MEASURING WATER USE IN A GREEN ECONOMY - UNEP

3 Data and information
sources
The core elements of any methodology
and information system needed to quantify
how water flows through the hydrological
system, ecosystems, and the economy are
the water inventory and temporal and spatial
assessment of the stocks and flows in and
through the different compartments. The data
flows and methodologies needed to tackle the
challenges of sustainable water management
need to be temporally and spatially specific
and need to address both the environmental
impacts and the socio-economic dimension
of water efficiency. It is also vital that the
sources of data and information become
well-established, along with the governance
structures that use them. Given the complexity
of management of freshwater today, the focus
of this report remains for the most part on
water resources on land. There are, however,
certain considerations to be made with respect
to some coastal and transitional ecosystems,
e.g. coconut groves and mangroves, which rely
on seawater to provide ecosystem services on
land. Where these occur, they are included in
the quantitative methods described below.
3.1 Elements of the water
balance
All methodologies that quantify relevant
aspects of the hydrological and economic
water cycle rely on sound basic data. Water
balances (also known as inventories or
registers) represent the fundamental approach
to accounting for the flow of water into and out
of a system. But to provide a robust basis for
analysis and decision-making, such accounts
must meet certain criteria.
When considering resource efficiency and
sustainable use, a hydrological balance must
include fluxes (flows available for immediate
use) and stocks (resource from past inputs) but
not future inputs. Future resources must not be
treated as currently exploitable. That means,
for example, that the time span for allocations
has to be chosen carefully. Availability and
accessibility shall be evaluated on a monthly
basis, depending on the specific climatic
variability in the eco-regions and catchments.
A seasonality signal may be stronger and
therefore can be used instead. Classical water
balances, which calculate an overall balance
over one year or over national territories larger
than natural hydrological units, may ignore
this point and should therefore be used with
caution.
The key components of the hydrological balance
have been specified since the second half of
the 20th century: evaporation, precipitation,
soil water, transfers to groundwater and
rivers, storage in lakes, aquifers and ice
caps. In classical balances, the volume in a
certain compartment is presented without
reference to its residence time, which can vary
significantly between different compartments.
For example, the global average residence
time in oceans (standing stock 1 370 million
km 3 ) is approximately 2 600 years, whereas in
rivers (standing stock 0.0012 million km 3 ) the
average residence time is 12 days. The main
elements of the water balance are shown in
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