REPORT ON THE HYDROLOGICAL CONDITIONS IN THE LOWER MEKONG BASIN FROM JANUARY TO JUNE, 2010 FOR THE FIFTEEN DIALOGUE MEETING MEKONG RIVER COMMISSION

Transcription

1 Attachment to Agenda E REPORT ON THE HYDROLOGICAL CONDITIONS IN THE LOWER MEKONG BASIN FROM JANUARY TO JUNE, 21 FOR THE FIFTEEN DIALOGUE MEETING 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 Regional rainfall from January to June Selected rainfall sites, compared to long-term average condition 3. Regional soil moisture condition-january to June Hydrological condition of Water Level and River Flow in Lower Mekong Basin 4.1 Flood Season of Low flow of 21 in the Lower Mekong Basin Flow and Water level in Mainstream from January to June, Tributary Inflow 5. The Flow Regime of the Tonle Sap System and its floodplain 6. Study on mainstream flow probability analysis at a selected station 7. Conclusions 2

3 1. Introduction In response to a request from the Twenty-second Meeting of the MRC Joint Committee in August 25 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 submitting to the fifteen dialogue meeting in 27 th August 21. The paper considers the hydrological situation within the Lower Mekong Basin from the 1 st January to 31 st July 21. In general conditions both meteorological and hydrological aspects in upper reach of the Mekong River were below long-term average which would be considered dry year or low flow season. The main cause of low water levels being experienced in the 21 dry season in the Mekong mainstream is a combination of an early end to the 29 wet season and low monsoon rainfall which has led to regional drought conditions. Based on the available information it appears that flows from tributary rvers in Lao PDR and northern Thailand are at levels that are amongst the lowest that have been observed in recent decades. Starting from the end of January to March 21, the water level of the Mekong River has dropped drastically into the lowest record, compared to the driest year in The 21 low flow season in the Mekong follows conditions during the flood season of 29 which were amongst the lowest record. The analysis is based upon the data currently available which are compared to historical hydrological and meteorological conditions. The rainfalls of last year 29 and January 21 were less than average, which could not significantly contribute to river runoff. This could help explain the low flow condition in early 21. Soil moisture displayed low values in the LMB beginning from January to April ( to 1%), corresponding to significant low amounts of rainfall in LMB. But from April to July 21, follows by monsoon rainfall saw gradually moisture high levels across the region to more than 2% in May and 5% in July. The inflow /reverse of the Tonle-Sap Lake at Prek Kdam were considered marginally below the average from January to March, whereas from May to July 21 the outflow of the lake is showed fluctuated in between its average 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 hydro-meteorological data from the upper Mekong in China is necessary. However, MRCS has been worked on this issue in-term of data sharing and exchanged information with China not only in flood season but also in low flow season. 2. Meteorological and Hydrological Conditions in the Lower Mekong Basin in 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, USDA) via operational daily data and the internet facilities. In this report, the hydro-meteorological data in quarter 3

4 1and 2 (January to June 21), NOAA rainfall from satellites were used for analysis of hydrological condition in the upper Mekong and LMB. Figure 1 shows some selected hydro-metrological stations in the entire Mekong/Lancang River Basin. 2.2 Rainfall in the Upper Mekong River (Lancang) from January to June 21 In the upper Mekong Basin, rainfall and air temperature data at Jinhong, Lincang, Simao and Lancang were obtained from NOAA ( Figure 2 shows the observed monthly rainfall, compared to its long term average (2-29), and air-temperature at the four selected stations from January to June21 at each site. There is a consistent pattern of below average rainfall in each month between January and June. The January 21 rainfall was also below average. Lancang Lincang Simao Fig. 1 Observed hydro-metrological stations over the Mekong/Lancang River Basin 4

5 Fig. 2 Monthly rainfall compared with long-term average (2-9), and monthly airtemperature data at selected four stations in the Upper Mekong Basin in Southern Yunnan province, China. ( Monthly air temperature data for China indicate consistent increasing trends in May 21. For instance, the temperature at Lincang (highest latitude selected station) increased from around 5 degrees at beginning of January to about 3 degrees in May. It is not clear how the increase in temperature effect snow-melt runoff from Tibet which contribute to the Mekong/Lancang flow. Ground rainfall obseration at the four selected stations confirmed the rainfall condition from Jan-Jun 21 is still below long-term avarge (2-9) for the upper stations, except at Jinhong rainfall is bout its long-term average (15 mm) in June. 2.3 Regional rainfall of the lower Mekong Basin form January to June, 21 The DSF tool was used to analyze the spatial areal rainfall distribution over the Lower Mekong Basin from 6 rain gauge stations (see Table 1) from January to June 21. Rainfall analysis was based on two sources: 1. Sixty rain gauge stations from the national line agencies and 2. Satellite observed rainfall by the Climate Prediction Center (CPC) of NOAA. 5

