For the IA approach, based on the work of de Ridder [7] and the EU SEA framework, four phases were identified [26]:

• Problem analysis: definition of the assessment situation;

• Finding options: identification of the objectives;

• Analysis: impact assessment;

• Follow-up: monitoring.

Our approach is expressed in the following Figure 1. The figure visualizes the interconnections between the natural resources and the socio-economic system, which have to be considered by the IA FEW nexus.

Figure 1.

The IA model for the FEW nexus

(source: UNEP 2016 and Authors 2017 based on http://www.unep.org/esm/Waterecosystems/Thematicareas/Water-Energy-Foodnexus/tabid/131727/Default.aspx, 2016)

The socio-economic system is part of the FEW nexus, a subsystem of the overall ecosystem, which provides the water supply, energy resources and food for the socio-economic system. Since 2000, the German socio-economic system has been engaged in a social discourse about the sustainability path for Germany [30]. In 2002, the German government developed a sustainability concept for Germany based on aggregated and disaggregated data for Germany [22]. In 2014, the German government set up its new research agenda for the transformation process of the green economy in Germany [31], which led to a completely revised sustainability strategy in 2017 [25] that is in line with the UN SDGs [32] as Table 1 shows. With its new sustainability strategy, the government also revised its sustainability goals and targets for a sustainable development of Germany [24] and in its green economy process the government outlined the political action fields to achieve these goals [33]. The indicators of the 2017 sustainability strategy represent important aspects for a sustainable society in the view of the government. The selection of the indicators also structures the social discourse about a sustainable development of Germany and reveals to the public the values the government associates with its strategy.

In the following, the FEW nexus issues are analysed with selected FEW-nexus-related sustainability indicators of the German strategy (Table 1) and the new FEW metrics [Fisher Nexus Index (FNI)]. The IA of the FEW nexus sketches the methodological framework for analysing the sustainability path of the German FEW nexus and reveals the values connected with the sustainability strategy.

On 25 September 2015, the United Nations adopted the “2030 Agenda for Sustainable Development” and thereby also 17 sustainable development goals (UN SDGs) to assess global development [32], and in 2017 the German government published its revised sustainability strategy [24], which implemented the UN goals in Germany and selected 36 sustainability measures to analyse the sustainable development of Germany. The government uses its targets to define its understanding of the sustainable development of German society and its nexus systems.

The German FEW sector can be described by 10 of the 17 UN SDGs and by 28 indicators of the 36 German sustainability measures (Table 1). This disaggregated indicator set for the FEW nexus demonstrates that it is possible to operationalize the UN goals in the analysis of the German FEW nexus, as the following table shows [25].

The government set up its 36 indicators and sustainability targets for these key issues to avoid the impression that its sustainable development strategy is merely a list of good intensions [25], and in this way it renewed its vision of a sustainable pathway for Germany of 2002 [22] and enabled a description of the sustainable development path for the FEW nexus sector. The goals and targets are new revised criteria for quality of life and well-being in Germany.

Both the UN and the German government are driven by the conception expressed in the definition of sustainable development in the Brundtland Report: ‟A development that meets the needs of the present without compromising the ability of future generations to meet their own needs [34]”. The UN committed itself again in 2015 to intergenerational justice by stating: “We are determined to protect the planet from degradation, including through sustainable consumption and production, sustainably managing its natural resources and taking urgent action on climate change, so that it can support the needs of the present and future generations” [32]. Hence, justice between the present generation (intragenerational justice) and justice between different generations (intergenerational justice) are still central guidelines for any sustainable development concept [35]. Thus, justice is the defining ethical norm for sustainable development.

The “Ideas of justice are the core normative guidelines with regard to the sustainable use and conservation of the services provided by ecosystems, such as food and fresh water production, flood protection and erosion control” [35], as Glotzbach und Baumgärtner wrote thereby stressing the necessity of justice for the FEW nexus and its central institutional framework. Because as Rawls put it: “Natural distribution is neither just nor unjust, nor is it unjust that persons are born into society at some particular position. These are simply natural facts. What is just and unjust is the way that institutions deal with these facts” [36]. According to Rawls’ reasoning, institutions are thus responsible for enabling a fair society. For Rawls “Justice is the first virtue of social institutions” [36] and of sustainable development and has to be reflected in the way development is measured.

In the next assessment step, the FEW Nexus Index is derived in order to calculate the degree to which sustainability has been achieved in the FEW nexus sectors on the basis of the German sustainability strategy and against the background of the UN SDGs.

Thus, inter- and intragenerational justice also represents a central issue for the development of the FEW nexus and this aspect is integrated in our FEW measuring concept by operationalization of the social discount rate.

