MEKONG RIVER COMMISSION

Transcription

1 Attachment to Agenda E REPORT ON THE HYDROLOGICAL CONDITIONS IN THE LOWER MEKONG BASIN FROM 15 th JANUARY TO 31 st JULY, 2011 FOR THE 16 th DIALOGUE MEETING By Khem Sothea MEKONG RIVER COMMISSION 1

2 Table of the content 1. Introduction 2. Meteorological and Hydrological Conditions in the Upper and Lower Mekong Basin in Data used for Analysis 2.2 The Upper Mekong /Lancang River at Yunnan Province, Regional rainfall and the Normalized Difference Vegetative Index (NDVI) of the lower Mekong Basin form January to July, Selected rainfall sites, compared to long-term average condition 3. Regional soil moisture conditions January to June, Hydrological Condition of River flow and water level in the Lower Mekong Basin for January to July Flood Season of 2010 and Onset of flood season in Probability Analysis on low flow at the selected stations in 2011 from January to June 4.3 Mainstream flows 5. Analysis on the Impact of the Dams in China on Mainstream Hydrology 6. The Flow Regime of the Tonle Sap System and its floodplain in Conclusions 2

3 1. Introduction In response to a request from the Twenty-second Meeting of the Joint Committee in August 2005 to the MRC Secretariat to continue the routine monitoring of the hydrological conditions in the Mekong Basin, this document was prepared as a bi-annual report on the current hydrological situation in the Lower Mekong Basin (LMB). This report is prepared for submission to the 34 th MRC Join Commission (JC) and Dialogue meetings in August 29, The report covers the hydrological situation within the Lower Mekong Basin (LMB) from the 15 th January to 31 st July In general dry season conditions in both meteorological and hydrological aspects were following the longterm average which would be considered a normal dry season year. However, the inflow /reverse of the Tonle-Sap Lake at Prekdam were considered seasonally as low flow. In order to monitor and analyse the interconnected factors of hydrological condition in the LMB in more detail and distinguish them from the natural variations in discharge and water level caused by storm rainfall it is recommended that the provision of hydro-meteorological data from the upper Mekong in China be extended to the low flow season in addition to the current provision during the flood season only. This Report considers the general hydrological conditions in the Lower Mekong region between the 15 th January and 31 st July 2011, which cover a complete dry season and the starting onset of flooding season. The justification is three folds:- 1) The hydrological preconditions do not appear to have had any ongoing effect upon the 2011 low flow season, at least until mid February, a period during which discharges and water levels have been more or less average or just slightly below average throughout the region. 2) The impact upon the hydrological regime of the mainstream reservoirs in Yunnan continues to manifest itself in the northern parts of the Lower Basin, where the hydrological regime illustrates a higher frequency of unnatural variability. These impacts are becoming equally evident during the flood season as they are during the low flow season between October and May. 3) The annual hydrological data observed on the Mekong mainstream at Kratie may be considered to provide the benchmark for describing the overall regional hydrological situation in any given year. At that river cross-section more than 90% of the total average flow of the Basin has entered the system therefore the conditions at Kratie can be regarded as the integral and summary of those upstream parts of the Basin which generate virtually all of the runoff. Conditions at Kratie also define those further downstream across the Cambodian flood plain, within the Tonle Sap system and in the delta in Viet Nam, since apart from the Tonle Sap only small amounts of water is added further downstream. 3

