Thailand plays a central economic and policy-making role in Southeast Asia. Although climate change adaptation is being mainstreamed in Thailand, a well-organized overview of the impacts of climate change and potential adaptation measures has been unavailable to date. Here we present a comprehensive review of climate-change impact studies that focused on the Thai water sector, based on a literature review of six sub-sectors: riverine hydrology, sediment erosion, coastal erosion, forest hydrology, agricultural hydrology, and urban hydrology. Our review examined the long-term availability of observational data, historical changes, projected changes in key variables, and the availability of economic assessments and their implications for adaptation actions. Although some basic hydrometeorological variables have been well monitored, specific historical changes due to climate change have seldom been detected. Furthermore, although numerous future projections have been proposed, the likely changes due to climate change remain unclear due to a general lack of systematic multi-model and multi-scenario assessments and limited spatiotemporal coverage of the study area. Several gaps in the research were identified, and ten research recommendations are presented. While the information contained herein contributes to state-of-the-art knowledge on the impact of climate change on the water sector in Thailand, it will also benefit other countries on the Indochina Peninsula with a similar climate.

In northeast Thailand, 17% of the total agricultural land is classified as salt-affected. In the future, climate change may exacerbate salt-affected soil problems. Therefore, in this study, we conducted a field survey to evaluate seasonal changes in soil electrical conductivity (ECe) in salt-affected paddy areas of Ban Phai District, Khon Kaen Province, northeast Thailand. Fifteen soil samples were collected every 2 weeks from October 2016 to December 2018, and the ECe, soil water content, and soil textures were analyzed. Then, the HYDRUS-1D model was applied to estimate seasonal changes in the salinity level, and the simulated results corresponded well with observed data. Using HYDRUS-1D and the global circulation model (MIROC5) outputs under the Representative Concentration Pathways 8.5 scenario, future ECe was predicted. Under a temperature increase of 2.8°C from 2016 to 2100, annual potential evapotranspiration increased from 1,430 mm (2016-2025) to 1,584 mm (2081-2100). The average ECe in cultivation season increased from 2.63 dS/m (2016-2025) to 3.31 dS/m (2081-2100). As a countermeasure to mitigate soil salt accumulation, a 5 cm reduction in groundwater level offsets the negative impact of climate change, and a 10 cm reduction significantly improves the soil ECe relative to the current soil salinity level.

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Field experiment was carried out at Agronomy Experimental Farm, Faculty of Agriculture, Khon Kaen University in 2015-2016 to investigate the response of cassava to supplementary irrigation during the dry season month. The experiment was laid out in split plot design with four replications. The main plots comprised two cassava varieties (Huaybong 80 and Rayong 11). The sub l included four levels of drip irrigation [I-20, EV-40 mm (crop received 20 mm of water when daily cumulative pan evapolation value reached 40 mm during the dry season months)]; [I-20, EV-60 mm] ; [I-10, EV-40 mm] ; [I-10, EV-60 mm] and [I-10] (cassava under rainfed condition without additional irrigation)]. Results indicated that irrigation at (I-20, EV-40 mm) produced maximum the fresh (52 t ha-1) and dry (22 t ha-1) storage root yield. Huaybong 80 variety a gave significantly higher the storage root yield than that of Rayong 11 variety. The highest starch content also was obtained in the (I-20, EV-40 mm) treatment. There was no significant difference in the starch content between the two cassava varieties. Water were applied in treatment [I-20, EV-40 mm], [I-20, EV-60 mm], [I-10, EV-40 mm] and [I-10, EV-60 mm] was an average 299 mm, 194 mm, 150 mm and 97 mm, respectively during the growing season. Water use efficiency was the highest (35.3 kg ha-1 mm-1) in the [I-20, EV-60 mm] treatment.