In the case of water quality regulation, the maximum potential (mitigation) benefit in a given region or watershed is the total amount of pollutant (i.e. nitrogen load) requiring retention by vegetation in that area. We use rural populations (within 100 km watersheds) as the population exposed because they are presumably less likely to have water treatment options. The potential benefit provided by nature is nitrogen pollution retention; and we can characterize nature’s contribution as the proportion of total nitrogen pollutant load retained by ecosystems.
The potential benefits provided by nature, which are often called “ecosystem services” (but should be thought of as the potential supply of a service, and only truly becomes a service when combined with human demand for the service, shown as the realized benefit) may be measured in terms of tons of pollutant retained. We emphasize that a proportional representation of nature’s contribution to providing potential benefits is important to track differences or changes across space and time; as realized benefits provided by nature could increase alongside (or due to) increases in maximum potential benefits or population exposed, though nature’s contributions may remain the same. That is, if pollutant run-off increases from elevated fertilizer use, a constant proportional contribution of nature would result in higher levels of the corresponding realized benefits, pollution retention, even if conditions for people (in terms of water quality) deteriorate. The relative proportion of nature’s contribution, along with people’s needs, especially for the most vulnerable people, are more useful metrics than realized benefits alone when considering change across several variables at once (stressors, people, and nature), as they reveal where and when nature plays a key role in delivering benefits.
We also examine the benefits not provided by nature, or benefit gaps, people depend upon for their well-being (which could be filled to some extent by other forms of capital, e.g., infrastructure), and the populations exposed to changes in benefit gaps for each NCP in future scenarios. We use nitrogen export (the amount not retained by vegetation that therefore enters waterways and drinking water supplies as pollution) as the measure for the benefit gap in water quality regulation. This benefit gap results in the outcomes people will actually face and perceive – drinking water pollution in this case— and is what will determine people’s well-being, the visible component of NCP. It does not by itself, however, reveal the role nature plays in contributing to that well-being.