Chao Phraya River is the most important and largest river flowing from Chai Nat Province to estuary at the Gulf of Thailand in SamutPrakan Province, located at central part of Thailand. The climate of the Chao Phraya River has a tropical wet and dry or savanna climate, which generates wet and dry seasons of more or less equal length. The monsoon season is usually from May until late September and/or early October. In the wet season, averagely 1 - 2 tropical depressions occur over much of the area from August to October of the year. The average annual discharge is 718 m³/s and rainfall varied between 1122 to 1511 mm, depending on monsoon direction and elevation. However, some parts of the catchment continue to suffer from drought problems due to the uneven distribution of rainfall. Some areas experience both flooding and drought conditions in a single year, due to temporal and spatial uncertainties in the monthly rainfall and/or the poor management of the conveyance infrastructure. The common practice in Thailand is to manage the risks after considering which areas are likely to be vulnerable to either flood or drought. Failure to manage risk by addressing one aspect at a time can lead to adverse results. Therefore, climate change and an association with managing flood and drought risks are new challenge in Thailand and becoming increasingly important.

The MIKE11 model has been used. This numerical model simulates water flow and salinity as a consequence of low flow conditions. The shallow water equation was used for simulation of 1-D unsteady water flow and transportation of mass was used for salinity. To modelling the river network of the Lower Chao Phraya, a digital elevation map (DEM) was applied. The model input data were cross-section, flow discharge, water level, side flow and salinity. The MIKE11 program, 6-points Abbott’s finite difference scheme was used to solve governing equations, consisting of separate modules each representing a different procedure in calculation process. A first module calculated hydrodynamics of river flow (HD module), and the next module of transportation of mass (salinity intrusion; AD module). The model setup of plan view and longitudinal profile of the Lower Chao Phraya are shown in Figure 1.

The comparison of time series of measured and simulated water surface level and salinity at two major gauge stations and Manning’s roughness coefficient (n) are shown in Figure 2 and Figure 3. Good agreement between the simulated and measured hydrographs for the low flow events was achieved by considering side flow and pumps in the areas. The coefficient of determinant (R²) and Index of agreement (IA) have been used as the main criteria to judge whether the data fitted between measurement and simulation. The comparison model was followed by adjusting important coefficient of two parts. The results of manning (n), global dispersion factor, global exponent and K_mix were in the range of 0.022, 500 - 1800 m²/s, 0.1 - 0.8 and 500 - 1800 hr⁻¹, respectively. The results of comparison between models and observation data revealed the order of forecasting error, R² and IA in the range of 0.88 - 0.91, 0.94 - 0.97 for water level and 0.86 - 0.96, 0.75 - 0.94 for salinity. These indicate well fitted between measured data and this proposed model. Good agreement between the simulated and measured water surface level was achieved by considering side flow and pumps in the areas for model verification. It was observed that the values of R² and IA for two major gauge stations in verification period were between 0.93 - 0.94, 0.97 - 0.98 for water level and 0.71 - 0.91, 0.72 - 0.87 for salinity, indicating well fitted between measured data and this proposed model (Figure 4 and Figure 5). Good performance of simulated results was observed in both water flow and salinity intrusion characteristics, therefore, indicating that model simulation is reasonable.

For model application, a scenario RCP8.5 from IPCC report was simulated. The comparison of time series of measured and simulated flow discharge at the major gauge stations are shown in Figure 6 and Figure 7. It was found that sea water level at the Chao Phraya estuary had rising and betaking tendency to intrusion of sea water level. Salinity was shown to be in the same tendency. For IPCC SRES in the year of 2100, sea water level rising in RCP8.5 scenario was 1.16 m, and salinity values and intrusion distant were in the range of 0.38 - 0.45 g/l and 142.0 - 157.0 km (from Chao Phraya Dam), and salinity at Samlae Station were 0.29 - 0.37 g/l. The value of 0.25 g/l exceeding standard and the pointed tip of salinity was at Bang Sai District, Ayutthaya Province. The worst case scenario, a constant value of salinity of 30 g/l was adopted at the Gulf of Thailand. It was found that salinity at Samlae Station was 0.99 - 1.70 g/l, the value of 0.25 g/l was at Pratuchai Sub-district, Ayutthaya Province, 133.0 km from Chao Phraya Dam. We could also observe from the simulation that the climate change would have more conspicuous effects in rising up sea water. However, Metropolitan Waterworks Authority reported that climate change affected salinity intrusion and raw water supply in Lower Chao Phraya River. This observation demonstrated that Samlae Pumping Station will not be available when salinity is over 0.25 and/or

0.50 g/l, and water supply shortage. The mitigation plan has suggested the diversion flow from Mae Klong River and construction of new pump station for short-term and long-term, respectively. The proposed locations of new raw water pump station along the Chao Phraya River from Ayutthaya to Singburi Province have been investigated. It was found that the most suitable sites when evaluation by multi-criteria analysis (MCA) is at KlongPongpheng, Ayutthaya Province (site 4).

Water level change will affected the water supply in urban, and agricultural areas. Salinity level less than 0.2 g/l was not impacted; it can use irrigation for any planting. Salinity level from 0.2 had little impact, while salinity level from 0.5, and salinity level more than 1.5 g/l were highly impacted (Figure 8). For agricultural sectors, the value of 0.20 g/l exceeding standard and the pointed tip of salinity was at Ban Mai District, Ayutthaya Province (123 km from Chao Phraya Dam).