An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

FINAL REPORT

Kratie

30 End of flood season 25 An Evaluation of Landuse Annual minimum End of dry season discharge 20 Start of dry season Start of flood season 15 and Climate Change on the

Frequency (%) Frequency 10 5 Recent Historical Regime 0 10 20 30 40 50 Week (k). k=1 of the Mekong

P.T. Adamson

INTEGRATED BASIN FLOW MANAGEMENT

Mekong River Commission

December 2006

Table of Contents

Objectives, Scope and Summary 1

PART I: Aspects of the Natural Flow Regime of the Mekong Mainstream, IBFM and Potential Landuse Impacts 5 1.1 General Overview. 5 1.2 The potential impacts of landuse change. 6 1.3 Aspects of the long term dynamics of the proposed flow indicators and hydro- biological seasons. 8 1.4 Aspects of the geography and variance of the flow seasons. 11 PART II: Aspects of Climate Change 15 2.1 Aspects of Climate Change and the Tibetan Plateau. 15 2.2 Climate Change and the SW Monsoon in the Mekong Region 16

PART III: Summary Conclusions 23

References 25

Annex 1. Metrics used to define the start and end of the four flow seasons 27

Objectives, Scope and Summary

There is no universal prescription for the maintenance of a river’s health, each hydro-ecological system has an implicit natural dynamic equilibrium that needs to be evaluated in a way that effectively results in environmental flow criteria that emerge self evidently. This requires a much more perceptive understanding of the structure and function of the hydrological regime than is commonly the case. From a hydrological perspective, three factors should initially govern the selection of environmental flow indicators:

1. Issues of scale. In a large international river basin such as the Mekong, the macro- geographic scale means that significant local impacts on flows soon become spatially imperceptible. For example, the hydrological effects of a tributary hydropower scheme tend to become screened out once the modified flows enter the mainstream. Significant impacts of this nature on the Basin scale will therefore tend to arise as a result of cumulative developments. Single interventions on the mainstream itself are a different matter, though even these have to be large scale in order to generate impacts that translate any significant distance downstream before being masked by natural tributary inflows. The impacts of landuse change also become undetectable as geographic scale increases, despite the fact that regional deforestation has been significant from the 1960s to date. Scale issues must therefore influence the type of indicator that is appropriate for environmental management. For example, an event based hydrological index such as the annual frequency and magnitude of flood hydrographs would only be appropriate for small tributary systems, where the scale is small enough for individual storm runoff events to be distinguished. In the larger tributaries and mainstream there is effectively only one annual flood hydrograph in response to the seasonal scale of the SW Monsoon. The onset and duration of this single annual flood is clearly one of the key determinants of the regional hydro ecological environment.

2. The structure of the hydrological regime. This simple single amplitude of the annual mainstream hydrograph is complemented by a highly predicable phase, which together form the definitive feature of the hydrological regime of large tropical monsoonal rivers. On the Mekong the mean annual volume of this flood hydrograph ranges from 65 km3 at the China border to 350 km3 in the Cambodian floodplain. The timing of the onset of the flood season, defined as the (usually) single upcrossing of the mean annual discharge in June, has a standard deviation of only two weeks. In other words it is highly predictable. Such temporal aspects of the hydrological regime must in turn influence the structure and function of aquatic communities and therefore would suggest themselves as key flow indicators, particularly when given their small inter annual variance, a modest man-induced change would be a significant intervention. On the other hand, volumetric flow indices would be almost meaningless in the face of the scale of the figures involved. Regulation storage could, on the other hand, affect the onset of the flood season (and its narrow temporal window) as reservoir storages refill. The nature of the hydrological regime also contributes to the physical template of rivers, such as channel form and the geomorphological processes

An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

that determine it. Appropriate hydrological indices in this respect would be linked to the characteristic duration of flows above and below critical hydraulic thresholds.

3. The major types of resource development. Over the next 20 to 30 years, the major area of water resource development is anticipated to be hydropower, the fundamental feature of which is the seasonal regulation of the hydrology, that is a reallocation of water from the wet to the dry season via reservoir storage. Since hydropower schemes are in principle non consumptive users of water , the ‘at site’ mean annual flow remains the same – unless of course the scheme involves inter-catchment diversion. The major impact is therefore an increase in the average dry season flows, an appropriate measure of which on the mainstream is the minimum discharge in each year averaged over 90 consecutive days. Statistical analyses of these data can define the structure and pattern of dry season hydrology and uncover appropriate environmental flow indicators. These would include temporal aspects such as measures of onset and duration. Other resource developments, such as water abstraction, principally for irrigation, would tend to reduce dry season flows, though the prospects are that these will merely serve to moderate the increases due to hydropower regulation.

Changes to such flow indices provide the framework for assessing the impacts of development and whether these are tolerable or not. However, the indices themselves have a natural variability and the need arises to incorporate this into the impact assessment process. Finally, before defining their natural or benchmark values, it is necessary to establish that the hydrological indices are themselves not already undergoing systematic modification in response to climate induced change, resource development or landscape conversion. Any detectable historical response of the proposed indices to man-induced change provides insights into how they may respond in the future and what the wider environmental and hydrological consequences could be.