4 Flow and water level conditions subsequent to the 2010 flood season from October onwards and into 2011 remain average at best. Soil moisture displayed low values in the LMB beginning from January to March (0 to 20%), corresponding to the significant low amounts of rainfall in LMB. But from April to July 2011, following monsoon rainfalls moisture levels across the region gradually rose to more than 20% in May and 90% in July. Some improvement is evident in late June as a result of more rainfall in some parts of the LMB, which increased moisture levels to above 90% within the eastern highland margins of the basin in Lao PDR and the southern part of Cambodia and the Mekong Delta. The fact that flows in the latter months of 2010 and in early 2011 do not reflect the severity of the draught of 2010 might be explained by the fact that low flows at Kratie and further downstream are dominated by the contributions from China and the northern regions of the Lower Basin rather than those of the southern tributary systems. This aspect of the regional hydrology merits further study and analysis. In addition, the early weeks of the monsoon up to the end of July did produce the kind of intense and sustained storm rainfall that would have generated significant flood runoff. Although in parts of the Basin rainfall conditions subsequently improved such that seasonal totals recovered to more or less average levels, in other areas this was not the case. This explains why soil moisture conditions in southern Lao PDR and northern Cambodia at the end of May were increased leading to considerable wet conditions, which is clearly evident from the satellite imagery. 2. Meteorological and Hydrological Conditions in the Lower Mekong Basin till July Data used for analysis The hydrological condition is analyzed based on data availability such as: rainfall, water level, stream flow, soil moisture, and vegetation index. These data were collected from various sources (MRCS, TMD, and USDA) via operational daily data and the internet facilities from January to July Figure 1 shows some selected hydro-metrological stations in the entire Mekong/Lancang River Basin. 2.2 The Upper Mekong /Lancang River at Yunnan Province, 2011 In the upper Mekong Basin, daily water level and rainfall data at Jinghong and ManAn stations, where MRCS installed telemetry systems, were obtained. Figure 2 shows the observed daily water level for June and July 2011, compared to its long term average ( ), and rainfall for the same period. The hydrograph at Jinghong was clearly below average between June and July, whereas in ManAn the actual hydrograph is fluctuating around its long term average condition. Average rainfall from the two stations showed the highest value in early July, preceded by significant rainfall events starting from middle of June

5 Fig. 1 Observed hydro-metrological stations over the Mekong/Lancang River Basin 5

6 Fig. 2 Daily Water Levels hydrograph in 2011 compared with long-term average ( ) against average rainfall at Jinghong and ManAn stations of the Upper Mekong Basin in Yunnan province, China. Fig. 2 The daily water level and rainfall conditions at upper part in Yunnan Province, China at Jinghong and ManAn stations 2.3 Regional rainfall and the Normalized Difference Vegetative Index (NDVI) of the Lower Mekong Basin form January to July, 2011 The rainfall distribution from January to July 2011 over the Lower Mekong Basin is available from the internet domain of USDA at precipitation analysis was based on two sources: 1. Sixty rain gauge stations from the national line agencies and 2. Daily precipitation is estimated by the Air Force Weather Agency (AFWA) and available at: These precipitation data from (AFWA) were calculated based on four different data sources: - Special Sensor Microwave /Imager satellite (SSM/I) - Geostationary satellites such as GOES, Meteosat, and GMS. 6

7 - Real Time analysis Cloud Model (RTNEPH) - WMO ground station data. Decadal precipitation is then calculated for each grid cell by adding the ten daily precipitation records. Figure 3 shows the daily precipitation over Mekong Basin from January to July 2011, based on AFWA data. It is shown that the precipitations from May to July in 2011 were generally varied from 1 mm to higher than 200 mm. The low soil moisture in the preceding period might be a factor contributing to drought condition in the succeeding period, particularly by looking at crop water stress condition. A state of the art technology makes it possible by using satellite data to study the seasonal cycle of vegetation growth, which includes leaf development, plant maturity, die back and winter dormancy. Each of these phases is of course linked to biological processes. The phase-wise water consumption by the plants can be monitored as a function of the plants reflective and absorptive characteristics in the electromagnetic spectrum. In this regard, there exists a commonly used vegetation indices known as the Normalized Difference Vegetative Index (NDVI), which enables the crop water stress to be monitored. Figure 4 shows images of NDVI for specific periods in March and July 2011 for SPOT-VEG NDVI and SPOT-VEG departure from short-term average, compared to SPOT-VEG NDVI Departure from the previous year (2010). The images show clearly that the vegetation in the agricultural areas (low lying and flat land) of LMB has gradually improved to be fairly dense in March and fully dense in July 2011 (0.10 to 1.00), compared to the drought year in 2010 (-1 to 0.1) at the same period. The vegetation in the agricultural areas of LMB was improved to be fully dense in July

8 A. Precipitation from January to April, 2011 B. Precipitation from May to July, 2011 Fig. 3 The daily precipitation covering the Mekong Watershed 8