6 Figure 3 shows the cumulative rainfalls over LMB from January to March and April to June 21, based on NOAA data. Figure 4 shows the quarterly rainfall based on ground station from 6 rain gauge stations. Method of interpolation-topo to raster in ArcMap was used to extrapolate rainfall map from points to raster. Figure 3: Figure 1 Cumulative rainfall over the LMB for the period 1 st January to 3 th June 21, based on NOAA-satellite rainfall. (Sources: Based on NOAA areal rainfall distribution, the South and Northern parts of the LMB have relatively low rainfall from January to March 21, whereas from April to June have relatively higher rainfall (2-3 mm) in some of areas (e.g. in Cambodia and Delta areas) which are characterized as floodplain area and high lands with mountainous. At the same period the ground based station rainfall indicated the low rainfall from South and Northern of LMB, ranging the same range of satellite rainfall ( to 5 mm). This indication revealed a drought year in 21 from January to March all over the LMB. The case study of low flow and 6

7 water level fluctuations at the end of January to April at Chiang Sean, Luang Prabang, Vientiane and Kratie of the Mekong River indicated the sudden non natural further reduction in Mekong mainstream water levels from late January onwards suggests reduced flows from China as reservoir releases could not be sustained as storage levels fell to critical levels in response to the drought. The current emphasis on climate change and adaptation may provide the necessary impetus to address the previous under-investment in regional, national and local capacities for drought management and mitigation. (Full report on Mekong low flow, available at: From April to June 21, the ground based stations observed rainfall showed similarly trend with NOAA data of increasing rainfall over the LMB from 1 to 3 mm and even more higher rainfall in floodplain areas (Cambodia and Mekong Delta in Vietnam), which varied from 1 mm to 148 mm. Table 1 summarizes the regional quarterly rainfall distribution over LMB from 1 st January to 3 th June 21, based on ground observed stations. Figure 4: Cumulative mapping of ground based rainfall from 6 rain gauge stations over LMB in quarter I and II, from Jan to June 21 7

8 Table1: Regional quarterly rainfall over the LMB based on ground observed stations Country No. Station Lat. Long. Monthly Quaterely Rainfall (mm), 21 Name Jan Feb Mar Apr May Jun Cambodia 1 Kratie Kg.Cham Basac-CTM Neak Luong Koh Khel Prek Kdam Kompong Speu Pailin Pursat Mung Russey Kravanh Sesan Laos 1 Luang Prabang Vientiane Paksane Thakhek Savannakhet Pakse Muong Mai Mahaxai Saravanne Veun Khen Muong Ngoy Sayaboury Xieng Ngeun Vang Vieng Xiengkhouang Kengkok Laongam Sekong Ban Donghene Thailand 1 Chiang Saen Chiang Khan Khong Chiam Nong Khai Nakhon Phanom Mukdahan Khon Kaen Roi Et Ubon Ratchathani Surin Chiang Rai Phayao Veitnam 1 Tan Chau My Thuan Chau Doc Vam Nao Ban Don Kon Tum Muong Te Sin Ho Lai Chau Tuan Giao Ba Don A Luoi Hue Pleiku An Khe Ayunpa Buon Me Thuoc

9 2.4 Selected rainfall sites, compared to long-term average condition Figure 5 shows the comparison between monthly observed rainfall from January to June 21 and long-term monthly average rainfall (196-29) from the selected stations at Chiang Saen, Luang Prabang, Chaing Rai, Vientiane, Pakse, Phnom Penh and Chau Doc. Based on the Fig. 5, the distribution of rainfall over the LMB can be concluded as follows: In the upper LMB, the rainfall from January to March 21 at Chiang Saen, Luang Prabang, Chaing Rai, and Vientiane varied from mm to 35 mm which considered lower than long-term average ( ). However, from April to June 21, rainfall in this area was considered a little higher than long-term average (25mm-26 mm). In the middle LMB: the January to March rainfall in 21 at Pakse is significantly dropped lower than its long-term average in ( ), varied from to 8 mm. However, in April to June 21, rainfall at Pakse seems to be about with its long-term average. In the lower reach from Phnom Penh to Chau Doc, the increases of rainfall at Phnom Penh in April to June were due to the impacts of monsoon rainfall. In general rainfall from January to March 21 showed more or less about its long-term average. Monthly rainfall data at Chiang Saen (Jan-Jun) Monthly rainfall data at Luang Prabang (Jan-Jun) m m l in fa in a R Mean Jan Feb Mar Apr May Jun 25 2 m m15 l in fa in 1 a R Mean Jan Feb Mar Apr May Jun 9