This Report therefore sets out to inform the IBFM process with regard to these hydrological aspects , acknowledging the rather obvious fact that the biological and ecological contributions need to be made on the basis of a reasonable understanding of the key aspects of the mainstream flow regime. It is laid out in two major sections:

• The first part briefly overviews those broad aspects of the hydrological regime considered to be relevant to the IBFM process within the Mekong context. The potential impacts of landuse change on the hydrological regime are also examined, specifically with regard to the significant regional deforestation that has taken place since the 1960s. This synopsis is considered particularly pertinent to IBFM since it underscores the issues of scale that have already been referred to as well as revealing how fragmentary types of change only appear to generate localised impacts. An awareness of the long term dynamics of the specific flow indicators and hydro-biological seasons that have already been introduced to the IBFM procedures is considered to be fundamental. This historical scrutiny of the indices reveals just how persistent the timing, onset and duration of the seasons have been over the last 90 or so years as well as the fact that there is virtually no geographical variation. Also exposed is the quasi periodicity of aspects of the hydrological regime and the dynamic nature of the relationship between the hydrology and the environment that could be at risk from human intervention.

An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

• The second part focuses upon the evidence within the observed hydrometeorological data that climatically induced changes are already taking effect. No confirmation that this is so was found for the hydrology studies reported in Part I, so the focus in Part II lies principally within the regional rainfall climate.

An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

PART I: Aspects of the Natural Flow Regime of the Mekong Mainstream, IBFM and Potential Landuse Impacts

1.1 General Overview.

Five components of the hydrological regime regulate the biophysical processes and the socio- economic activities that operate within and depend upon a river system. These are, according to Richter et al. (1996):

• the magnitude of the flows;

• the frequency of flows and water levels above and below critical thresholds;

• the duration and severity of these above and below average events;

• their timing within the year and their frequency of occurrence from year to year, and;

• their rate of change or how quickly one state changes to another, which is linked to aspects of resource reliability and exposure to the risk of extreme conditions, for example.

Separating out the constituent elements of the flow regime in this way not only allows a better understanding of the physical nature and statistical structure of the process but also enables the consequences of human interventions and climate change to be considered explicitly in terms of their potential impacts on each of the components and their relationships with each other.

In the context of monitoring or forecasting potential changes to river regimes or setting environmental flows to sustain ecosystems and livelihoods there has to be some benchmark measure against which the magnitude of potential change can be measured and the tolerance of the system to change itself evaluated. Ideally this benchmark state is the natural flow regime, though the degree to which any river system anywhere remains in an entirely natural / pristine state is open to question. In a practical sense the natural or reference state refers to the hydrological regime being in some form of dynamic equilibrium with the river basin climate and landscape and that ecological functions and socio-economic activities remain unimpaired. Few river system regimes in the developed world remain entirely unaffected by human disturbance, thus the emphasis lies with sound environmental management and the implementation of recovery and rehabilitation programmes to restore their natural ecological and geomorphological integrity.

The Mekong system presents a hydrological regime that can in fact be regarded as natural, which echoes the fact that it is in long term dynamic equilibrium with the regional climate and landscape. In order to establish this though, it is necessary to demonstrate that there are no monotonic (systematic and in one direction) trends in the magnitude and pattern of the flow

An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

regime and that all hydrological variability and fluctuation at the seasonal, annual, interannual and interdecadal time scales is the result of natural hydroclimatic variability.

That the flow regime of the Mekong mainstream remains ‘undisturbed’ is not, however, an instinctive supposition in the face of, for example, regional land use and land cover change. This has primarily been in the form of tropical deforestation and forest degradation and has, according to Giril et al. (2001), been occurring at an unprecedented rate and scale, particularly from the 1960s onwards. Other more recent influences include hydropower development and the expansion of water abstractions and diversion for irrigation.

1.2 The potential impacts of landuse change

The conventional viewpoint is that deforestation results in a decrease in the natural water storage capacity of a river catchment, which in turn leads to an increase in water yield, the magnitude of which varies with the local rainfall climate, the topography and the proportion, type and density of the removed forest cover (Newson 1997). In principle therefore, there are two potential hydrological impacts of deforestation that might be distinguished:

1. total water yield is increased as annual evapo-transpiration decreases, and the

2. seasonal distribution of flows is modified as flood runoff increases and dry season flow decreases.

On a country by country basis in the Mekong region the decline in the national area prescribed as forested from 1960 onwards is indicated in Table 1. The instinctive conclusion from such figures would be that the river regime has already undergone considerable change. This would have significant implications for the selection of the benchmark criteria for defining environmental flow requirements.

