WATER RESOURCE DEVELOPMENTS AND FLOW REGIMES ON THE MEKONG RIVER

Transcription

1 Hydrology for the Water Management of Large River Basins (Proceedings of the Vienna Symposium, August 1991). IAHS Publ. no. 201,1991. WATER RESOURCE DEVELOPMENTS AND FLOW REGIMES ON THE MEKONG RIVER B. S. PIPER &. A. GUSTARD Institute of Hydrology, Wallingford, Oxon OX10 8BB, UK C. S. GREEN Oxford Scientific Software, Benson, Oxon OX10 6RR, UK PÂCHERN SRIDURONGKATUM Mekong Secretariat, Bangkok 10330, Thailand ABSTRACT The river Mekong is one of the ten largest international rivers in the world. Analysis of historic flow records, made available through the offices of the Mekong Secretariat, shows that man-made developments on some tributaries have only had a small impact on the downstream flow regime. This situation is likely to change as mainstream projects are commissioned, and as development on the left-bank tributaries intensifies. This paper discusses simple flow indicators, derived from historic records, that can be used to compare flow regimes in different parts of the basin. Benchmark statistics are derived that can be used to detect changes in downstream flow regimes. NOTATION ADF AM(D) T HYDATA MAM(D) MAR Q75 Average Daily Flow Annual Minimum; duration D days, return period T years Hydrological database and analysis system Mean Annual Minimum; duration D days Mean Annual Runoff 75 percentile flow BACKGROUND Geography The upper reaches of the Mekong river are fed by snow from up to m above sea-level in the eastern part of the great plateau of Tibet in the Himalayas. The river then flows down through the mountains of Yunnan province in China, until it reaches the border of the Union of Myanmar and the Lao PDR. It forms much of the boundary between Thailand and the Lao PDR, before reaching the Cambodian border a further 900 km downstream. Downstream of Phnom Penh, the river flows through the delta in Viet Nam and into the sea (Fig. 1). Inflows 45

2 B. S. Piper et al. 46 into the delta are made up of two components; flows from upstream and the outflow from the Great Lake. The total length from source to mouth is some km. The catchment area of the lower basin, which extends from the border between Thailand and the Union of Myanmar down to the sea, is about km 2 which represents over 15% of the total catchment area. The catchment areas within each of the four riparian countries are : LaoPDR Thailand Cambodia Viet Nam Mekong basin(km 2 ) Country totaloan 2 ) Fig. 1. Location map.

3 47 Mekong water resources developments andflowregimes The lower basin can be divided into two contrasting parts, namely the relatively flat Korat plateau of northeast Thailand, and the mountainous regions of northern Thailand and the Lao PDR. Climate The lower basin lies entirely within the tropical zone of the northern hemisphere. Its climate is controlled largely by the seasonal monsoon winds with two distinct seasons, separated by short transition periods. The southwest monsoon and the rainy season occur mainly between mid-may and mid-september or early October the northeast monsoon or dry season occurs mainly between mid-october or early November and March. Mean annual rainfall ranges from less than mm in parts of northeast Thailand to as much as mm over some parts of the mountains in the northeast of the Lao PDR. With the exception of the latter area, which has no clearly defined dry season, the rainfall that occurs between December and May is a very small percentage of the annual total. Estimates of annual potential evaporation range from mm to mm. Hydrology The climatic pattern of distinct wet and dry seasons is reflected in the streamflow records of the Mekong and its tributaries which show marked seasonal variations. Upstream of Chiang Saen the distribution of runoff is much more uniform, as flow is influenced more by snowmelt than by rainfall. Generally the main river begins to rise following the start of the rains in May, and at upstream locations reaches its maximum level in August or September; further downstream the maximum occurs later in September or October. The lowest discharges occur in March or April. This pattern of runoff is also characteristic of the tributaries, and in many catchments where groundwater storage is small, the rivers dry up completely soon after the end of the wet season. Land use Shifting or slash-and-burn cultivation has been a feature of the lower Mekong basin for many generations. Most of the forest will have been cleared to some extent at some time in history; some areas may have been cleared many times. Large areas of secondary forest have arisen from the practice of replacing forest with temporary rainfed crops, which were abandoned once the soil fertility had fallen. Away from large-scale irrigation, the main features of the agricultural areas are the small, traditional bunded paddy fields located in flat low lying areas. Significant expansion of agriculture into more of the upland regions for dry-foot crops such as cassava, kenaf and sugar cane, has been occurring in recent years.

