Global Freshwater Targets
The reduction targets quantify the extent to which current water consumption must be reduced to remain within sustainable water availability limits while also maintaining environmental flow requirements.
Calculation Framework
The freshwater target calculation begins with Blue Water Runoff (BWR), which represents the natural water availability generated within a basin. For additional details, see Global Environmental Flows.
Environmental Flow Requirements (EFRs) are then estimated from BWR using three environmental flow methods: Richter et al. (2012), Smakhtin et al. (2004), and Pastor et al. (2014).
Each method produces an independent estimate of the flow required to sustain ecosystem functioning. The final EFR is obtained as the median of the nine estimates generated from the combination of three hydrological models and three environmental flow methods.
- Blue Water Availability
The amount of water available for human use, referred to as Blue Water Availability (BWA), is calculated as:
\[ BWA = BWR - EFR + US \]
where:
- BWR is Blue Water Runoff;
- EFR is the Environmental Flow Requirement; and
- US is the upstream surplus.
The upstream surplus represents unused sustainable water availability entering a sub-catchment from connected upstream basins.
BWA therefore represents the volume of water that can be sustainably allocated to human activities after environmental flow requirements have been satisfied.
- Historical Blue Water Footprint
The amount of water currently consumed within each basin is represented by the Historical Blue Water Footprint, denoted as H-BWF.
H-BWF is calculated as the average monthly consumptive water use during the period 1990–2010. It combines water consumption from irrigation, domestic uses, industrial uses, electricity generation, and livestock.
See SBTN’s Freshwater Quantity Target Version 2 methodology for additional details.
- Blue Water Footprint Cap
A sustainable consumption threshold, known as the Blue Water Footprint Cap (BWFC), is derived from the historical BWA record.
BWFC is defined as the 25th percentile of monthly BWA:
\[ BWFC = P_{25}(BWA) \]
where \(P_{25}\) represents the 25th percentile.
BWFC therefore provides a conservative estimate of the amount of water that can be consumed sustainably.
- Blue Water Footprint Overshoot
The extent to which current water consumption exceeds the sustainable threshold is quantified using the Blue Water Footprint Overshoot, denoted as \(BWF_{os}\):
\[ BWF_{os} = \max \left( H\text{-}BWF - BWFC,\, 0 \right) \]
Positive values indicate that historical water consumption exceeds the sustainable threshold.
When:
\[ H\text{-}BWF \leq BWFC \]
then:
\[ BWF_{os} = 0 \]
Monthly Reduction Target
The Monthly Reduction Target expresses the percentage reduction in historical monthly water consumption required to eliminate the overshoot:
\[ \text{Monthly Reduction Target} = \frac{BWF_{os}}{H\text{-}BWF} \times 100 \]
For basin (b) and month (m), this can be written as:
\[ RT^{monthly}_{b,m} = \frac{BWF_{os,b,m}} {H\text{-}BWF_{b,m}} \times 100 \]
where:
- \(RT^{\mathrm{monthly}}_{b,m}\) is the monthly reduction target for basin \(b\) and month \(m\).
- \(BWF_{os,b,m}\) is the monthly Blue Water Footprint Overshoot.
- \(H\!-\!BWF_{b,m}\) is the historical monthly Blue Water Footprint.
Annual Reduction Target
The Annual Reduction Target is calculated using the total annual overshoot rather than the average of the twelve monthly reduction targets:
\[ \text{Annual Reduction Target} = \frac{ \sum_{m=1}^{12} BWF_{os,m} }{ \sum_{m=1}^{12} H\text{-}BWF_m } \times 100 \]
For basin (b), this can be written as:
\[ RT^{annual}_{b} = \frac{ \sum_{m=1}^{12} BWF_{os,b,m} }{ \sum_{m=1}^{12} H\text{-}BWF_{b,m} } \times 100 \]
where the summations are performed across all twelve months of the year.
The annual reduction target is weighted by the total historical water consumption in each month. Therefore, months with greater water consumption contribute more strongly to the annual target than months with lower consumption.
Interpretation
A reduction target of:
- 0% indicates that historical water consumption is already within the sustainable threshold;
- a positive value indicates the proportion of historical water consumption that must be reduced to eliminate the overshoot; and
- a higher percentage indicates a larger gap between current consumption and sustainable blue water availability.