Table 1. Forest cover over Yunnan and Indo-China from the 1960s to 2000. (after Stibig et al 2004)

Period Country 1960s – 1970s circa 1980 circa 1990 circa 2000 Cambodia >70%1 >70%1 67%1 53%4 Lao PDR 60%1 - 47%1 41%5 Thailand 53%2 34%2 28%2 29%4 Viet Nam 42%1 - 28%1 30%4 Burma 58%3 - 52%4 Yunnan 55%6 33%6 Notes: 1 Meyer and Panzer (1990); 2 Klankamsorn and Charuppat (1994); 3 Perrson (1974); 4 FAO (2001); 5 MRC 2003; 6 Chinese National Bureau of Statistics

Shifting cultivation is often pointed out as a leading direct cause of forest loss in tropical regions. It has been practised in the Mekong Basin as elsewhere since pre-history and there is a general consensus that as a traditional tropical land use practise, shifting cultivation does not destroy forests so long as fallow periods are long enough, some shade trees are left standing and the felled patches are small and have a low density. However, as population pressure increases these conditions cannot be sustained and since the late 1970s such activities have been

An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

increasingly viewed as unsustainable. The regionally sustainable balance between relatively small upland populations and large forests has been lost in the face of resettlement policies, migration and forest land development for commercial agriculture.

Superimposed on these increased population and commercial pressures over the last three decades has been the expansion of the regional forestry sector in response to the rapid international escalation of commercial demands for tropical timber. Pressure on the forests of Lao PDR, Cambodia and Burma was intensified in 1989, when Thailand introduced a logging ban within natural forests, and consequently sought increased imports from its neighbours. Demand from China, fuelled by the pace of national economic expansion from the early 1990s, is the latest and potentially the greatest challenge to regional tropical forest conservation.

Such rates of deforestation might be expected to produce a long term, relatively smooth and systematic trend with regard to aspects of the Mekong regime. Annual flow volumes might be expected to increase and dry season flows would be expected to decrease. Figure 1 shows the time series of the percentage deviations (anomalies) above and below the long term mean annual flows at Vientiane and Kratie over the 46 years between 1960 and 2005. The plots reveal just how difficult it would be to confidently isolate any systematic pattern in the data that could be attributed to human activity, while the appropriate statistical tests for trend (Mann Kendal, for example) indicate nothing of any significance.

40 40 % above Vientiane % above Kratie average average 30 30

20 20

10 10

0 0

-10 -10

-20 -20

-30 -30 % below % below average average -40 -40 1960 1965 1970 1975 1980 1985 1990 1995 2000 1960 1965 1970 1975 1980 1985 1990 1995 2000

Figure 1. Mekong at Vientiane and Kratie: Percentage deviations of annual flows above and below the long term mean. 1960–2005. The smooth lines are the 3 year moving average

Figure 2 shows the results for the wet and dry seasons, treated separately. Again there is no evidence of any trend – the split same means of the dry season flows are identical while sub- sample differences in mean annual flood peak are climate related.

It appears to be the case that forest removal has to be on a very large scale and attributed to clear-cutting to create agricultural land in order for there to be unequivocal impacts on the hydrology of large catchments with areas in excess of circa 1000 km2. In the Mekong region, although clear-cutting has been a factor in forest removal, it has not been on the scale that has been seen elsewhere in SE Asia, in Indonesia for example. Rather, upland forests, where they have been removed or degraded, have been replaced by fragmented landscapes consisting of remnant forest patches and various human-disturbed land covers.

An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

60 % above Vientiane average

40 40 % above Vientiane 30 average

20 20

10 0 0

-10 -20 -20

-30 % below average -40 -40 % below average -60

60 % above Kratie 50 average

40 40 % above Kratie 30 30 average

20 20

10 10

0 0

-10 -10

-20 -20

-30 -30 % below % below average average -40 -40

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Mean Annual Flood Mean Annual Dry Season Sub-Period Peak(cumecs) Sub-Period Flow (cumecs) Vientiane Kratie Vientiane Kratie 1960-1982 17 400 54 300 1960-1982 1 250 2 450 1983-2005 15 000 47 500 1983-2005 1 220 2 450

Figure 2. Mekong at Vientiane and Kratie: Percentage deviations of annual maximum flows and mean dry season flow above and below their long term mean values. 1960 – 2005. The dry season flow is defined as the annual minimum 90 day discharge

Such fragmentation results from a myriad of activities, including timber extraction, shifting agriculture, permanent cultivation, forest gathering, dwelling construction, road building, and in some cases, re-vegetation (Ziegler et al. 2004). The hydrological functionality of such fragmented landscapes is only distinguishable from that of the natural forests that they have replaced at the local scale. Claims therefore that land use changes have historically had a detectable influence upon the regime of the Mekong cannot be substantiated by data analysis.

1.3 Aspects of the long term dynamics of the proposed flow indicators and hydro-biological seasons

The proposed flow indicators and flow seasons will only be effective if changes to them imposed by human intervention can be distinguished from their natural variability. Most hydro-climatic variables have both long and short term quasi periodicities and oscillations superimposed on them and a characteristic variance from year to year. It turns out that this variance, measured - say - in terms of the annual standard deviation of the process, is quite small. As will be demonstrated, this is especially so with regard to the onset/end of the flow seasons. This narrow

An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

variance means that small changes will generally be significant statistically and potentially significant ecologically, which makes the proposed indicators efficient detective mechanisms and effective indices of hydrological modification.

The time frame over which the hydro-climatic and hydro-environmental conditions in the Mekong region have been relatively uniform would be expected to cover the late Holocene and extend back approximately 6000 years. Figure 3 gives a clear indication of the pattern, frequency and range of selected annual flow vectors over the last 80 to 90 years.