Introduction – social discount rate. The social discount rate expresses the preferences of the current generation for the future (time preference) [37], i.e. the view of current society of the needs of future generations and the willingness of the current generation to consider future needs in their current decisions [38]. In our measuring concept, the discount rate based on the neo-Austrian capital theory [39] is interpreted as the difference in the price of a good at different points in time [40]:

where p is the price of good G1 in period 1 and 2 and r⁰ is the discount rate. The discount rate expresses the Marginal Rate of Substitution (MRS) of contemporary for future consumption [40], [41]. Hence, the price difference is considered in our measuring concept in order to engage with intergenerational justice issues as shown in the SDGs of Table 1.

The Fisher Quantity Index (FQI). The Laspeyres Quantity (LAQ) Index assesses the current (U⁰) and future household utility (U^k) with the current (x⁰) and future consumption (x^k) of goods at current prices (p⁰), as the following equation shows [42]:

In contrast, the Paasche Quantity (PAQ) Index assesses the current and future consumption of goods represented by the sustainability indicators at future prices:

The FQI [43]-[45] is the geometric mean of the Laspeyres Price Index and the PAQ Index [46]:

Hence, an index is needed that will assess the current and future consumption of the goods represented by the sustainability indicators at both current and future prices.

The Fisher Nexus Quantity Index (FNQI). The FNI is now based on 28 measurable indicators covering the key issue of sustainable development of the FEW sectors, where x⁰ represents the value of the indicator (i) at the starting period of time 0, x^k represents the value of the sustainability target of the sustainability indicator at period k.

The discount rate is included in parts of the Fisher Index to obtain the FNI and consider intergenerational issues in measuring sustainability.

Hence, the FNI is constructed in three major steps:

• The Laspeyres Nexus Quantity Index (LANQ): LAQ 0,k,i,n = p i,n 0 − x i,n k p i,n 0 − x i,n 0 , (5) where 0, k is point in time, i is indicator 1... 28 and n is nexus sector 1... 3. By introducing the discount rate: LANQ 0,k,i,n = ( p i,n k ⋅(1+r) )− x i,n k ( p i,n k ⋅(1+r) )− x i,n 0 (6) where 0, k is point in time, i is indicator 1... 28 and n is nexus sector 1... 3.

• Paasche Quantity Index: PAQ 0,k,i,j,n = p i,j,n k − x i,j,n k p i,j,n k − x i,j,n 0 (7) where 0, k is point in time, i is indicator 1... 28 and n is nexus sector 1... 3.

  By introducing the discount rate in the Paasche Index, the Paasche Nexus Index (PANQ) is obtained: PANQ 0,k,i,j,n = ( p i,j,n 0 1+r )− x i,j,n k ( p i,j,n 0 1+r )− x i,j,n 0 (8) where 0, k is point in time, i is indicator 1... 28 and n is nexus sector 1... 3.

• FNI: The FNI is the result of combining the Paasche and the Laspeyres Index: FQNI 0,k,i,n = p i,n k ⋅(1+r)− x i,n k p i,n k ⋅(1+r)− x i,n 0 ⋅ p i,n 0 1+r − x i,n k p i,n 0 1+r − x i,n 0 (9) FQNI 0,k,i,n = LANQ 0,k,i,n ⋅ PANQ 0,k,i,n (10)

The analysis of the FEW nexus sector represents a new scientific research area [47] and a new local, national [48] and international governance field which needs quantitative data to develop new political options in a political discourse [48]. The democratic institutions are at the centre of this discourse process, because for far-reaching political decisions the political decision makers need sufficient political legitimation based on democratic approval by the citizens [49]. This legitimation can only be effected by elected parliaments, which are the institutional centre of the political life of a country [50]. Voßkuhle stresses that the political decisions of parliament need statistical data and a ‘culture of reflected numbers’ [50], which uses the quantitative research results as a starting point for political reasoning and debate. The research results are not simply to be executed by the political institutions [50]. Especially the sustainability indicators [51] are an important tool for initiating a political debate and social learning process.

We have to consider that no matter how well the indicators are calculated, they cannot replace a democratic political decision-making process. In particular, the target values of the numerical sustainability indicators are simply the outcome of a social discussion process resulting in political targets approved by parliament [52]. Hence, also the new FEW nexus policy process needs a ‘culture of reflected numbers’ as Voßkuhle puts it [50] to discuss the policy options and values behind the measurement concept.

However, Voßkuhle also stresses [50] that for such a culture of reflected numbers an open political and social discourse in the sense of Karl Popper [53] is needed because democracy lives first and foremost from and in a climate of functioning public opinion [53], which discusses the measured numbers [54]. This culture can help to avoid “short-term and ‘common-sense’ solutions (which) often bear unintended consequences that may produce worse situations over the long term than if no action had been taken at all” [55].

The result of the culture of reflected numbers can be that the targets of the indicators will be readjusted or new indicators selected to engage more effectively with German social reality. The goal of the culture of reflected numbers is to avoid the automatism that the measuring results have a self-executing character, which can no longer be questioned by society. The validity of the results has to be scrutinized and the meaningfulness of the indicators has to be analysed in an ongoing monitoring process [52].