9 SPOT-VEG NDNV, NDNVI 2011 SPOT-VEG NDVI Departure from last year 2010 Mar. 1-10, 2010 Fig. 4 Normalized Difference Vegetation Index (SPOT-4), available at (Source: 9

10 2.4 Selected rainfall sites, compared to long-term average condition In Figure 5 the comparison between monthly observed rainfall from January to July in 2011 and 2010 is shown and also compared with its long-term monthly average rainfall ( ) from selected and representative stations in LMB, namely at Chiang Saen, Luang Prabang, Vientiane, Mukdahan, Pakse, Phnom Penh. Evaluating the presented results in Fig. 5, the following can be concluded about the distribution of rainfall over the LMB: In the upper LMB, the accumulated rainfall from January to April 2011 at Chiang Saen, Luang Prabang, and Vientiane varied from 0 mm to 25 mm which are considered lower than long-term average ( ). However from April to July 2011, accumulated rainfall values were sharply increased from 30 mm to 800 mm which indicated higher than long-term average (35mm-760 mm). In the middle LMB: the March to July rainfall in 2011 at Pakse has significantly dropped lower than its long-term average in ( ). In general, rainfall over the Lower Mekong Basin from March to April shows more or less about its long-term average, whereas from May to July it showed the increasing rainfall due to the impacts of monsoon rainfall. 10

11 Fig 5: Accumulated rainfall pattern from January to July 2011 at selected stations: Chiang Saen, Luang Prabang, Vientiane, Mukdahan, Pakse, Phnom Penh, and within the Lower Mekong Basin, compared with its long-term monthly average rainfall ( ) 11

12 3. Regional soil moisture conditions January to July, 2011 In Figure 6 the mapped status of the regional soil moisture conditions over the Lower Mekong Basin from January to July 2011 are presented. The following description outlines the typical pattern 1 : From January to March: the general picture shows very low levels of soil moisture (0 to 10%), corresponding to the significant low amounts of rainfall in LMB. There are large pockets of significantly low residual moisture (less than 10%) in the whole LMB mostly in March, which indicates the driest month. From April to June: following the monsoon rainfall gradually the moisture levels increase to little higher levels across the region starting from 20% in April to 50% in May, and in June the soil moisture is up to 90%. Some improvement is evident in late June as a result of higher rainfall conditions in some parts of the LMB, which increased moisture levels to above 90% within the eastern highland margins of the Basin in Lao PDR and southern part in Cambodia and the Mekong Delta. This indicates the onset of the Monsoon, which increased soil moisture levels over 90% for a much wider proportion of the Basin. Figure 6: Regional soil moisture conditions between January and July, (Source: 12

13 4. Hydrological Condition of River flow and water level in the Lower Mekong Basin for January to July Flood Season of 2010 and Onset of flood season in 2011 The general weakness of the 2010 SW Monsoon meant that flows during the flood season were well below normal, particularly in these northern parts of the Mekong basin. In Figure 7 the indicated low trend of the flood season in 2010 is shown. The onset of the flood season is the date of the up-crossing of the long term mean annual discharge (or water level) and the end, the down-crossing. In a typical year there is only one such crossing in either direction. It would generally be expected therefore that due to the natural catchment storage in particular in northern Lao, the significantly below normal flows at the end of the wet season would be followed during the subsequent dry season with flows which are also below the seasonal average. Not only were discharges low but also the flood season ended almost two months early at Chiang Saen, reflecting early Monsoon withdrawal. This early withdrawal of the Monsoon brought about an early end to the flood season in the Lower Basin by as much as five weeks. It also, as indicated above, led to very low levels of natural catchment storage to sustain flows during the dry season and the early onset of the flood recession leading in turn to very low tributary flows by January Fig. 7 Low trend of flood season in 2010 at Kratie Station 13