30 Vientiane

25 Vector

20 Discharge 103 cumecs annual maximum 15

10 annual discharge exceeded 25% of the time. 5 annual median annual mimimum 0 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

80 Kratie

Vector 60

Discharge 103 cumecs

40 annual maximum

annual discharge 20 exceeded 25% of the time.

annual median 0 annual minimum 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Figure 3. Mekong at Vientiane and Kratie. Timeseries plots of selected annual flow vectors. None indicates any evidence of a statistically significant monotonic trend, though the frequency pattern of the annual maximum discharge does appear to show a change-point in the early 1980s

These data reveal:

• No significant monotonic trends, but a sequence of oscillations and change points, particularly evident in the annual maxima. These periodic fluctuations are climate related and are considered in detail in the climate change studies in the context ENSO periodicities.

An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

• Evidence of a more pluvial period between the late 1930s and the early 50s. This is revealed more persuasively in the climate change studies where it is considered along with the available regional rainfall data.

Of the potential impacts of water resources development upon the regime of the Mekong, those concerning the dry season hydrology are generally recognised as likely to be amongst the most manifest and ecologically important. In this regard, the degree to which the mean level of the dry season flows is quasi periodic is particularly significant. Figure 4 confirms that these dynamic oscillations are substantial and must be an integral aspect of the natural flow regime that organizes and defines the river ecosystems and its overall environment.

1600 µ + two standard deviations Dry season discharge. Vientiane 1500 (cumecs)

1400 µ + one standard deviation

1300 µ = long-term mean = 1250 cumecs 1200

1100 µ - one standard deviation 1000

900 µ - two standard deviations

800 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

3200 Dry season discharge. Kratie 3000 (cumecs) µ + two standard deviations

2800 µ + one standard deviation 2600

2400 µ = long-term mean = 2300 cumecs 2200

2000 µ - one standard deviation

1800

1600 µ - two standard deviations

1400 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Figure 4. Historical time series of annual dry season discharges at Vientiane and Kratie, indicating the quasi periodic oscillations in the mean level of the process. These shifts in the mean can be as much as +/- 15% and the average ‘run length’ is 14 years (the red lines indicate the average discharge in a ‘run’)

The charts show a number of interesting features:

• As expected, the departures above and below the long term mean are in general accordance along the mainstream. The typical length of a ‘run’ or ‘anomaly’ is 14 years and over such a period the average dry season flow can be systematically as much as 15 to 20% from the long term value.

10 An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

• The significance of any impacts from human activity upon the dry season hydrology is not therefore simply an issue of a modification to the long term seasonal average flow. A reduction of the year to year variance of the dry season flows and a decrease in the amplitude of these mean periodicities will presumably have an influence on the longer term ecological equilibrium of the system. Doubtlessly this change will be imperceptible at first but the consequences will accumulate as the shift in the mean level becomes a permanent one as opposed to the natural cycle of temporary fluctuations.

1.4 Aspects of the geography and variance of the flow seasons.

An analysis of the historical onset and duration of the four proposed flow seasons at Vientiane and Kratie over the last 80 to 90+ years reveals two important features:

• The timing of the onset and the duration of the seasons is virtually identical at Vientiane and Kratie (Figures 5 and 6) despite the fact that the hydrology of the former is dominated by the so called Yunnan component of the Mekong regime (see Chapter 5 of the Overview of the Hydrology of the Mekong Basin, MRCS, 2005), while at Kratie the hydrological regime is largely dictated by flows entering the mainstream from the large left banks tributaries in Lao PDR, downstream of Vientiane. The system is therefore entirely homogenous with regard to these temporal aspects of its hydrology.

• The tabulated seasonal onset values in Figure 5 disclose a very narrow range, expressed in terms of their standard deviations from year to year. The point has already been made that any small change to these seasonal onsets and durations would be not only statistically significant but may potentially be of an ecological and environmental consequence that is quite disproportionate to the magnitude of the temporal shift. A delay of two weeks, or one standard deviation, to the onset of the flood season is an anticipated impact of the refilling of large hydropower reservoir storages, for example.

As Figure 7 implies, the onset and duration of these hydro-biological seasons has been remarkably consistent and unchanged over the last century and almost certainly over the last 6000+ years. These seasonal timings are arguably one of the key factors in determining the hydro-biotic equilibrium of the system and yet, potentially, they are the most exposed and susceptible aspects of the flow regime to modification from regulation and reservoir storage.

