Abstract:Collaborative environmental governance seeks to engage diverse stakeholders to tackle complex challenges efficiently, sustainably, and equitably. However, mixed empirical evidence underscores a need to understand the conditions under which particularly equity is or is not achieved. Here, we use the empirical case of California Sustainable Groundwater Management to quantify the extent to which vulnerable small and rural drinking water users' needs are addressed in collaborative groundwater planning. Drawing on a diverse array of mixed method data, we then employ Boosted Regression and Classification Trees (BRCT) to assess potential driving factors including collaboration, representation, elite capture, stakeholder engagement, and problem severity/salience. We find each to be influential, highlighting their relevance for equitable planning. We also find evidence that these relationships are complex and outcome specific. Nonetheless, the overall effect on the three equity measures is modest at best. More institutional analysis of collaborative governance regimes from diverse contexts is needed to build a comprehensive understanding of how to meaningfully advance social and environmental equity in such decentralized reforms. Based on our results, we suggest the answer, if there is one, may transcend current focal domains such as stakeholder representation and engagement.