4 B. S. Piper et al. 48 Modem irrigated agriculture is practised in the delta and parts of northeast Thailand, and is becoming more widespread in the Lao PDR. WATER RESOURCE DEVELOPMENT The Mekong Committee The Mekong and its tributaries provide valuable water resources for the riparian countries, though the distribution over time and space is uneven. For example in northeast Thailand the main constraint to irrigation development is water, whereas there is adequate water in the Lao PDR, but not enough cultivable land. To date, some major water resource and irrigation schemes have been implemented; there is ample scope for further development to increase power generation and food production, improve navigation on the main river, and provide some means of flood control. It is estimated that the water resources of the lower basin could generate some GWh per year, and irrigate some ha of cultivable land for rice and other crops (Interim Committee, 1989). The rate of future water resource development in the basin will be driven by the need to balance the demands for electric power and rice production with environmental considerations. Secondary factors such as fisheries, aquaculture potential, navigation, flood control and forestry will also be important. The development and management of an international river is a complex and delicate balance between possibly mutually exclusive objectives and requires agreement between the riparian countries. The Mekong Committee was established in 1957 by the Governments of Cambodia, Laos, Thailand and Viet Nam, when the member countries decided to work together. The objective of the Committee was to promote, coordinate, supervise and control the planning, investigation and implementation of water resource development projects in the lower Mekong basin (Interim Committee, 1988). Despite the profound political and economic upheavals in the region that have occurred since 1957, the Mekong Committee has compiled and maintained a source of hydrological and meteorological data, which have been the basis of many planning studies, including the study on which this paper is based. In recent years there has been an acceleration in the pace of technical and social change, and demands on water resources are increasing. Existing development Since 1965, nine major reservoirs have been built in the lower basin. The area regulated by these reservoirs is 5% of the catchment area down to Kratie. The main consumptive use of water in the basin is for agriculture, with rainfed paddy cultivation the most important farming activity. In 1985 the total area of paddy cultivation in the lower basin amounted to 8.5 million ha (Interim Committee, 1988), that is about 13% of the catchment area to Kratie. A small percentage of the paddy area is served by formal gravity or pumped irrigation systems.

5 Potential development 49 Mekong water resources developments andflowregimes The priorities for development are different for each of the riparian countries. In the Lao PDR, the potential for large-scale irrigation development is relatively small, and at present most of the electricity produced through hydropower is exported to Thailand. In northeast Thailand, water is the main constraint to agricultural development. Plans for the expansion of irrigation can only be successful if reliable supplies of water can be assured for dry season cropping, for proper management of potentially saline soils, and for the alleviation of drought periods during the wet season. Such supplies of water are only available from outside the northeast, by diversion or transfer of water from the Mekong. In Viet Nam, expansion of rice production in the delta is foreseen. This will require assured freshwater flows into the delta during the dry season. Freshwater is needed not only for consumptive use by the rice crops, but also for the management of acid sulphate soils and for the prevention of damaging saline intrusion up into the delta from the sea. At present, the requirements for Mekong water in Cambodia are unknown. AVAILABLE DATA Mekong Secretariat Hydrologie and Meteorological Database (HMDB - ) The Mekong Secretariat maintains a large database of hydrological and meteorological data for the lower basin. Many of the data are published in annual yearbooks (Mekong Secretariat, ). More recently the data have been incorporated into the HMDB database (Mekong Secretariat, 1986) on the Secretariat's VAX 11/730 computer. The data used for this work were downloaded directly from the VAX onto a PC using the Institute of Hydrology professional software, HYDATA (Institute of Hydrology,, Farquharson & Green, 1989). HYDATA provided a 'front-end' to the HMDB by offering easy to use graphical and hydrological analysis programs. Rainfall and soil data are also available and were used to develop a procedure for estimating dry season flows at ungauged sites (Institute of Hydrology, 1988). Flow data The analysis of dry season flows required daily mean flow data from a number of mainstream and tributary stations throughout the basin. The stations were chosen to give as good geographical coverage as possible, to have at least seven years of mean daily flow data and to have data of at least reasonable quality, particularly at low flows. A thorough investigation and revision of the rating relationships on the mainstream gauges (Institute of Hydrology, 1988) was the basis for selection of the mainstream gauges. Selection of tributary gauging stations was based on discussion with hydrologists at the Mekong Secretariat.

6 B. S. Piper et al. 50 FLOW REGIMES Flow measures The term "flow measure" is used in this paper to describe a simple statistic that characterizes the flow regime at a given point on a chosen river. Three simple measures have been calculated from the time series of mean daily flow: the mean discharge, the flow duration curve and the flow frequency curve (Institute of Hydrology, 1980). Mean discharge When expressed as runoff in mm over a catchment, comparisons between different catchments can be made. Changes in mean runoff will usually be the result of variability in climate (particularly rainfall), changes in land-use (which may influence evaporation, interception losses from vegetation, and infiltration rates) or artificial control of the river (net imports or exports of water). Flow duration curve The cumulative frequency distribution curve of mean daily flows, or flow duration curve, is a convenient measure for describing the complete range of flows from the dry to flood season. Fig. 2 shows the flow duration curve for the Mekong at Pakse for the period 1923 to 1965 before any major regulation on the tributaries. Other flow indices, derived from the flow duration curve, are useful for comparing stations and for comparing different time periods at the same station. HYDATA was used to calculate seven different indices, defined as the discharge exceeded 95, 90, 75, 50, 25, 10 and 5% of the time ^ v x s i % Time flow exceeded Fig, 2. Flow duration curve at Pakse (1923 to 1965).