11 An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

Vientiane

30 Annual minimum discharge 25 End of dry season Start of flood season 20 End of flood season Start of dry season 15

Frequency (%) Frequency 10

0 10 20 30 40 50 Week (k). k=1

Week of occurrence Standard Annual calendar variable average deviation (weeks) Minimum discharge 14 2.1 Dry season end 21 1.9 Flood season start 25 2.2 Flood season end 45 2.1 Dry season start 47 2.4

Kratie

30 End of flood season

25 Annual minimum End of dry season discharge 20 Start of dry season Start of flood season 15

Frequency (%) Frequency 10

0 10 20 30 40 50 Week (k). k=1

Week of occurrence Standard Annual calendar variable average deviation (weeks) Minimum discharge 14 2.0 Dry season end 20 1.7 Flood season start 25 1.9 Flood season end 44 1.7 Dry season start 47 2.3

Figure 5. Mekong mainstream at Vientiane (1913 -2005) and Kratie (1924 – 2005). Historical percentage frequency of the weeks during which the annual minimum discharge and flow season transitions occurred, along with their sample means and standard deviations

12 An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

Week Number 10 20 30 40 50 100 TRANSITION SEASON 2 Vientiane Key TRANSITION SEASON 1 1 Sample probability of the 80 timing of the annual

w minimum discharge o l f

n 2 Dry/flood transition season 60 o s a 3 Flood/dry season transition e DRY SEASON s FLOOD SEASON y r

d ( k=1,52 )

40 m u

m i

n i

M Probability (%) that a flow season a flow (%) that Probability will start before the end of week k. will start the end of week before 20 1 2 3

0 Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

Week Number 10 20 30 40 50 100 TRANSITION SEASON 2 Kratie TRANSITION SEASON 1 80

o l f

o 60 s a

s DRY SEASON y FLOOD SEASON r

d ( k=1,52 )

40 u

m i

n i

Probability (%) that a flow season a flow (%) that Probability 20 will start before the end of week k. will start the end of week before 1 2 3

0 Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

Week (k) in the year before which the flow season changes with probability P( K>k) Annual minimum Start of flood End of flood Start of dry P( K>k) End of dry season discharge season season season Vient’n Kratie Vient’n Kratie Vient’n Kratie Vient’n Kratie Vient’n Kratie 10% 10 12 18 18 22 22 42 41 43 43 25% 12 13 19 19 23 23 44 43 45 45 50% 14 14 21 20 25 25 45 44 47 47 75% 16 16 23 22 26 26 47 46 49 49 90% 18 17 24 23 27 27 49 48 50 50

Figure: 6 Mekong mainstream at Vientiane (1913 -2005) and Kratie (1924 – 2005). Historical timing of the annual minimum discharge and the transitions between the flow seasons. The graph and summary table indicate the probability (P( K>k)%) that in any given year the minimum discharge and seasonal transitions will have occurred before week k ( k=1,52 )

13 An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

50 Dec.

Nov. 40 Oct. Sep. FLOOD SEASON Aug. 30 Jul.

Month Jun. 20 May Apr.

10 DRY SEASON Mar. TRANSITION SEASON 2 Feb. TRANSITION SEASON 1 Vientiane Jan. 1920 1930 1940 1950 1960 1970 1980 1990 2000

50 Dec.

Nov. 40 Oct. Sep. FLOOD SEASON Aug. 30 Jul.

Month Jun. 20 May Apr.

10 DRY SEASON Mar. TRANSITION SEASON 2 Kratie Feb. TRANSITION SEASON 1 Jan. 1930 1940 1950 1960 1970 1980 1990 2000

Figure 7. Onset of the seasons in the Mekong mainstream at Vientiane (1913–2005) and Kratie (1924–2005)

14 PART II: Aspects of Climate Change

2.1 Aspects of Climate Change and the Tibetan Plateau.

The Tibetan plateau accounts for almost a quarter of China’s area and is one that climatologists have identified as a ‘tipping point’ for the impacts of global warming. Such regions will, it is argued, show a sudden, dramatic and precipitous response to climate change. It is the headwater of rivers that flow down to half of humanity. The Yellow River and the Yangtse start in northeastern Tibet and flow across China, the Mekong originates in eastern Tibet as do the Irrawady and Salween that traverse down to Burma, Laos., Thailand, Cambodia and Vietnam The Tsang Po starts near Lake Manasarovar and travels eastwards for nearly 2000 km before cutting through the Himalaya to become the Brahmaputra and empty into the Bay of Bengal. Most of the major rivers in Nepal originate in the Tibetan plateau and cut deep gorges to flow down to the Ganga. And there is the Indus and its tributaries which also start near Lake Manasarovar and flow westwards into Pakistan and empty in the Arabian Sea.

In a warmer world, the reflective ice and snow of the Tibetan plateau will slowly turn to brown and grey as it melts and retreats to reveal the ground beneath. As the ground warms, melting will accelerate. Tibet will become a much warmer place.

100 Vientiane

Percentage of monthly 70 flows originating from Tibet and Yunnan 60

maximum 30 Jan. Feb. Mar. Apr. May June July Aug Sep. Oct. Nov. Dec. upper quartile median lower quartile 60 minimum Kratie

Percentage of monthly flows originating from 30 Tibet and Yunnan

0 Jan. Feb. Mar. Apr. May June July Aug Sep. Oct. Nov. Dec.

Figure 8. Historical contribution of flows from Tibet and Yunnan to monthly flows on the Mekong mainstream at Vientiane (1913 – 2005) and Kratie (1924 – 2005)

15 An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

For many regional rivers such as the Mekong any sustained change in the hydrological response of the plateau of Tibet is likely to have significant, if not dramatic, long term impacts on the flow regime. As Figure 8 indicates runoff from Tibet and Yunnan in China provides a dominant component of the low flow hydrology throughout the Mekong system:

• In the upper part of the Lower Mekong system, at Vientiane, the so called ‘Yunnan Component’ not only provides most of the dry season flows but in addition most of the floodwater during the majority of years. Average contributions range from over 75% during the low flow months in April and May, to over 50% during the peak flow months of July, August and September. The year to year range of the contributions is, non the less, quite wide and indicates a complex and varying nett contribution.

