The measurement of water scarcity: Defining a meaningful indicator

Metrics of water scarcity and stress have evolved over the last three decades from simple threshold indicators to holistic measures characterising human environments and freshwater sustainability. Metrics commonly estimate renewable freshwater resources using mean annual river runoff, which masks hydrological variability, and quantify subjectively socio-economic conditions characterising adaptive capacity. There is a marked absence of research evaluating whether these metrics of water scarcity are meaningful. We argue that measurement of water scarcity (1) be redefined physically in terms of the freshwater storage required to address imbalances in intra- and inter-annual fluxes of freshwater supply and demand; (2) abandons subjective quantifications of human environments and (3) be used to inform participatory decision-making processes that explore a wide range of options for addressing freshwater storage requirements beyond dams that include use of renewable groundwater, soil water and trading in virtual water. Further, we outline a conceptual framework redefining water scarcity in terms of freshwater storage. Electronic supplementary material The online version of this article (doi:10.1007/s13280-017-0912-z) contains supplementary material, which is available to authorized users.

. Overview of selected indicators as they relate to water scarcity and stress Index Reference Scale Description Specific notes on water and scarcity components Water Requirements Inverted Falkenmark Falkenmark (1986) National Amount of people competing for 1,000,000 m 3 of water Available freshwater resource is based on Mean annual river runoff Traditional Falkenmark (Water Stress Index(WSI)) Falkenmark (1986,1989) National Available freshwater resources per capita. See Table 1 for more details.
Available freshwater resource is based on Mean annual river runoff Gleick Gleick (1996) National Drinking = 5 Litres/person/day (l/p/d); sanitation = 20 l/p/d, bathing = 15 l/p/d, food preparation = 10 l/p/d; Total = 50 l/p/d. However, a global complete range is estimated to be 27 -200 l/p/d and the paper finds "Falkenmark's 100 l/p/d falls well within the middle of this bracket".
Acknowledges that available sources of water differs across the world and can be culturally and societally determined.

National
Measures magnitude of water deficit necessary to be returned into the natural system in order to sustain a balance between available water and water demand. Incorporates population growth rate, water availability, and domestic, industrial and ecological water usage Available freshwater resource is based on Mean annual river runoff Available Freshwater resources availability based on mean annual river runoff on a log-scale.

National WTA ratio accounts for Environmental Water Requirements
Water Accounting Frameworks Water Footprint Hoekstra and Hung (2005) National, river basin, local The water footprint (WfP) is the virtual water (embedded water) in production of a good. A global trade can be visualised as an adaptive capacity Originally available freshwater resources is based on mean annual river runoff

Local and watershed
This assessment indicator uses the WSIEWR combined with traditional Life Cycle Assessment approaches to measure environmental stresses.
The WTA ratio is applied as the hydrological component. A later study, using the same methods (Pfister and Bayer, 2014) recognises the importance of considering the temporal variability of freshwater availability. Available freshwater resource is based on mean annual river runoff Water Impact Index (WII) Bayart et al., (2014) Local Adopts the LCA and WfP approaches with the aim to integrate issues that relate to water scarcity and quality in a single indicator in order to assess the water footprint of human uses of freshwater on the environment.
Water scarcity component of WII applies WSIEWR methodology. Available freshwater resource is based on mean annual river runoff

Water Sustainability Metrics
Watershed Sustainability Index Chaves & Alipaz (2007) Watersheds below 2,500 km 2 WSIndex incorporates hydrology (H), environment (E), life (L) and policy (P), each with the parameters "pressure, state and response". The WSIndex value (ranged 0-1) is calculated as the average of HELP, all of which are also scored on a scale from 0-1. (See Table S3 below for more information) Water quantity parameter applies Falkenmark threshold of 1,700 m 3 /capita/year and state water stress occurs under this level and applies five levels of per capita water availability in relation to multiples of this minimum standards. Available freshwater resource is based on Mean annual river runoff Available freshwater resources is based on Mean annual river runoff: Applies Falkenmark thresholds where a score is assigned of 100 is assigned to any value over 1,700 m 3 /capita/year and 0 of 500 m 3 /capita/year; indicator for supply serves as a proxy for the vulnerability of the community's freshwater supply by addressing the variability of surface water flows and/or trends in ground water reserves.
Demand indicator: demand on the resource is the amount of water annually allocated relative to the total amount of renewable fresh water.

