This paper investigates the effect of social capital between irrigation canal head-enders and tail-enders on their water allocation problem. In irrigation management, the water allocation problem between head-enders and tail-enders is one of the serious problems. Using unique natural and artefactual field experiment data as well as general household survey data collected by JICA, this study finds that social capital, especially trust toward their tail-enders, has a significantly positive effect on satisfaction with water usage among head-enders. Considering the fact that the incentive structure of irrigation water allocation for head-enders closely resembles that in the dictator and trust games, this finding also supports the validity of experimentally measured social capital. In addition, this study deals with the simultaneity bias between satisfaction level and experimentally measured social capital, and finds that OLS estimators are downward biased, which is consistent with the hypothesis that scarcity of resources enhances social capital.