Baseline water stress measures the ratio of total water withdrawals to available renewable surface and groundwater supplies. A higher ratio indicates more competition among users. These ratios were then converted into risk scores based on the thresholds defined in the methodology. These risk scores range from low water stress (<10%) to extremely high water stress (>80%). Water withdrawals include domestic, industrial, irrigation and livestock consumptive and non-consumptive uses. Available renewable water supplies include surface and groundwater supplies and the impact of upstream consumptive water use and large dams on downstream water availability. Water stress values are available per sub-basin (HydroBASIN level 6).
Aqueduct used the global hydrological model, PCRaster Global Water Balance (PCR-GLOBWB 2), to calculate five indicators. Baseline water stress is one of those indicators and was calculated using the gross and net total withdrawal and available water per sub-basin time series from the default model run.
The model was first used to calculate a time series of water stress per catchment per month. To calculate water stress, total withdrawals were divided by the available renewable surface and groundwater supplies. Net consumptive water use was subtracted from the total available renewable water supply before the ratio was calculated. The results were output in 12 monthly time series time series of water stress per sub-basin. Water resources in delta sub-basins were pooled.
Next, the time series were converted into baseline values using an ordinary least squares (OLS) regression. For example, the regression output for January 2014 represents the chronic, long-term condition of water stress in January’s from 1960 to 2014. This was repeated for every month and then averaged into an annual value. All raw values were limited to a maximum of 100% and a minimum of 0%.
Sub-basins that were classified “Arid and low water use” were handled separately and were given a raw value for baseline water stress of -1.
Finally, raw baseline water stress values were converted to risk scores between 0 and 5, based on quantiles. These scores were placed into categories indicating how high the risk for this indicator is. The thresholds for raw baseline water stress values and the corresponding risk scores that were used in each category are defined below as Risk Category (Risk Scores): Raw Value Thresholds.
Low (0-1): < 10%
Low to medium (1-2): 10-20%
Medium to high (2-3): 20-40%
High (3-4): 40-80%
Extremely high (4-5): >80%
Arid and low water use (5): -1
For the full documentation, please see the source methodology.
1960 to 2014 Baseline Water Stress: Baseline water stress measures the ratio of total water withdrawals to available renewable water supplies. Water withdrawals include domestic, industrial, irrigation and livestock consumptive and non-consumptive uses. Available renewable water supplies include surface and groundwater supplies and considers the impact of upstream consumptive water users and large dams on downstream water availability. Higher values indicate more competition among users. Values are calculated based on data from years 1960 to 2014.
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Baseline Water Stress, Aqueduct 3.0
Although the underlying models have been validated, the results were not. Water stress remains subjective and cannot be measured directly. The lack of direct validation makes it impossible to assess some of the parameters in our calculation such as the length of the input time series, regression method and optimal moving window size.
The water stress indicator presented here did not explicitly take environmental flow requirements, water quality or access to water into account. Multiple views exists regarding what to include in a water stress indicator. Views differ regarding what to include in a water stress indicator (Vanham et al. 2018).
Coastal and island sub-basins were often grouped to make the area of the sub-basins more homogeneous. The assumption of shared water resources might not hold in aggregated coastal sub-basins.
Water resources in PCR-GLOBWB 2 were pooled in abstraction zones. This assumption differs from the sub-basin approach in Aqueduct. This is one of the prime reasons for further processing of the PCR-GLOBWB 2 data.
Results were tailored towards large scale comparison of water related risks. The indicators have limited added value on a local scale.
Hofste, R., S. Kuzma, S. Walker, E.H. Sutanudjaja, et. al. 2019. “Aqueduct 3.0: Updated Decision-Relevant Global Water Risk Indicators.” Technical Note. Washington, DC: World Resources Institute. Available online at: https://www.wri.org/publication/aqueduct-30. Accessed through Resource Watch, (date). www.resourcewatch.org
Hydrological sub-basin (HydroBASINS 6)