The MRB is a large basin with specific physical environmental and complex socioeconomic conditions. Therefore, the basin is vulnerable to changes in climate and other related environmental factors. The most significant effects of these changes are in water resources. Adaptation to these changes should be considered in an integrated assessment context. Such integration is needed because individual countries, regions, resources, sectors and systems will be affected by climate change not in isolation but in interaction with one another. By referring to country studies on specific issues the geographical integration can be realized through linkages between different functional regions and analysing the transboundary issues. This geographical integration is particularly important because six countries with different socio-economic conditions are sharing the Mekong water.

Fortunately, the riparian countries recognized the need for cooperation in natural resources management. The Mekong River Commission of LMB has a long history of over 50 years in the coordination of studying and planning for resource development. Recently, the concept of the Greater Mekong Sub-region also helps in bringing the two upper basin countries closer to the LMB countries in terms of sharing resources and cooperating in regional development.

From the comparison of observed data and data from the HadCM3, we conclude that adjustment is needed for a large basin like the MRB, in particular for precipitation. The observed data, therefore, become important in this type of study. Data from the HadCM3 show that there is significant change in temperature in the MRB, about 4% in 2010-2039 and 12% to 16% in 2070-2099 in different scenarios. During 2010-2039, precipitation is not much different to the baseline 1961-1990. However, during 2070-2099, precipitation can increase by up to 10%. Changes in temperature are more homogeneous throughout the whole MRB, while changes in precipitation are very different from one location to another.

In this study, modelling and analyses were done for the MRB to identify the adaptation measures for the current as well as possible future issues in food security and environmental management related to climate changes. Even higher crop yield is expected; crop intensification will be the trend of agricultural development in the MRB to satisfy the food demand of future population under both A2 and B2 scenarios (about 1.3-1.4 times the current level in 2025 and 2.1-2.9 times in 2085). At present, crop intensification has begun in many parts of the MRB.

Outputs from the SLURP model for the 'business as usual' scenario without climate change but with crop intensification show that the expansion of the crop calendar into the dry season will have significant impacts on the minimum flow by a reduction of about 15-17% in 2010-2039 and 90-100% in 2070-2099. This reduction will cause serious water shortage at upstream sub-basins and salinity intrusion in the Delta, therefore measures at different scales will be needed, such as applying water-saving irrigation techniques at a field scale or reserving more water for the low flow season at the sub-basin level. A future decrease in the average stream flows of 10-15% also implies that less water will be available for hydropower.

Under climate change scenarios with different adaptation strategies for food and the environment, an increase in average monthly flow is expected, in particular at the most downstream sub-basins. However, an increase of about 10% will not be enough to compensate the higher irrigation requirements for food in the future. On the other hand, climate change will increase the maximum monthly and daily flows that cause higher flood levels up to 30-40% in 2070-2099. Moreover, minimum monthly flow will decrease about 20-30% during that period, i.e. more serious water shortage at upstream sub-basins and deeper salt water intrusion in the Delta will occur during the low flow season. The adaptation strategy of reforestation on shrub land in the whole basin will not help much in reduction of severe flood under climate change (less than 10%), possibly due to the high slope of the basin and the large agricultural area in the basin. However, other effects of this strategy on the environment such as reducing watershed degradation and maintaining biodiversity have been noticed. To implement this adaptation strategy, a reforestation plan is required with the support from government authorities at both national and local levels, i.e. the key roles should be given to those people who are living in the forest land.

Further analysis will be needed to incorporate changes in land use at field and basin levels as well as incorporate socio-economic conditions into the modelling activities. Also more adaptation strategies such as changing cropping pattern and the irrigation calendar, or increasing the storage capacity, etc. in different sub-basins will need to be considered in detail to deal with the higher flood level in the rainy season and the lower flow in the dry season.

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