Economic valuations of ecosystem services often transfer previously estimated global unit values to the geographical setting of interest. While this approach produces quick results, its reliability depends on how representative the large-scale average unit values are for the given local context. Here, we estimate the values of three ecosystem services (ES)—water filtration, nutrient cycling, and carbon sequestration—in the Grand River watershed (GRW) of southern Ontario, Canada. The watershed covers nearly 7000 km2, has a humid continental climate and a population of close to one million people. Land cover is dominated by agriculture. We compare ES valuations using locally derived (i.e., GRW-specific) unit values to valuations based on unit values from a regional database and those compiled in the global Ecosystem Services Valuation Database (ESVD). The regional database includes mean unit values from three case studies within southern Ontario and one boreal watershed in British Columbia. As expected, the regional database yields average monetary values for the three ES that are close to those obtained using the local unit values but with larger associated uncertainties. Using the ESVD, however, results in significantly higher monetary values for the ES. For water filtration, the ESVD value is more than five times higher than the regional and local estimates. We further illustrate the effect of the extent of aggregation of forested and agricultural land categories on the ES values. For example, by subdividing the forest category into three subcategories (deciduous, coniferous, and mixed forest), the estimated value of the carbon sequestration service from forested areas within the GRW decreases by 7%. Overall, our results emphasize the importance of critically assessing the origin of unit values and the land cover resolution in ES valuation, especially when ES valuation is used as a policy-guiding tool.

• Potential and realized values of a bundle of six ecosystem services are estimated for Southern Ontario, Canada. • The realized value of the ecosystem services averages 51% of the potential value. • Within the Greenbelt, a protected area surrounding the Toronto conurbation, 61% of the potential eco-services are realized. • The spatial distribution of realized ecosystem services helps inform environmental policy-making. The full production of a given ecosystem service is called the potential ecosystem service; the fraction of the potential ecosystem service that is actually used by society is referred to as the realized ecosystem service. Because they are directly contributing to human well-being, the realized ecosystem services are of particular socio-economic importance. A key challenge faced by the economic valuation of ecosystem services is how to differentiate between realized and potential ecosystem services. Here, we address this challenge for Southern Ontario, which is the most densely populated region of Canada. We apply the Co$ting Nature model to generate the combined spatial distribution and use intensity of a bundle of six ecosystem services: water provisioning and supply, water quality, carbon sequestration, carbon storage, flood regulation, and nature-based tourism. The relative distribution of the potential ecosystem services is then combined with region-specific unit values for the land covers supplying the ecosystem services. The unit values are expressed in 2017 Canadian dollars per hectare and per year. Our analysis yields a total potential value of the bundled ecosystem services of $19 billion per year for Southern Ontario. To estimate the value of the realized ecosystem services, the potential values are scaled by the corresponding relative use indices. The resulting value of the realized ecosystem services is $9.7 billion per year, that is, about 50% of the value of the potential ecosystem services. The importance of accounting for the use intensity of ecosystem services is illustrated for the Greenbelt, a protected area of about 7600 km 2 surrounding the Greater Toronto-Hamilton conurbation, which is home to more than nine million people. Within the Greenbelt, 61% of the value of potential ecosystem services is realized, significantly higher than the regional average. Of particular importance is flood regulation by the Greenbelt, given the growing threat of urban flooding in the Toronto area.