The hydropower potential in Yunnan is unquestionable especially at a time when there is a global need to reduce reliance on carbon-based energy production but there remains major concern about the negative impacts of the dams on the Lower Mekong Basin. Many have discussed and explored these issues (Mogg, 1997; Roberts, 2001; Whiting, 2001; China Daily, 2002; Hawiset, 2002a and 2002b; Richardson, 2002; Kummu and Varis, 2007; Kummu et al., 2004b; Xu and Moller, 2004; Salidjanova; 2007). Major concerns relate to: increasing flow fluctuation downstream, increasing average downstream dry-season flows, decreasing wet-season flows, shifting of the flooding period, decreasing flux of nutritious sediments crucial for fisheries and agriculture production, and geomorphological changes such as bank erosion and bed degradation due to sediment trapping. Proponents argue that besides hydropower generation the dams offer flood control, more assured dry-season flows, increasing navigation options, reduction of salinity intrusion and provision of irrigation opportunities for downstream countries (Plinston and He, 2000; Hori, 2000). Other positive impacts offered in the schemes’ defence include attenuation of flooding in the wet season by storing the water in upstream reaches and releasing it when needed in dry season for irrigation and navigation purposes which would benefit downstream countries (Hawiset, 2002a and 2002b; Richardson, 2002). The two completed dams (Manwan and Dachaoshan dams) were relatively small and had no noticeable impacts on the Mekong’s level (MRC-WUP-JICA, 2004). Although these projects are provided with large reservoirs for hydropower generation, they are operated under the mode of run-of-river (maintaining the inflow to be equal to outflow) without seasonal regulation. However, the third ongoing dam construction at Xiaowan (4,200 MW) over the Lancang River with a storage capacity of 11,500 million m3 for seasonal flow regulation, is expected to increase dry season flow by around 555 m3/s. Further, after completion of all the cascade projects, the low flow is expected to increase by around 1,230 m3/s (MRC-WUP-FIN, 2004). Chapman (1996) reported that when Xiaowan is completed in about 2010 the mean dry season flow at the Yunnan-Lao PDR border will increase by 40% and by 170% when the Nuozhadu dam is added to the system. The International Rivers Network (IRN) also analysed the run-of-river cascade dams in the Lower Mekong and found that the six dams and reservoirs recommended by China are on comparable scale to the Bonneville Dam on the Columbia River in the US Northwest and such massive dams cannot be considered run-of-river projects (Lawrence et al., 2007). Increasing dry season and decreasing wet season flow can be seen as helping to overcome water shortage during the dry season and contributing towards flood mitigation during the wet season (Kummu et al., 2004b). As already summarised, there is concern about the negative impacts of the dams. The filling of any reservoir with water requires extended diversions of river flows into the reservoir, diminishing the flows that reach downstream areas, and in the case of big or multiple dams, this can irreversibly change the ecosystem and water table downstream (Salidjanova, 2007). For example, unusual dry conditions in downstream reaches were observed in 1992 and this was perceived by many to be caused by the operation of Manwan dam probably when the dam was closed to fill its reservoir (Nguyen, 2003; Pearce, 2004; Salidjanova, 2007). As the hydrology of the local rivers is distinct from the Mekong mainstream, any long-termed decrease in mainstream Mekong discharge, which may now be inevitable given the unilateral decision to undertake the program of large mainstream dam construction of China, will result in the contribution to local catchment runoff increasing as a percentage of the total water supply to Tonle Sap Lake to possibly 40 - 50% from its current level of 25 ñ 38% (ADB, DOF and FAO, 2003). On average, the operation of the Manwan dam has significantly increased the discharge at Chiang Saen (the most upstream gauge station in the Lower Mekong River) because of the release of water from the dam during the dry season. The completion of the Xiaowan dam would increase the average river flow to the Lower Mekong River by 40% (Nguyen, 2003). However, the impacts of the construction and operation of the Manwan dam on the hydrological condition at Chiang Saen have not been observed at downstream locations (Nguyen, 2003). At Pakse, long-term average wet season discharge may be decreased by 12% due to large and small dam construction in the tributary rivers since the mid 1960s (ADB, DOF and FAO, 2003). One counter argument is that the total impounded mass of water is estimated to be approximately four percent of total annual Mekong flow, about half of this is live storage, and a fraction of that is consumed for irrigation or lost through evapotranspiration. Thus, the decline in total Mekong discharge to the Tonle Sap Lake may only be around one percent (ADB, DOF and FAO, 2003). However, Adamson (2006) raised awareness by an analysis of data trends. His evaluation looks at the annual rainfall at Vientiane from the 1920s until 2005 and shows that caution in trend analysis is 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. For example, during the 1950 to 2005 period it might be justified to conclude there was a decrease in annual rainfall, particularly after the late 1960s. However, if the longer period of
20
Change of Hydrology and Fishery Impacts in the Tonle Sap