A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Water Engineering and Management

Thesis (Ph.D.) - Asian Insitute of Technology, 2025

Climate change is expected to significantly impact agricultural production, though the extent depends on factors such as its severity, crop types and farming practices. The Nan River Basin (NRB) in Thailand{u2014}a key maize cultivation area{u2014}is increasingly vulnerable to climate induced drought. This study adopts a multi-faceted approach to assess drought severity, its effects on maize suitability and potential adaptation strategies in the upper NRB. Historical and future drought patterns were analyzed using the Standardized Precipitation Evapotranspiration Index (SPEI), based on four CMIP6 Global Climate Models (GCMs) under SSP2-4.5 and SSP5-8.5 scenarios. Projections suggest continued warming and increased rainfall variability, leading to more frequent, prolonged and severe droughts. Notably, a 4.2{u202F}°C rise in minimum temperature is expected in the far future (2063{u2013}2097) under SSP5-8.5. SPEI results indicate the wetter wet seasons and harsher dry seasons, particularly under SSP5-8.5. Maize suitability models show a shift in viable cultivation areas under both scenarios. Historical data highlight frequent droughts, exposing the vulnerability of rain-fed systems. SWAT model results project increased wet-season flows but reduced dry-season water availability, posing risks to maize during critical growth periods{u2014}especially under SSP5-8.5{u2014}exacerbated by higher evapotranspiration. AquaCrop simulations forecast near future yield increases (+5.1% under SSP2-4.5; +2.6% under SSP5-8.5) but declines in the far future ({u2212}5.2% and {u2212}7.7%, respectively), emphasizing the need for adaptation. Supplementary irrigation and delayed planting (early May) can help reduce yield losses. Adaptation strategies are framed around four pathways: resource optimization, technological innovation, crop diversification, and conservation. Measures such as drought-tolerant maize, adjusted sowing dates and improved water and crop management proved effective in reducing climate impacts. These findings highlight the urgency for integrated, location-specific adaptation planning. The research offers actionable insights for policymakers, farmers and researchers to support sustainable maize production under evolving climate conditions.