observed and simulated levels is 0.967.
It can therefore be concluded that the simulation of water
levels in Tonle Sap Lake is sufficiently accurate for the model of lake levels
to be used to study flooding around the lake.
Projected levels
In a separate part of the project flows of rivers within
Cambodia and the whole of the Mekong were
simulated using the HYSIM rainfall/runoff model (http://www.watres.com/software/HYSIM/).
The hindcast values of all climate
models reported on in the IPCC Technical Assessment Report of 2012 were
analysed and the models rated on 4 factors: representation of observed monthly temperature,
representation of observed monthly rainfall, representation of monthly temperature
anomaly, representation of monthly precipitation anomaly. It was concluded that
the MIROC model was most
appropriate for Cambodia.
Using the calibrated hydrological model and the climate projections,
flows were projected for a 30 year period centred on 2045.
The following chart shows the observed (or more strictly,
simulated using observed meteorological data) and projected levels.
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Figure 11 - Observed and projected
water levels in Tonle Sap Lake
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The following table shows the change in the level of Tonle
Sap Lake for different return periods.
Return period
(years)
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Current conditions
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Projected 2045
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1-in-2
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8.60
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9.43
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1-in-5
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9.20
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10.13
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1-in-10
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9.60
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10.60
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1-in-25
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10.11
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11.18
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1-in-100
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10.48
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11.62
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1-in-100
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10.85
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12.05
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Acknowledgement
The work described above was performed while the author was
working for SweRoad under a contract providing support to the Ministry of Rural
Development of Cambodia, financed by the Nordic Development Fund and supervised
by the Asian Development Bank. Any views expressed or those of the author and
do not necessarily represent those of the other parties.
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