MEASURING WATER USE IN A GREEN ECONOMY - UNEP

Measuring water use in a green economy
outlined above this refers to the overall
available water minus the amount to be
reserved for environmental requirements.
The term is used following the approach of
‘Human Appropriation of Renewable Fresh
Water approach’ (HARFW). 1 However, accounts
differ from that approach in several respects,
in particular because they record returns of
water and water stored in dams as accessible
although it has been ‘appropriated’.
Ecosystem capital accounts are based on
physical statistical units (e.g. hydrological
units, land-cover units) and not on economic
units defined from their institutional status
(e.g. enterprises, governments or households).
In the case of terrestrial systems, they are
basic land-cover units (e.g. forests, cropland
areas, urban areas) which are in a second step
combined into socio-ecological landscape units.
For water, they are hydrological units (rivers,
lakes, aquifers…). Rivers are considered as
basic reaches re-combined into hydrological
networks. River basins and sub-basin limits are
considered explicitly in mapping terrestrial and
hydrological statistical units.
4.2.5 Key accounting variables in ECWA
In addition to SEEA-W, the key accounting
variables in ECWA relate to accessible water:
• accessible freshwater stocks in aquifers
(i.e. not including stocks that are
inaccessible due to physical constraints,
salinity, costs, etc.);
• accessible freshwater stocks in lakes,
dams and reservoirs (reservoirs increase
accessibility by storing water and decrease
it by generating additional evaporation);
• actual evapotranspiration (ETA), including
the use of water by rain-fed agriculture and
managed forests, which is a measure of
rain water accessed in situ;
1 Human Appropriation of Renewable Fresh Water
(HARFW) is defined in Vitousek et al. (1986). An estimation of
HARFW was undertaken by Postel et al. (1996).
• accessible ecosystem water flow, which is
the available hydrological effective rainfall
net of the water which is inaccessible
because of the water regime (most of
the flood water in temperate countries),
pollution (river runoff needed to dilute
pollution to acceptable levels and/or to
maintain life in rivers), additional ETA
induced by irrigation, and other uses.
4.2.6 Aggregated accounting balances
in ECWA
The first aggregate is Total Ecosystem
Accessible Water (TEAW), which summarises
the various positive and negative changes in the
water resource: flows and changes in stocks.
This indicator can be computed by ecosystem
units and river sub-basins and basins and
aggregated at the level of administrative
regions and countries, as well as according to
any geographical or climatic zoning.
TEAW will vary according to factors such as
precipitation (positive or negative impacts);
spontaneous evapotranspiration by crops
or tree plantation (negative); additional
evapotranspiration by irrigation (negative);
storage in reservoirs (positive) and additional
evaporation from reservoirs (negative);
salinisation of groundwater (negative); pollution
of rivers (negative); and transfers of water
received (positive) or supplied (negative).
The TEAW aggregate is not sufficient to
assess the availability of water completely.
The temporal variability of the meteorological
conditions also needs to be taken into account,
including a succession of wet and dry periods
and the possible temporary severe stress that
may result for people, agriculture and nature.
ECWA could capture this risk by using a stress
coefficient based on the number of days
when plants cannot access any water in their
growing season, recently calculated by EEA.
Net Ecosystem Accessible Water is obtained
by multiplying TEAW by the water stress
coefficient. On that basis, a headline indicator
derived from ECWA is Ecosystem Accessible
Water Surplus, which compares withdrawals
of water (abstraction, diversion to electricity
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