Agriculture sustainability standards and certification are increasingly used by the private sector and civil society to incentivize and support environmental conservation and improved rural. However, evidence of impact is limited by methodological challenges that hamper the quantification of certification-induced changes, especially beyond farm level. This paper aims to explore the changes to soil and nutrient regulation ecosystem services from the adoption of Rainforest Alliance tea certification in the Kenyan Upper Tana River watershed. In this study we: i) apply ecosystem service models to simulate the effect of farm-level practices for before and after-certification scenarios, and; ii) evaluate the model applications for their ability to guide future decision making. Our scenario results indicate that a widespread adoption of agricultural practices prescribed in the certification standard reduces sediment export into watercourses. However, an increase in fertilizer use by certified farmers is estimated to result in greater nitrogen and phosphorous loads. Our scenario analyses are highly sensitive to input data and model choice, but show similar relative impacts of tea certification. Opportunities to improve spatial impact measurements to support decision making can be found in the systematic accounting of land management practices by certification organizations and increased remote sensing image accessibility.