The simplest version of a sustainability indicator related to the model in the previous section is calculated by dividing the number of inconsistencies by the total number of flows, resulting in an index between 0 and 1 (in our example, 9/23 = .39). A lower number implies a higher level of sustainability and vice versa. This indicator is independent of the current state of the stocks or related indicators. Similar to existing sustainability indicators, the QSSI consists of two layers of information: an index (i.e., a single number that summarizes the information contained in the underlying system) and the body of information that in this case consists of a model in matrix form and in more common approaches of lists of indicators. In both cases, a sound interpretation of the index is impossible without an insight into the underlying body of information. By the logic of the QSSI, the number of inconsistencies in the system can initially be decreased in three ways.
• We can try to find a way to make an inconsistent flow disappear (decoupling of stocks). In our example there is an inconsistency as buildings take up retention capacity. Both are desired, and if there was a possibility to build houses above the potential flood water level (e.g., on poles), then the negative flow between the two stocks would disappear. This would result in a QSSI of 8/22 = .36.
• The QSSI is reduced even more if we manage to turn the sign of the flow. If we could stimulate such agriculture that can be applied in retention areas and actually profit from occasional flooding of these areas, then the QSSI in our example would decrease from .39 to 8/23 = .35.
• A third way to decrease the QSSI is to add consistent flows. If we can find a way to give space to water in such a way that agricultural activity is stimulated at no cost for nature development (admittedly a far-fetched example), then we would add a consistent flow. The QSSI would then decrease from .39 to 9/24 = .38.
These interventions for increasing the ability of the system to sustain itself on a desired trajectory in the long run are all concerned with the structure of the system. By changing the system structure (adding, removing, or changing flows) in a desired manner we make the system more sustainable. Considering the enormous resources needed to actually change existing flows in the real world, the QSSI is likely to change very slowly. This reflects the fact that the indicator is concerned with the slowly changing system structure and its direction rather than short-term symptoms in the form of flows.
But not all inconsistencies can be solved this way. At some point no further system improvements are possible. We will have to make choices between different sets of inconsistencies. In our example one of these choices could be to weigh the importance of agriculture and the economic benefits of it against the importance of retention capacity and the potential damage done to the agriculture by floods and nature (all the stocks that agriculture is influencing). It is at this point that the desired direction of a stock dependent on the desired direction of other stocks is discussed. In the Dutch case, policymakers have made a very explicit choice and decided that agriculture should recede in order to provide room for nature (RIVM 1997).
At the cost of the foregone economic gain from agriculture, we resolved our conflicts with the retention capacity, the potential agricultural damage caused by floods, and nature. We thus add one inconsistency and remove three others. This results in a QSSI of 7/23 = .30. The QSSI is reduced through a change in our goals. This choice is not a choice to be made in a sustainability assessment but one that should be delegated to democratic processes.
The ultimate consequence of this is that we can achieve a higher degree of sustain-ability if we are willing to give up some of our goals and thus redefine what is desired. Traditional lists of indicators can help us make an informed choice about desired states and the relative urgency of different inconsistencies.
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