7 51 Mekong water resources developments andflowregimes Flow frequency curve The flow frequency curve shows the proportion of years, or equivalently, the average interval between years (return period) that a flow taken over a given number of days is exceeded. Fig. 3 shows the flow frequency curve for the same gauge and the same period as Fig " Analysis over the period i Jan 1923 to 31 Dec 1965 X 60 day mlrtisa Return Period o œ E,0, X CD x «.... X " X x X ;0 Reduced variate «Fig. 3. Flow frequency curve at Pakse (1923 to 1965). Benchmark statistics Four flow indices were selected to describe the variation in flow regime found in the lower Mekong basin both under natural and regulated conditions: (a) MAR the mean annual runoff expressed in mm; (b) Q(P) seven percentiles from the flow duration curve expressed as m 3.s' : 95, 90, 75, 50, 25, 10, 5; (c) MAM(D) the mean annual D day minima in ntf.s" 1, for five different durations of 10, 30, 60, 120, 180 days; (d) AM(60>r the 60 day annual minima of T year return period, for four different return periods of 2, 5, 10, 25 years, with discharge expressed as a ratio of the 60 day mean annual minima. Table 1 summarizes some of the results of this analysis. Mean annual runoff ranges from less than 150 mm in northeast Thailand to over mm in me mountains of the Lao PDR. There is also a wide range in both MAM(60) and Q(P) reflecting the variation both in the size and climatic conditions of each catchment. In order to compare indices between gauging stations, the values can be standardized by dividing by the average flow.

8 B. S. Piper et al. 52 Table 1. Benchmark flow statistics. Area (km 2 ) Start End ADF (m 3.s') MAR (mm) MAM(60)Q75 (m 3.s')(m 3.s') Natural catchments Mekong at Chiang Saen Mekong at Luang Prabang Mekong at Vientiane Mekong at Mukdahan Mekong at Pakse Mekong at Kratie Nam Ngum at Tha Ngon Nam Chi at Tha Phra Nam Chi at Yasothon Se Bang Fai at Se Bang Fai " Regulated Catchments Mekong at Mukdahan Mekong at Pakse Nam Ngum at Tha Ngon Nam Chi at Yasothon Nam Mun at Ubon insufficient data for calculation Upstream mainstream stations have similar indices to nearby tributary basins with MAM(60) around 30% of the mean flow, and Q75 about 37%. The corresponding values for the station furthest downstream are 14 and 19% respectively. This reduction is caused by the low dry season flows from the intermediate tributaries. CHANGES IN FLOW MEGIMES Historical Mean annual runoff, annual 60 day minima, and flow duration curves were compared. An initial inspection of the data suggested that the main changes in flow regime could be related to regulation of natural flows following implementation of reservoir storage or irrigation. An analysis of natural flow records was also carried out to identify whether natural dry season flows had changed and also whether dry season flows could be related to seasonal rainfall. Six gauging stations (three on the mainstem and three on tributaries) downstream of one or more of the major reservoirs have sufficiently long records

9 53 Mekong water resources developments andflowregimes before and after the regulation for meaningful comparisons to be made. A summary of the changes in flow indices is given in Table 2. Table 2. Changes inflow regime after regulation (%). Mekong Nam Ngum Nam Chi Nam Mun Mukdahan Pakse at Tha Ngon at Yasothon at Ubon Flow index MAR MAM(IO) MAM(30) MAM(60) MAM(120) MAM(180) AM(60)2 AM(60)5 AM(60)10 AM(60)2S Q95 Q90 Q75 Q50 Q25 Q10 QS indicates insufficient data for annual minima calculation The impact of regulation on the flow indices is clear: MAR reduces as there are evaporation losses from the reservoir surfaces; the MAM are increased for all durations; the annual minima at different return periods have in general shown an increase; discharges for given flow percentiles from the flow duration curve have increased at low flows (at or below Q75) and decreased in the higher flow range (at or above Q25). Note that as a greater proportion of regulated river flow is abstracted for irrigation, the drainage and return flows into rivers will reduce. This will tend to cause a reduction in the MAM. The total catchment area of the tributaries that feed existing reservoirs is about km 2. Nevertheless the effects of regulation on flows are evident as far downstream as Pakse where the total catchment area is km 2 ; here the MAM(60) shows an overall increase of 18%, an increase of 10% in Q75, and a reduction in MAR of 8 %. Comparison of the time series of annual rainfall and runoff indicated that mere had been no apparent change in flow regime due to climate. It was also concluded from the observed flow records that there was insufficient evidence to identify any impacts from land use change or increased consumptive use by pumped irrigation schemes.