• Much further down the system at Kratie, the large left bank tributaries in Laos provide most of the flood season flow on the mainstream such that the ‘Yunnan Component’ is reduced to a modest 15% to 20%. However, its remains a significant source of dry season discharge, reaching a maximum average contribution in excess of 40% in April.

The implication of this key aspect of the Mekong regime is that it is not the flood season hydrology that is potentially the most vulnerable to climate change impacts, but the low flow regime. This is particularly noteworthy within the IBFM context as it is arguably the low flow regime of the system that is most exposed to modification by resource development, by reservoir regulation for example. This will tend to increase the dry season hydrology, whereas in the longer term it is at risk of decreasing as the reliability and contribution of snow and glacial melt waters declines. Initially, global warming may enhance these melt water contributions but such increases will be relatively short lived as the glaciers and snowfields retreat to higher altitudes.

There appears to be no significant evidence as yet that any such changes are manifesting themselves in the observed Mekong low flow hydrology (see Figure 3), but they may be anticipated as the Tibetan ice and snow fields retreat. The evidence is accumulating of accelerating rates of glacial and snowfield recession (WWF, 2005). On the Qinghai-Tibetan Plateau during the past 40 years or more glacial extent has shrunk by some 6600 km2 out of a total of 110 000 km2, which is significant. Presently, 95% of glacial systems are in retreat, such that the long term implications for the freshwater resources of much of Asia are immense. In a study of future water resources availability in the Sutlej River system, with is major sources in the Himalayan glacial snowfields, Singh and Bengtsson (2002) found that the impact of climate change to be more prominent on seasonal rather than annual water availability. Reduction of spring and summer meltwater would have severe implications on future regional water resources at times of the year when hydropower and irrigation demand are at their peak

2.2 Climate Change and the SW Monsoon in the Mekong Region

The Tibetan Plateau also plays a major role within in the climate system of Asia and in particular upon the timing of the SW monsoon system, through both thermal and mechanical (uplift) influences. This in turn affects global climate and global climatic change (Wu and Zhang. 1998). Consequently, any change in the thermal regime of the plateau of Tibet through

16 An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

snowmelt and glacial retreat has the potential to significantly disrupt the pattern and intensity of the monsoon itself.

Two aspects of the monsoon are relevant in the hydrological and water resources context and therefore to the BFM process, namely its onset and duration and its intensity, in terms of the seasonal runoff and river flow it generates. At the much broader regional scale Kripalani, and Kulkarni (2001) examined seasonal summer monsoon (June-September) rainfall data from 120 east Asian stations for the period from 1881 to 1998. A series of statistical tests revealed the presence of short-term variability in rainfall amounts on decadal and longer time scales, the longer ‘epochs’ of which were found to last for about three decades over India, Indo-China and China and approximately five decades over Japan. In spite of year-to-year fluctuations and the decadal variability in the rainfall records, no significant long-term trends were observed in the data. The authors concluded that the observational history of summer rainfall trends in east Asia does not support claims of intensified monsoonal conditions in this region as a result of CO2- induced global warming. As for the decadal variability inherent in the record, it ‘appears to be just a part of natural climate variations’.

These conclusions are broadly supported by studies specific to the Mekong basin, though the scale of these regionally data based analyses needs expansion and the application of more powerful and refined statistical techniques. The first of the evaluations undertaken here centres on the timing and duration of the monsoon based on criteria taken from Khademul et al. (2006). The annual onset is defined as the week receiving more than 20 mm of rainfall in 1 or 2 consecutive days, provided that the probability of at least 10 mm of rainfall occurring in the subsequent week is more than 70%. The latter component of the criterion screens out isolated storm events earlier in the year that do not fully indicate the start of ‘true’ monsoonal conditions. The date of monsoon withdrawal is defined as the day up to which at least 30 mm of rainfall accumulates over a sequential seven day period, with no subsequent rainfall for at least three consecutive weeks. Criteria such as these have found wide application across the Indian subcontinent.

Figure 9 shows the results, year by year, at four locations in Laos and Thailand that have suitably long uninterrupted daily rainfall records. Table 2 summarises the average historical date of monsoon onset and withdrawal at each one:

Table 2. Historical (1950-date) average date of onset and withdrawal of the SW Monsoon at selected locations in the Lower Mekong Basin.

Site Monsoon onset Monsoon end Day Date Day date Chiang Rai 129 9th May 311 7th Nov Khon Khaen 128 8th May 289 16th Oct Vientiane 125 5th May 284 11th Oct Khorat 128 8th May 298 25th Oct

• The average annual onset dates, according to the selected rainfall criterion, are remarkably consistent between the four sites.

17 An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

• This is not the case with respect to the average annual date of withdrawal, which indicate a regional variation in the average timing of almost a month. The most significant indication is that the termination of the Monsoon towards the north in Chiang Rai is several weeks later than elsewhere.

