Under intensifying climate pressures and accelerating water scarcity, the long-term viability of agriculture increasingly depends on farmers' capacity to implement effective adaptation measures. In many arid and semiarid regions, sustaining agricultural production is contingent upon proactive adjustments in water management practices. This study investigates the determinants of farmers' adoption of water-scarcity adaptation strategies in Dezful County, Khuzestan Province, southwest Iran. Drawing on the Social Identity Model of Collective Action (SIMCA) and extending it by incorporating impact size as an additional explanatory construct, the study develops a theoretically grounded framework for analyzing collective adaptation behavior among farmers. The findings demonstrate that the proposed model explains 68.7% of the variance in farmers' intentions to engage in adaptation initiatives. Group efficacy, social identity, negative emotions, and perceived impact size exert significant positive effects on adaptation intentions. By advancing a collective-action perspective on climate adaptation in agriculture, this study contributes to the refinement of theory-driven explanations of pro-adaptive behavior. It also provides actionable insights for policymakers seeking to design targeted, socially embedded interventions to enhance sustainable water resource management in water-stressed agricultural systems.