Arab Water Ali et al. (2008) National
Four theme-based components were proposed to Available freshwater resource is based on Mean annual river Sustainability Index (Arab region) reflect a meaningful representation of the situation in the region: water crowding, dependency, scarcity and environmental sustainability.
runoff. The WSI is adopted to portray "water crowding" and the WTA ratio to measure water scarcity. This is done in the context of agricultural impact on water resources availability.
West Java Sustainability Index Juwana (2012) West Java, Indonesia Composite indicator measuring components of Conservation; Water Use and Policy & Governance, incorporating water availability, demand and quality.
Available freshwater resources is based on Mean annual river runoff. The WSI is applied to portray water availability and the WTA to reflect water demand.

Water
Resources Sustainability Evaluation Model Kang & Lee (2011) --Aqueduct Water Risk Tool Reigh et al. (2013); Gassert et al. (20130 National, Global Publicly available global database that provides information on water-related risks worldwide for businesses, using three categories of indicators: Physical Risks: Quantity; Physical Risks: Quality; Reputational and Regulatory Risks Available freshwater resource is based on mean annual river runoff WTA ratio approach to identify areas of water stress. The issue of seasonality in water supply between months is acknowledged as being a challenge.

National
Years to Sustainability is the time it will take to reach predefined sustainability goals. Years to Sustainability is the time it will take to reach a sustainability goal, which is calculated as the difference between a predefined sustainable level of impacts and the current level of environmental impacts from a specific pressure.
. In progress. Available freshwater resource is based on mean annual river runoff Severe stress occurs when WTA >40%.

Water Security
Water Security Status Indicator approach Norman et al. (2013) Community It's method rather than an indicator; integrates variables pertaining to water quantity and quality as they relate to aquatic ecosystems and human health.
Available freshwater resource is based on mean annual river runoff.

Climate vulnerability index
Sullivan & Meigh (2005) Resource (R), Access (A), Capacity (C), Use (U), Environment (E) and; Geospatial (G) divided by eight risk factors Available freshwater resource is based on mean annual river runoff Governance and Climate Vulnerability index Jubeh & Meigh (2012) Combined Climate Vulnerability and Governance Index Available freshwater resource is based on Mean annual river runoff . Applies the WSI.
Water Vulnerability Index Sullivan (2011) Municipal scale supply-driven vulnerability (from water systems) (SDWV) and demand-driven vulnerability (from water users) (DDWV) dimensions are combined Available freshwater resource is based on mean annual river runoff .
Bagmati River Basin Vulnerability Assessment Babel et al. (2011) Combination of water stress sub-index and adaptive capacity sub-index Available freshwater resource is based on Mean annual river runoff. Applies the Falkenmark and WTA thresholds.
Arctic Water Resources Vulnerability index Alessa et al. (2008) Communities in circumpolar Arctic An index to assess resilience toward changes in freshwater resources: 2 sub-indices: physical (quality and quantity) and social.
Physical water supply: measured via precipitation as average annual rainfall over 30 years. For the river flow indicator, the average annual runoff in the watershed and the Coefficient of Variance for that run-off over a 30-year time series are measured; seasonal variation in water supply, the difference in monthly maximum and minimum river discharge, normalised by the monthly mean river discharge is calculated in order to determine a measure for the intra-annual water supply variation.
Physical Water Supply: the ability to use infrastructure to continuously ensure that there is 20-100 l/capita/day available of water. AWRVI recognises the importance of the ability to store water to ensure resilience against times where natural supply may not be adequate to meet demands. Groundwater Sustainability Metrics Groundwater Sustainability Infrastructure Index Pandey et al. (2011) National existing knowledge, practices and institutions whose adequate strengthening helps to achieve groundwater sustainability is necessary infrastructure in evaluating progress in achieving groundwater sustainability