Water is essential for life and connects many parts of Earth’s complex system. Yet scientists still struggle to predict how the water cycle will change in the future because current models do not fully and accurately capture how water moves through and is transformed by the Earth system. Stable water isotopes---special forms of oxygen and hydrogen in water---carry unique "fingerprints" that reveal where water comes from, how it travels through the atmosphere, ocean, and land surface, and how it interacts with the environment. However, most of today’s Earth system models do not simulate these isotopes, leaving valuable data from field campaigns, satellites, and ancient geological records underused. Past efforts to build isotope-enabled Earth system models have faced several obstacles; outdated software and software practices make the models difficult to maintain, and a lack of training resources makes it challenging to help new researchers collaborate effectively to enhance water cycle science. This project aims to solve these problems by using advanced software concepts and practices to develop the isotope-enabled Community Earth System Model (iCESM). The innovative Earth system model is rigorously validated using diverse observational datasets. The project demonstrates the profound opportunities afforded by iCESM by providing new insights into atmospheric dynamics, plant-water and ecosystem hydrology, and changes to ice sheets and glaciers. It enables broader research on wat