14 The onset of the flood season on the Mekong mainstream in 2011 occurred at the usual time during the latter half of June or in the Delta later in July as shown in Table 1 (Adamson et al 2010). Table 1: Onset of the flood season in LMB Site Onset of flood season Historical average Standard deviation 2011 Chiang Saen 28 th June 13 days 15 th June Vientiane 3 rd July 14 days 28 th June Pakse 29 th June 16 days 28 th June Kratie 1 st July 16 days 22 nd June Phnom Penh 10 th July 14 days 30 th June Chau Doc 23 rd July 17 days 13 th July 4.2 Probability Analysis on low flow at the selected stations in 2011 from January to June Based on the outcome of the twenty-eighth Meeting of the Joint Committee, further detailed hydrological analysis was requested to be added to this report. This includes risk analysis and data preparation for the development of the Drought Management Programme. In response, the analysis of mainstream flow based on probability was also taken into account. The analysis is based on daily recorded water levels, using lognormal probability density function method to assess and understand the periodically low water level conditions. This can provide the information of low water level when exceedance of the actual level goes below a certain probability of exeedance (expressed in annual recurrence interval; or simple once in so many years) at each selected station. This helps significantly to evaluate the drought of The Annual Recurrence Interval (ARI) is used to express the low flow condition. The statistics to determine the probability is considered on the use of Annual Recurrence Interval (ARI), e.g. 1:5 year (equivalent to 20 times in 100 years) means 80% probability of exceedence of daily flow. Additionally, there are some explanatory notes in the graph as follows: The Average is equal to a probability of P = 50% (P 50) 1:4 years means P = 75% (P 75) probability (25 times in 100 years) 1:5 year means P = 80% (P 80) probability (graph indicates this significantly-low flow pattern) 14

15 1:10 year it means P = 90% (P 90) probability ( or only 10 times in 100 years) 1: 20 year it mean P = 95% (P 95) probability (5 times in 100 yeas; graph indicates also this extremely-low flow pattern) In Figure 8a and 8b the actual values of daily water level in 2011 from January to June are illustrated, and compared to 50%, 80% and 95% probability of exceedance at Chiang Sean, Luang Prabang, Vientiane and Kratie Stations. The 95% probability of exceedance graph can be adopted as an extremely-low flow pattern. In the Mekong Basin and along the mainstream there are a range of environments and water demands that respond differently to the various and complex elements of a drought event. It is shown that water level hydrographs at all selected stations from January to March 2010 were considered more or less within the characteristic range of P 50 and P 95, which indicated low flow conditions. The rapid fall in water levels from January to March, as shown by evaluating the data, is not a common natural feature of the dry season recession flows and suggests reduced releases from reservoirs upstream of Chiang Saen in response to the very low levels of storage that have been reported by Chinese news agencies. However, from April to June the increased water levels at each station truly reflect the regional rainfall contribution over the LMB and are shown in the following figures. Figure 8a: Daily water level at Chiang Sean and Luang Prabang on the Mekong mainstream for the period 1 st January to June, 2011 compared to their historical average and characteristic range 15

16 Figure 8b: Daily water levels at Vientiane and Kratie for the period 1 st January to 30 th their average and characteristic range. June, 2011 compared to In terms of their risk of occurrence deficient annual flood conditions comparable to those of 2010 are in fact not that rare. In Figure 9 the characteristic, wide ranging bi-variate distribution of flood season peak and volume at Kratie are shown in a probabilistic framework. Four comparable events occurred in 1988, 1992, 1998, and 2004, giving an estimated recurrence interval of such conditions between once in ten and twenty years according to this type of statistical analysis. It was shown that the 2010 flood at Kratie was the lowest on record (1992) in terms of flood volume and amongst the lowest ever in terms of maximum discharge. According to Adamson 2011, during the low flow season the contribution of flows from China increases in importance as the Lower Basin tributaries with their limited aquifers contribute proportionally less and less to the mainstream low flow regime. During the first six weeks of 2011 this Yunnan Contribution amounted to 35% of the average discharge, which is the average figure. Later in the season, in April, this proportion increased to 45%. 16

17 Fig. 9: Scatter plot of the joint distribution of the annual maximum flood discharge (cumecs) and the volume of the annual flood hydrograph (km 3 ) at Kratie on the Mekong mainstream. The boxes indicate one ( 1δ ) and two ( 2δ ) standard deviations for each variable above and below their respective means. Events outside of the 1δ box might be defined as significant flood or drought years and those outside of the 2δ box as historically extreme flood or drought years. 4.3 Mainstream flows Confirmation of the severity of the regional hydrological conditions is provided by an analysis of the flows in 2011 from January to July. The discharges at Chiang Sean, Vientiane and Kratie, compared to it s longterm average plus/minus 1 standard deviation range are shown in Figure

