Water retention plays a critical role in terrestrial ecosystem service. However, regarding its definition and calculation, there is a long debate in academia, which illustrates its importance in practice, and simultaneously demonstrates the complexity and vagueness of this essential concept. Thus, there is an urgent need to clarify its definition and calculation method based on basic ecological and hydrological theories, and eventually promote science-based decision-making and integrated water management. Interestingly, we observed that for the same term of "water retention function", ecologists intended to concern the terrestrial ecosystem's water storage capacity (Smax), while hydrologists concerned more about the water yield from the catchment (Q). Both perspectives have their own rationality, but with totally different vision and emphasis. By theorical discussion and data analysis, we found that water storage capacity (Smax) and water yield (Q) indeed have strong connection, but they are definitely two different concepts. The Smax of terrestrial ecosystem determined the separation of precipitation into either evaporation (green water) and Q (blue water). The size of Smax in most cases trades off with the amount of Q. We further revealed that the root zone storage capacity (SRmax) of ecosystem is at the heart of water retention function assessment, and plays a key role linking blue and green water. The SRmax is the result of ecosystem's adaption to its climate, and can be derived by the classic method to design reservoir, i.e. the Mass Curve Technique (MCT). Lastly, we gave three recommendations: (1) simultaneously evaluating green water retention capacity and blue water yield in practice; (2) further investigating the water retention functions of more water bodies, e.g. glacier, snow cover, and groundwater; (3) synergizing natural and artificial water retention capacities to enhance the water use efficiency in both the ecosystem and our economic-social system.

Keywords： water retention function; ecohydrology; root zone storage capacity; water balance equation; blue water and green water