• This characteristic later termination at Chiang Rai is very evident from Figure 10. The difference with the other sites is quite consistent over the 50+ years of analysis, while there is also an indication of a higher variability in the date of the monsoon withdrawal in these more northern regions.

• A clear conclusion to be drawn from all four plots is that over the last 50+ years there is no indication that the timing and duration of the SW Monsoon has undergone any change at all, as part of any climate change process.

350 CHAING RAI THAILAND Dec 350 VIENTAINE Dec Nov Nov 300 300 Oct Oct Sep Sep 250 End of SW Monsoon 250 End of SW Monsoon Aug Aug Day (t) Day (t) t = 1, 365 200 Jul t = 1, 365200 Jul Jun Jun 150 May 150 May Apr Apr 100 Onset of SW Monsoon 100 Onset of SW Monsoon Mar Mar 50 Feb 50 Feb 11 year running mean of day(t) Year 11 year running mean of day(t) Jan Year Jan 0 0 1950 1960 1970 1980 1990 2000 1950 1960 1970 1980 1990 2000

350 KHORAT THAILAND Dec 350 KHOEN KAEN THAILAND Dec Nov Nov 300 Oct 300 Oct Sep Sep 250 End of SW Monsoon 250 End of SW Monsoon Aug Aug Day (t) Day (t) 200 t = 1, 365 Jul t = 1, 365 200 Jul Jun Jun 150 May 150 May Apr 100 Onset of SW Monsoon Apr 100 Onset of SW Monsoon Mar Mar 50 Feb 11 year running mean of day(t) 50 Feb Year Jan 11 year running mean of day(t) 0 Year Jan 1950 1960 1970 1980 1990 2000 0 1950 1960 1970 1980 1990 2000

Figure 9. Historical onset and withdrawal dates of the SW Monsoon at selected sites in the Lower Mekong Basin, using the rainfall criteria described in Khademul et al. (2006)

Turning to a consideration of rainfall totals during the course of the regional Monsoon, generally speaking the results reported by Kripalani, and Kulkarni (2001), as summarised above, are supported, though caution and further analysis are necessary in equal proportion.

For example, Figure 10 shows plots of the historical time series of annual rainfall totals observed at Vientiane for two periods:

1. 1950 to 2005. During this period it might be justified to conclude a decrease in annual rainfall, particularly after the late 1960s. Statistical tests would confirm this.

18 An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

2. However, if the longer, though discontinuous period of record fro 1923 to date is considered the conclusion of a decreasing trend becomes much more difficult to support. There is a strong suggestion that the wetter years between the early 1950s and late 1960s are merely part of the longer term quasi periodicity of the rainfall climate, supporting the wider regional conclusions of Kripalani, and Kulkarni (2001).

2500 annual time series 1951 - 2005 median annual total in each third 11 year moving average.

2000 Annual Rainfall (mm)

1500

1000 1950 1960 1970 1980 1990 2000 Year

2400

2200

2000

Annual 1800 Rainfall (mm) mean + / - 1 1600 standard deviation no data no data

1400 11 year running mean.

1200 No significant trend detected

1000 1920 1930 1940 1950 1960 1970 1980 1990 2000 Year

Figure 10. Annual rainfall at Vientiane, for the continuous period of record from 1950 to date (above) and for the interrupted period of record from 1923 to date (below). Statistical test indicate a decreasing trend in the shorter period of record, but long-term means over the discontinuous record (indicated by the dashed lines) are insignificantly different.

This example, serves to illustrate the caution required when apparent trends are detected in hydro meteorological time series data. Such processes almost always have multi decadal periodicities embedded within them and it is easy to confuse these with systematic trends when only a part of such long period cycles is evident in the sample used for analysis.

A second example, may on the face of it appear more indicative of a climatically induced change in rainfall regime, but once again care and circumspection are recommended. Figure 12 shows aspects of the annual rainfall regime at Korat in the upper Mun River system in Thailand (see Figure 11, below).

19 An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

Figure 11: Selected rainfall stations in NE Thailand

The plots reveal a substantial change point in the rainfall climate at Korat, centred upon the late 1960s, when:

• Annual rainfall totals decreased significantly, reducing the mean annual value between 1951 and 1970 of over 1250 mm, to around 1000 mm for the years following.

• This shift was associated with a decline in the annual number of storm days, defined as one upon which 25mm or more rainfall occurred.

• The annual maximum rainfalls over durations such as 10 days also decreased, though perhaps not as distinctly as the above.

It would be impulsive to suppose that such a result is linked in any way to climate change for several reasons:

1. The downward shift in annual rainfall at the end of the 1960s parallels the decrease which occurred at the same time for the data observed at Vientiane, where the longer term data indicates that such changes are a natural component of the long term periodicity of the regional rainfall climate.

2. The change in the number of storm days is at least as likely (as not) to be part of this same periodic process. Such aspects of rainfall climate as storm frequency and severity have not yet been considered as intensively as annual and season rainfall amounts in studies of long term climate variability and the detection of climate change. Their links with annual total rainfall are certain to be significant and mutually deterministic.