18 Fig. 10 Daily discharges for the period 1 st Jan to 31 July, 2011 compared to their -term average plus/minus 1 standard deviation range 18

19 The major over seasonal influence between the flood season flow situation and conditions during the following low flow season is not whether flood volumes during the flood season were above or below average. The inter-season (wet to dry) dependence in terms of flow volumes, though statistically significant, does not provide any meaningful basis for prediction. For example at Kratie only 25% of the year to year variability in mean dry season flow can be explained by the total volume of flow during the previous flood season, as shown in Fig. 11. Figure 11: Mekong at Kratie - the over-seasonal (flood to low flow) dependence of flow volumes. Flood season volumes explain just 25% of the variance of low flows in the subsequent dry season. The relationship therefore has no predictive value. 5. Analysis on the Impact of the Dams in China on Mainstream Hydrology In general, daily discharge hydrographs at Chiang Sean reveal considerable variations (in the literature also called noise ) during the low flow and also during the flood seasons. There are a great number of short term fluctuations in discharge with a frequency and pattern that appears to be inconsistent with the hydrological response to rainfall that might be expected over the upstream drainage area which amounts to almost km 2. At this scale the expected hydrograph should be much smoother and not show these short term variations. The oscillations, mostly not very pronounced, also occur during the low flow season when there is little if any rainfall to explain the variations. In order to clarify the impact from the upper Mekong River, Peter A. et al in 2011 analyzed the impact of upstream hydro-power dams based on historical hydrological data from 1960 to

20 Taking into account the consideration on the fluctuations of time series discharge data which occurred on day t-1 and t+1 compared with recent day t (t=1,365), the defined value of both can be greater than or less than based on the form of: q(t-1) < q (t) > q(t+1) or q(t-1) > q (t) < q(t+1). These fluctuations we might term discharge reversals. In Figure 12 the annual number of discharge reversals at Chiang Saen, Luang Prabang and Vientiane, , based on pre and post hydro-power dams in 1993 and 2003 respectively are illustrated. Pre 1993 the hydrology may be considered to have been natural and indicates that the mean annual frequency of discharge reversals decreased downstream as drainage area increases and the hydrograph becomes more coherent day by day. There is a clear change in their frequency around 1993 and the commissioning of Manwan dam. Post 1993 the mean annual changing rate doubles at Chiang Saen. At Luang Prabang the increased rate remains significant, while at Vientiane the change is much more modest. Clearly the short term hydrological impacts of reservoir operation are moderated downstream as tributary inflows exert an effect and damp them out, but the picture that emerges from this simple analysis is that the operational impacts of the dams in China on the flow regime of the Mekong are clearly manifested upstream of Vientiane. The principal emerging impact of mainstream hydropower is unmistakable, however, it remains mainly localized to the northern parts of the Basin between Chiang Saen and Luang Prabang. Fig. 12 The annual number of discharge reversals at Chiang Saen, Luang Prabang and Vientiane,

21 6. The Flow Regime of the Tonle Sap System and its floodplain in the first half of 2011 The most unique hydrologic feature occurs during the flood season from May to October, when massive floodwater from the Mekong River flows into Tonle Sap Lake. Additonal water comes also directly from its tributary rivers which swell the lake surface up to six times its normal size (from 2410 km 2 to km 2 ). The flow of water between the Mekong River and the Tonle Sap Lake is seasonal, and its flow direction changes depending on the water level of the Mekong River. When the water level of the Mekong River becomes high during the flood season, water is pushed into the lake (reverse flow), and when the water level of the Mekong River recedes in the dry season, water flows from the lake to the Mekong River (outflow). The water level at the gauging station in Phnom Penh port is a good indicator for these situations. As described, if the Mekong level is higher than the water level in the lake, the water starts to flow towards to the lake. In November, usually the water level in the lake is highest and starts to be higher than the water level in Phnom Penh Port, therefore also to flow out of the lake back to the Mekong River begins. The reverse-flow in the Tonle Sap River can be calculated using a discharge rating curve developed by MRCS and observed water levels at Phnom Penh port, Prek-Kdam and Kampong Luong stations. The rating curve has been checked against more than 50 discharge measurement results from Prek-Kdam during the years Prek-Kdam station is regarded as the outlet point of the lake. The calculated outflow and reverse-flow from January to December of the Tonle Sap River compared to its maximum, minimum and average flows in the Tonle Sap River are summarized in Figure 13. The flow in 2011 shows lower than average condition ( ) from January to April, whereas from May to July the flow is fluctuating around its long term average. On average the outflow stops around mid of May and then slowly the reverse flow started, which indicated the normal condition of inflow into the Tonle Sap Lake (similar process as flow in 2009). Figure 13 Observed cumulative reverse flow at Prek-Kdam