10 Fig 5: Monthly average rainfall pattern from January to June, 21 at Chiang Saen, Luang Prabang, Vientiane, Khon Kean, Pakse, Phnom Penh and Chau Doc, compared with its long-term monthly average rainfall ( ) 3. Regional soil moisture conditions January to June, 21 Figure 6 presents the mapped status of the regional soil moisture conditions over the Lower Mekong Basin from January to June 21. Each general term represents the typical pattern 1 : From January to March: the general picture shows very low levels of soil moisture ( to 1%), corresponding to significant low amounts of rainfall in LMB. There are, however, pockets of significant residual moisture (less than %) in the whole LMB mostly in March, which indicated the driest month. 1 It is intended in future to apply the results of research from the Geoinformatics Department at Khon Kaen University on the mapping of regional soil moisture status to further understand the regional annual pattern. 1

11 From April to June: follows by monsoon rainfall saw gradually moisture a little higher levels across the region of more than 2% in April and 5% in May, and in June the soil moisture is up to 9%. Some improvement is evident in late June as a result of sporadic rainfall conditions in some parts of the LMB, which increased moisture levels to above 9% within the eastern highland margins of the Basin in Lao PDR and southern part in the Cambodia and the Mekong Delta. This indicate the onset of the Monsoon, which increased soil moisture levels increase up to 9% over a much wider proportion of the Basin. Jan , 21 Feb , 21 Mar , 21 Apr. 21-3, 21 May , 21 Jun. 21-3, 21 Figure 6: Regional soil moisture conditions between January and June, 21. (Source: 11

12 4. Hydrological Condition of River flow and water level in the Lower Mekong Basin for January to June Flood Season of 29 The general weakness of the 29 SW Monsoon meant that flows during the flood season were well below normal, particularly in these northern parts of the Mekong basin. Figure 7 showed the indicated low trend of flood season in 29. The peak and the total volume of the 29 flood at Vientiane, for example, were the 5 th lowest over the last 98 years. It would generally be expected therefore that natural catchment storage in northern Lao in particular would be significantly below normal at the end of the wet season with the followon effect that the subsequent dry season flows would also be below the seasonal average. Not only were discharges low but 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 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 Mean daily discharge. (cumecs) CHIANG SAEN Mean daily discharge. (cumecs) VIENTIANE / NONG KHAI 6 5 average (196-28) average ( ) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Mean daily discharge. (cumecs) average ( ) 29. KRATIE Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Fig. 7 Chiang Saen, Vientiane and Kratie: The 29 daily discharge hydrograph on the Mekong mainstream compared to the long term average. 12

13 4.2 Low flow of 21 in LMB The definitive feature of the drought process is the accumulation of a moisture deficit, which may be measured in terms of rainfall, stream flow or soil moisture. The term Monsoon is virtually synonymous with torrential rainfall, moisture surplus, floods and climatic predictability. Drought on the other hand is more generally associated with the marginal rainfall climates of arid and semi arid regions, which show high variability and low reliability from year to year, such that rainfall deficits are common and often severe (Adamson et al 21). 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. For example:- below normal flows during the flood season affect the timing, depth and duration of wetland inundation and the inflows to the Great Lake / Tonle Sap system with negative consequences upon fisheries and therefore upon the economy of Cambodia. deficient flows during the dry season months permit more extensive saline intrusion in the Viet Nam delta which reduces water availability for irrigation. This leads to lower yields, reduced planted area and significant economic loss for one of the world s largest producers of rice for the global market. Low water levels also restrict navigability and reduce river-borne trade. Rainfall shortfalls and unexpected precipitation patterns in both the wet and dry seasons can result in agricultural losses, which can become so large that they lead to reductions in regional and national economic growth targets, as has happened in north east Thailand in Flow and Water level in Mainstream from January to June, 21 Figures 8a and 8b present daily discharges and water levels at selected mainstream sites for the period 1 st January to 3 th June 21, compared to their mean and characteristic range. The latter is defined as the mean daily mean +/- one standard deviation. At all sites the 21 discharges and water levels were more or less within the characteristic range but generally below average. The levels at Luang Prabang and Vientiane being lower than 1992/3 more truly reflect the regional drought conditions from September 29 onwards and the very low contributions to the mainstream by the large tributaries in northern Lao PDR. The rapid fall in water levels from late January, as has been indicated, is not a 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. This reduction appears to have further contributed to what was already developing into a serious regional hydrological drought. 13

