Using group-level and household-level data collected in 2006-2007 in two districts in Hubei Province of China, we investigate social capital and water source variables and assess the factors influencing the formation of a sub-group, which is a sub-set of the original water user group (WUG). We use a probit model and regression analysis to examine the impact of sub-group formation on the adoption of alternate wetting and drying (AWD). Results show that when social capital is strong, farmers tend not to form sub-groups because strong social capital can prevent overuse of water. However, when social capital in a group is weak, farmers need to avoid overuse of water by forming a sub-group. Therefore, if farmers in the group of weak social capital are physically constrained and thus fail to form a sub-group, overuse of water is more likely. Such failure occurs when the existence of many water sources physically prevents farmers from forming a sub-group. We also found that when social capital is strong, farmers adopt the AWD practice even without forming a sub-group. ©2010 Elsevier B.V. All rights reserved.