22 To verify the low flow into the Tonle Sap Lake, water levels at Phnom Penh Port, Prek-Kdam and Kompong Luong stations were taken into account. From January to March 2011, water levels at Komping Luong station were higher than Prek-Kdam station, indicating the out flow (reserved flow) of the lake. From May to July, water levels at Prek-Kdam were higher than Kg. Luong indicating the inflow into the lake. In Figure 14 the daily change in water level (above mean sea level, MSL) of the lake in 2011, compared with observed water levels at Kampong Luong, Prek-Kdam and Phnom Penh Port stations are presented. 6 Water Level of the Tonle Sap River System from 1st January to 31st June Jan-11 Feb-11 Mar-11 Apr-11 May-11 Jun-11 Jul-11 Water level, [m, MSL] WL at P/Penh Port WL at Prek-Kdam WL at Kampong Luong Figure 14 The hydrographs of water level at Kampong Luong, Prek-Kdam and Phnom Penh Port The analysis of the flood phenomena of the Tonle Sap Lake shows the specific times and durations of inundation and recession, including the areas of inundation and non-inundation. This analysis also provides useful information for assessing multi-functional roles of hydrology not only in agricultural context but also in fishery production in the floodplain area of the Tonle Sap Lake. 7. Conclusions and recommendations The hydrological condition along the Mekong mainstream during the first half of the year 2011 was generally slightly increased above the year 2010 s range that might be defined as normal year condition, with the exception of the Tonle Sap Lake in Cambodia, which exhibited a rather low reversed flow in Between Chiang Sean and Kratie, the dry season in 2011 began earlier than usual with offset of the flood season in middle of November

23 The sudden, consered non-natural further reduction in Mekong mainstream water levels from late December 2010 onwards, suggests reduced flow from China as reservoir releases could not be sustained as storage levels fell to critical levels.. It is therefore recommended to obtain also dry season flow data from China in the future. It has been acknowledged that the operation of the Jing Hong hydroelectric power plant some 3 km upstream from the Jing Hong gauge is likely to have some impact on the downstream fluctuations of water levels, and that there is some evidence of this in the dry season daily water levels transmitted to the MRCS and available on our website. ( It can be concluded that the operational impacts of the upstream hydropower plants are detectable but generally quite small at the diurnal and weekly timescales. They are most evident at Chiang Saen but are filtered out by natural hydrological processes towards Luang Prabang and Vientiane. On the basin scale, there is no real effect of the short term variance on the annual flood hydrograph. The analysis of regional rainfall conditions was limited to those observed within the Mekong-HYCOS networks including some selected stations in the four countries, which are not fully representative. None the less, through the satellite imagery available for changes in regional soil moisture do not suggest that regional rainfall conditions from July onwards were abnormal. The inflow of the Tonle Sap remained normal. The analysis of reverse flow and inflow is suitable for providing useful information on the multi-functional roles of hydrology and the floodplain area of the Tonle Sap Lake. References: Peter A., Jeremy B (2010). The Mekong- A drought prone tropical environment. A special report to be submitted for MRCS publication. Peter A., Jeremy B (2011). Hydrological Condition Report on the Lower Mekong Basin. A report for 33 rd JC meeting in Shihanuk Vill, Cambodia. April, MRCS,IKMP/TSD (2010). Preliminary report on low water level conditions in the Mekong mainstream. A special report for drought condition in the LMB, done by hydro-team. MRCS (2010). Annual Mekong Flood Report Draft report published in May