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Box 4.1 the Water Exploitation Index+ (WEI+)
The Water Exploitation Index (WEI) was developed to formulate a harmonized message for awareness purposes on the state of the water resources, to provide an EU overview of water stress conditions, a hot spot analysis, and to be able to communicate the problem of overexploitation to other EU policy areas. Identifying the fact that the original WEI presented some limitations due to its simplified view of the water balance and its highly aggregated scale of implementation (i.e. country level), the EEA worked with the WFD CIS Expert Group on Water Scarcity & Drought towards an improved formulation of this indicator (the so called WEI+) with the purpose of better capturing the balance and critical thresholds between natural renewable water resources and abstraction, in order to assess the prevailing water stress conditions in a catchment. The proposed WEI+ aims mainly at redefining the actual potential water to be exploited (i.e. availability), since it incorporates returns and accounts for changes in storage, tackling as well issues of temporal and spatial scaling and proposing the use of environmental requirements for the formulation of adequate thresholds.
The WEI+ is formulated as follows: WEI+ = (Abstractions – Returns) / Renewable Water Resources
For the calculation of the Renewable Water Resources (RWR) two options have been suggested and selection relies on the available information and certainly (minimisation of bias) associated with each option.
Option 1 refers to the calculation of RWR based on the hydrological balance equation, using precipitation, external inflow, actual evapotranspiration and change in natural storage as components:
RWR = ExIn + P – Eta – ΔS
Option 2 refers to the calculation of RWR based on the naturalization of stream flow, using outflow, abstraction, return and change in artificial storage as components:
RWR = Outflow + (Abstraction – Return) – Δsart
Environmental Flows should be conceptually considered in the WEI+. At the moment, due to the absence of a harmonized and comparable method for calculation, eflows should be left out of the WEI+ formula itself, and be considered instead in the definition of the relevant thresholds. For more information on these thresholds: see Box 4.2.
Figure 4.7 Variability of the Water Exploitation Index (WEI+) at Morava RB in Czech Republic for the period 2005-2009 at monthly scale.
Source: EG WSD, provided by the representative of Czech Republic
To further enhance interpretation of the acuteness of water stress conditions, a satellite index to the WEI+ is proposed, defined as the ratio of water abstraction to the actual water use. This indicator can depict cases where water use is higher than abstraction and met by other means (e.g. desalination) so that freshwater resources are not overexploited, or cases where abstraction is much higher than the actual use due for instance to high losses.
Figure 4.8 Precipitation versus agricultural demand patterns
Source: Jucar Pilot RBMP
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When considered on an annual time basis, the Water Exploitation Index used as a stand alone indicator can be fully relevant in some situations, and irrelevant in others. For instance, where the major consumptive use of water is very seasonal (irrigation in southern but non arid regions, which can account for much more than 50% of total yearly net abstraction), in addition if it takes place in a usually significantly dry season, the annual WEI considered alone completely disregards currently existing problems over weeks of months every year. And the economic and environmental costs of possible storages which could structurally "compensate" the seasonal unbalances may be high.
figure moved to introduction where seasonallity is mentioned
idea behind the comment taken and rewritten
1) Please define RWR
2) Proposing 2 options for one and the same balance requires some explanation, as they should provide identical results in theory, but differ in practice. Theoretically, the sum of the inflows must be equal to the sum of outflows (Total In = Total Out) and then RWR = Total In or RWR = Total Out. In practice, this basic equation is "disturbed" by return flows, storage of water volumes, artificial recharge etc., which need to be included to avoid significant errors to calculate water availability. In this respect, it is correct to include Eta and Return, correcting P and Abstraction respectively. The way Storage parameters are included and the reason they are subtracted is not very clear.
3) The definition of WEI as the ratio of Abstraction over Avaialbility (p. 29) is different from the formula on p. 33 : WEI = (Abstractions - Returns) / RWR. Including Returns in some cases is correct, in particular in cases of direct discharge in the same water body. However, this formula indicates that RWR is identical to Water Availabilty, which is not the case since availability is calculated as the difference between In- and Outflow (Av = In - Out).
4) Leaving out E-flows involves that Availability = human availability.
Fig. 4.7.
1) the ratio W.Abstraction to actual W.Use is not coherent with the definition of WEI (p. 29) since W.Use is different from either RWR or W.Availability. As such, this formula is calculating the Water Use Efficiency.
2) W.Use can never be higher than the volumes abstracted. When including desalination, also the abstracted marine water volumes should be taken into account. If not, the figures represent the dependency of W.use on salinated sources.
The definitions of all terms included should be precicely defined, also the "formulas" should be well discussed to exclude missunderstanding. Also to consider how/where/if to include water losses due to leaking (pipelines leaking problems) as this water is abstracted and it (in a way) "returns" back.