14 7 6 Vientiane Mean daily flow [cumecs] Characteristic range Mean 21 1 Jan Feb Mar Apr May Jun Figure 8a: Daily discharges at selected sites on the Mekong mainstream for the period 1 st to 3 th June, 21 compared to their historical average and characteristic range January 14

15 6. 5. P r e k K d a m C haracteristic range M ean 21 ] s c e m u [c w flo ily a d n a e M J a n F e b M a r A p r M a y J u n Figure 8b: Daily discharges and water levels at selected sites for the period 1 st January to 3 th 21 compared to their average and characteristic range. June, 15

16 4.2.2 Tributary flows Confirmation of the severity of the regional drought conditions is provided by an analysis of the flows so far in 21 on two large Mekong tributaries in northern Lao PDR, the Nam Ou and Nam Khan, Figure 9. Discharges on the Nam Ou are amongst the lowest observed over the period of record, while those on the Nam Khan are unprecedented, falling well below anything observed over the last 5 or so years. 25 Mean daily discharge. (cumecs) NAM OU AT MUONG NGOY. Catchment area: 19 7 km Historical data: January February Mean daily discharge. (cumecs) NAM KHAN AT BAN MIXAY. Catchment area: 6 1 km 2 Historical data: January February Fig. 9 Nam Ou and Nam Khan in Northern Lao PDR: Daily discharges for the period 1 st Jan to 23 rd February, 21 compared to their historical range 16

17 5. The Flow Regime of the Tonle Sap System and its floodplain in 21 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 and also from its tributaries rivers which swell the lake surface up to six times its normal size (from 2414 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 in 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 Phnom Penh port is an indicator. If it is higher than water level in the lake, the water starts to flow towards to the lake. In November, after the water level in the lake is higher than water level in Phnom Penh Port, it starts to flow back to the Mekong River. 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 5 discharge measurement results from Prek-Kdam during the years Prek-Kdam station is regarded as an outlet point of the lake. Figure 1 summarized 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. The flow in 21 shows lower than average condition ( ) from January to April 21. Based on Fig.1, the flow had been flowing out from the lake until end of June 21. On average the outflow stops around mid of May and then the reverse flow starts. In comparison with average condition, the 21 reverse flow began late, in the middle of June, 21 (similar process as flow in 29). Tonle Sap flow (outflow positive) Inflow+reverseflow, [cumecs] Max 97-9 Min97-9 Aver Jan 1-Feb 1-Mar 1-Apr 1-May 1-Jun 1-Jul 1-Aug 1-Sep 1-Oct 1-Nov 1-Dec Figure 1 Observed cumulative reverse flow at Prek-Kdam

18 Figure 11 presents the daily change in water level (above mean sea level, MSL) of the lake in 21, compared with observed water level at Kampong Luong, Prek-Kdam and Phnom Penh Port stations. 1 Water Level of the Tonle Sap River System from 1st October 29 to 3th June 21 Water level, [m, MSL] WL at P/Penh Port WL at Prekdam WL at Kampong Luong 2 Oct-9 Nov-9 Dec-9 Jan-1 Feb-1 Mar-1 Apr-1 May-1 Jun-1 Figure 11 The hydrographs of water level at Kampong Luong, Prek-Kdam and Phnom Penh Port The digital elevation data was developed from the elevation contour map with a 1-meter interval was used to identify the Tonle Sap Lake Basin and its stream network (see Fig. 12). DEM of the Lake H-V relation of the Lake S W Prek-Kdam UTM-Coordinates Figure 12 The digital elevation data was developed from the elevation contour map of the Tonle Sap Lake 18

