Indicator System Indicator Aggregation

Indicator calculation requires multiple sources of data, like hydrologic models, local knowledge and data, global datasets and stakeholder surveys. Once calculated, indicator values are normalized to a scale between 0 to 100, with 100 being more natural or sustainable and 0 being highly modified or less sustainable).

Aggregating indicators occurs in two steps: 1) spatial aggregation of individual indicators across sub-basins (where relevant) to provide basin-level indicator values; and 2) aggregation of all indicators evaluated at the basin-level to provide the index value for a given component. In the first step sub-basin indicator values are be weighted according to the proportion the sub-basin area makes up of the total basin area. These weights should then be normalized to sum to 1. Sub-basin values are then aggregated as the weighted geometric or arithmetic mean of all sub-basins for which the indicator was evaluated.

In the second step, where indicators are aggregated at the basin level to form a component index, weights can be applied to each indicator to denote greater or lesser importance of the role of the indicator for assessing freshwater health in the basin. There are a variety of methods for assigning weights including, but not limited to, expert elicitation, the Delphi method (Brown 1968, http://www.rand.org/topics/delphi-method.html) or the Analytic Hierarchy Process (Saaty 1990). Each method encourages stakeholder participation. It is not necessary to apply weights to the indicators at this aggregation step; they should only be applied if there is good reason to believe that the indicators play disproportionate roles in measuring freshwater sustainability. However, we do not recommend that weights be applied to the Ecosystem Vitality indicators. Weights should only be applied if there is strong evidence that some ecosystem processes or attributes play a greater role in ecosystem functioning than others. This is an empirical question rather than a subjective one, which requires a great deal of understanding of ecosystem process and pattern

Values are then be aggregated to give a central tendency (e.g., arithmetic or geometric mean) to provide a separate index for each of the three components of Ecosystem Vitality, Ecosystem Services, and Governance & Stakeholders. We recommend using the geometric mean over the arithmetic mean as it is more sensitive to changes in multiple indicator values. For instance, under the arithmetic mean, a change for the better in one indicator can be offset by a change for the worse in another - resulting in no change in the aggregated index. Under the geometric mean, such a change would be reflected in the index value. The indices are not to be aggregated across the three components due to differences in their resultant interpretations and methods of evaluation. For instance, we recommend conducting surveys to derive subjective values for the Governance & Stakeholders indicators, whereas the indicators within Ecosystem Vitality and Ecosystem Services should be based on empirical data and models wherever possible. Furthermore, treating the indices for the three components separately can highlight where the greatest problems, or the greatest contributors to sustainability, lie.