3. Rainfall in tropical monsoonal regions has a high spatial variance, such that observed conditions at a single location are not necessarily indicative of a large area. None of the

20 An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

other data investigated for NE Thailand (see their locations on Figure 11) showed such significant shifts, though overall there was a common pattern linked to periodic components intrinsic to the data and therefore the process. The strengths and weaknesses of this pattern are, however, quite variable, from point to point.

1500 20

1250 15

Annual Rainfall Annual number of 1000 (mm) storm days 10 1951-2004

750 5

annual timeseries 1951 - 2004 median annual total in each third 11 year moving average. 5 year moving average 500 0 1950 1960 1970 1980 1990 2000 1950 1960 1970 1980 1990 2000 Year

500

1 day annual maximum. 10 day annual maximum. 400 Median value in each third

Weak downward trend detected in the 1 day maxima. For the 10 day maxima the 300 downward trend is much stronger and Annual maximum significant at the 1% level ‘n-day’ storm rainfall (mm)

200

100

0 1950 1960 1970 1980 1990 2000

Figure 12. Korat. NE Thailand. Aspects of the annual rainfall climate over the last 50+ years, showing a significant decrease in annual rainfall, the number of storm days in each year and the annual maximum 10 day rainfall since the late 1960s

It seems clear therefore that unravelling the natural periodic changes in rainfall climate from the more systematic modifications that may come about as a result of global warming will be extremely difficult when the focus lies entirely with the observed hydro-meteorological data. Claims that the impacts of climate change are already manifest are often based upon modest samples of data which contain incomplete medium and longer term natural periodicities. It is also a fact that human memory is relatively short and therefore concepts of climatic ‘normality’ are based upon very short periods of experience. Understandably therefore, natural shifts with relatively high periodic frequencies are often seen as a consequence of global warming.

21 An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

22 An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

PART III: Summary Conclusions

The major conclusions of the work reported here are as follows:

• Within the context of IBFM and the processes and activities associated with protecting the regime of the Mekong and its associated ecosystem, this regime as it is at present may be regarded as ‘natural’, in the sense that on the mainstream there is as yet no significant manifestation of man induced change.

• This applies both to changes in the landscape and the hydrological consequences of water resources development. In particular, the significant regional deforestation that has taken place since the 1960s has not, as is often claimed, generated any detectable change in the flow regime. This, it is concluded, is intimately linked with issues of scale and the fact that the regional process of deforestation leaves a fragmented landscape which functions hydrologically in ways similar to natural forest.

• The hydrological flow indicators developed within the IBFM process have a natural variability, but there is no evidence to suggest that they themselves are undergoing systematic change.

• Within the observed data, both hydrological and meteorological (specifically rainfall) there is no significant evidence to support claims that climate change impacts are already manifest within the region.

23 An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

24 References

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Giri1, C., Shrestha, S. and M. Levy (2001) Assessment and Monitoring of Land Use/Land Cover Change in Continental Southeast Asia. Paper prepared for presentation at the Open Meeting of the Global Environmental Change .Research Community, Rio de Janeiro, 6-8 October. Khademul M, Islam Molla, Rahman S, Sumi A and P Banik. (2006) Empirical Mode Decomposition Analysis of Climate Changes with Special Reference to Rainfall Data. Discrete dynamics in Nature and Society 2006, 1-17.

Klankamsorn, B. and T. Charuppat (1994) Deforestation in Thailand. Royal Forest Dept. Bankok, Thailand. Kripalani, R.H. and Kulkarni, A. (2001) Monsoon rainfall variations and teleconnections over south and east Asia. International Journal of Climatology 21, 603-616.

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MRC (2003) Mekong River Commission annual Report 2003. Vientiane, Lao PDR.

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Richter B. D., Baumgartner, J. V., Powell, J. and D. P. Braun. (1996) A method for assessing hydrological alteration within ecosystems. Conservation Biology 10, 1163 -117. Singh P and L Bengtsson (2002) Hydrological Sensitivity of a Large Himalayan Basin to Climate Change. Hydrological Processes 18 (13), 2363 – 2385. Wu G. and Y. Zhang (1998) Tibetan Forcing and Timing of the Monsoon Onset over South Asia and the South China Sea. Monthly Weather Review 126 (4), 913 – 927. WWF (2005) An Overview of Glaciers, Glacier Retreat, and Subsequent impacts in Nepal, India and China. Himalayan Glacier and River Project, WWF Nepal, Kathmandu.

Ziegler, A.D., Giambelluca, T.W., T. Liem et al. (2004) Hydrological consequences of landscape fragmentation in Mountainous northern Vietnam: evidence of accelerated overland flow generation. Journal of Hydrology 287, 124–146.

25 An Evaluation of Landuse and Climate Change on the Recent Historical Regime of the Mekong

26 Annex 1. Metrics used to define the start and end of the four flow seasons

Season Start End Dry Season Average daily flow recession Twice the minimum daily (decrease) is 1% or less discharge for the current dry over 15 consecutive days, season occurs, indicating that indicative of base flow discharges have increased conditions. significantly and the low flow season is at an end. Transition Season 1 End of dry season Start of flood season Flood Season Daily discharge exceeds mean Last date upon which daily annual discharge for the first discharge falls below mean time. annual discharge. Transition Season 2 End of flood season Start of dry season