10 B. S. Piper et al. 54 Identifying future change in flow regime A simple procedure was derived to identify any future change in flow regime. The method compares the time series of mean annual minima against the benchmark MAM(60) statistic. The series of 60 day duration annual minima were standardized by the benchmark MAM(60), and then plotted; cumulative departures from the mean clearly indicate any change with time. Fig. 4 shows the resulting plot for Chiang Saen, the point where the Mekong enters the lower basin. No significant change with time is evident. In contrast the plot for the Nam Chi at Yasothon (Fig. 5) shows the effect of upstream regulation very clearly. Note that the benchmark MAM(60) was calculated from data up to 1965, when upstream reservoir construction started. An increase in the MAM(60) series can also be detected on the mainstem at Pakse (Fig. 6), but the change is not as dramatic. The start of the increase coincides with impounding at Nam Ngum in Note that there was also a substantial increase in the departure from the mean in the 1940's. However it is only with the 20 years of data that are now available since 1965 that the recent increase in dry season flows has been identified as a real change at Pakse and not just part of the natural variability in annual flows. CONCLUSIONS The collection and processing of routine hydrological and meteorological data is an expensive and difficult task. In many large river basins, such as the Mekong, the catchments with the greatest influence on flow regimes may be those where access is difficult or impossible. Consequently continuous long-term record of good quality data are rarely available for all the major tributaries. In some parts of the Mekong basin, access has only become possible in recent years; some regions have been isolated for reasons of security. Nevertheless, the Mekong Secretariat has been able to assemble and maintain a comprehensive archive of data, both for tributaries as well as the main river. Without co-operation between the Mekong Secretariat and the national Agencies this success would not have been possible. This paper shows that it is possible to undertake analysis and interpretation of flow indices on a regional basis, using the data that are available. It has been possible to identify changes in flow regime that correspond to major water resource schemes on the tributaries. The analysis of flow records has shown that releases for power generation can significantly increase dry season flows at points some way downstream. Although reservoir regulation is likely to remain the most important cause of changes in flow regime, it is expected that as the consumptive use of water on downstream gravity and pumped irrigation schemes increases, this effect will become less pronounced. The size of the gauged catchments means that it has not yet been possible to identify the impacts from land-use change or climate change separately. Routine monitoring should continue, with more intensive campaigns on selected catchments, so that possible impacts can be investigated further.

11 55 Mekong water resources developments and flow regimes 6 a «P t/3 rt 1a O c ^^ 03 ~l ann Yaso T) D "O CS 13 ca V3 io 0 i ( (x. c o 4= -t-> a J3 u fi <n Z â B ea Iff S.S.ifl ca -< ea e o A! ca OO ^ 6 t <

12 B. S. Piper et al. 56 ACKNOWLEDGEMENTS Much of the work described in this paper was carried out in the Offices of the Mekong Secretariat during the three phases of the Lower Mekong Basin Water Balance Study financed by the United Kingdom Overseas Development Administration (ODA). The authors gratefully acknowledge the help and encouragement of many of the Staff of die Secretariat, whose knowledge of the region and constructive advice has proved invaluable. The authors also acknowledge the support of the Mekong Secretariat through the provision of office facilities and access to data. The views expressed in this paper are those of the authors and are not necessarily those of ODA or of the Mekong Committee. REFERENCES Farquharson, F. A. K. & Green,C. S. (1989) The Use of Personal Computers for the Analysis and Management of Hydrometeorlogical and Groundwater Data. Proceedings of the Sahel Forum, Ouagadougou, UNESCO, Institute of Hydrology (1988) Lower Mekong Basin Water Balance Study, Phase 3 Report. Institute of Hydrology () HYDATA: Hydrological Database and Analysis System. Institute of Hydrology (1980) Low Flow Studies Report. Interim Committee for Coordination of Investigations of the Lower Mekong Basin (1988) Perspectives for Mekong Development - Revised Indicative Plan (). Interim Committee for Coordination of Investigations of the Lower Mekong Basin (1989) Annual Report, Mekong Secretariat ( ) Lower Mekong Basin Hydrologie Yearbooks (A series of annual publications). Mekong Secretariat (1986) Hydrologie and Meteorological Database - User's Manual.