19 From the digital elevation data, the relationship curve between flooding area and water levels (H-A) was established. The flooding area was estimated based on water elevation of the Lake, and then converted to flooding area based on the relationship H-A (see Figure 13). Flooding area (A), [km 2 ] The relationship of A-H curve A = H H R 2 =.9959 AREA Poly. (AREA) Water elevation (H), [m,msl] Figure 13 The relationship between flooding area and water elevation of the Tonle Sap Lake (Sources: Digital elevation data, DEM) Due to the fluctuation of intensity and timing of the flood, rice cropping in floodplain suffers from floods and droughts, which drastically affect rice production on the Tonle Sap floodplain. Since the flood water begins receding after mid-october and reaches its minimum level in April, therefore only the period of flood recession was taken into account. During this flood recession, the lands covered by floods gradually appear and recession rice is planted during these land appearances. Based on the estimated flooding area from the (H-A) curve, land appearance from November 29 to June 21 in the Tonle Sap Lake is presented in Figure 14. The land appearance area sharply increased from bout 788 km 2 in November 9 to about 7,65 Km 2 in February 21, and then to about 7434 Km 2 in April and maintained stable until June Flooding area Land Appearance Monthly change in flooding area, [km2] Monthly change in Land appearance, [km2] Nov-9 Dec-9 Jan-1 Feb-1 Mar-1 Apr-1 May-1 Jun-1 Figure 14 The monthly change in flooding area vs. land appearance of the Tonle-Sap Lake 19

20 Due to deep inundation during long periods (about 3 months), the agricultural land was limited. Due to limitation in water management and additional constrains of inundation, the lowland rice was only suitable for single recession-rice cropping in the Tonle Sap Lake floodplain. The analysis of flood inundation of the Tonle Sap Lake showed 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 floodplain area of the Tonle Sap Lake. 6. Study on mainstream flow probability analysis at a selected station Based on the twenty-eighth Meeting of the Joint Committee, more analysis in the report is requested. This includes risk analysis and data preparation for the development of the Drought Management Programme. In response, the mainstream flow data is analyzed in order to estimate the flow probability of exceedance. This exercise is useful to give an insight into dry season stream flow data and water availability. Based on availability of time and resources, the data from Chiang Khan Station is selected in this report. Figure 15 shows time-series of discharge for 21 and those of 1%, 2%, 5%, 8% and 9% probability of exceedance at Chiang Khan Station. The 9% exceedance graph can be adopted to be an extremely-low flow pattern. The 8% graph is a significantly-low flow pattern. The 5% graph could indicate the average pattern. The 2% and 1% graphs could indicate the significantly and extremely high flow patterns respectively. The probability analysis indicates that the flows from January to July 21 in a selected station at Chiang Khan are lower than the 5%, having serious hydrological drought condition. Discharge (m 3 /s) Discharge Probability of Exeedance for CHIANGKHAN 1% 2% 5% 8% 9% Year Julian Days Figure 15 Time series of flow probability of exceedance at Chiang Khan based on historical flow data ( ) 2

21 7. Conclusions and recommendations The main cause of low water levels being experienced in the 21 dry season in the Mekong mainstream is a combination of an early end to the 29 wet season and low monsoon rainfall which has led to regional drought conditions. Based on the available information it appears that flows from tributary rvers in Lao PDR and northern Thailand are at levels that are amongst the lowest that have been observed in recent decades. This situation represents a regional hydrological drought affecting all countries in the Basin. The sudden non natural further reduction in Mekong mainstream water levels from late January onwards suggests reduced flows from China as reservoir releases could not be sustained as storage levels fell to critical levels in response to the drought. Further analyses and discussion with China are planned. Hydrological conditions along the Mekong mainstream during the second half of 21 were generally lower than the range that might be defined as drought. Discharges and water levels were marginally below average in upper reach of the Basin with the exception of the Northern Laos PDR at Chiang Saen, Luang Prabang, Vientiane in early July where they were significantly above average. The analysis of flow and water level in mainstream showed that in this year the hydrological conditions in mainstream were generally marginally below average for the reach from Chiang Saen to Pakse, but it fluctuated in between average from Kratie down to delta areas. The analysis of regional rainfall conditions is limited to those observed within the ANHIP and HYCOS networks including some selected stations in the four countries, which are not fully representative. None the less, through the satellite imagery available of changes in regional soil moisture there is nothing to suggest that regional rainfall conditions from July onwards were abnormal. The outflow of the Tonle Sap remained normal. Based on the analysis of the flow regime and its relationship of water level and flooding area of the lake, the conclusions are as follows: The suitable land for recession-rice was gradually appeared up to 7547 km 2 in April, after peak flood recessed. The analysis in this paper was considered to provide a better way for seasonal water balance, local drainage and for floodplain management in the Tonle Sap Lake area. The analysis of outflow and reverse flow is suitable for providing useful information on multifunctional roles of hydrology in floodplain area of the Tonle Sap Lake. References: Peter A., Jeremy B (21). The Mekong- A drought prone tropical environment. A special report to be submitted for MRCS publication. MRCS,IKMP/TSD (21). 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 (21). Annual Mekong Flood Report 29. Draft report published in May