MEASURING WATER USE IN A GREEN ECONOMY - UNEP

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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 34
Measuring water use in a green economy Figure 3.1, however a static model is shown, not including the time-space variability of the water cycle impacted by climate change. In a further development of Figure 3.1, blue water resources are referred to as the sum of surface and groundwater; green water resources are referred to as rainwater insofar as it does not become run-off. The large differences in residence times make it necessary to address the stocks and flows from the perspective of their potential use. Residence time is an intriguing indicator. It expresses the claim that a storage project makes into the future, as a kind of temporal footprint. It also expresses, in a temporal unit, its spatial dependency and area of influence. ‘The larger the residence time of a reservoir, the greater its dependence on the water resources generated in the upstream catchment area, and the larger the impacted area downstream [cf. Vörösmarty et al., 1997].’ Van der Zaag and Gupta, 2008). Large reservoirs and their operations for agriculture and hydropower can cause large changes in flow regime which can be a major cause of ecosystem degradation. There is no simple method to depict the complexity of the situation. The role of all quantification methods included here is to present the balances at levels of temporal and spatial disaggregation that are practically useful (both in themselves and in supporting other methodologies). They should be as free as possible from misleading errors resulting from misunderstanding of the flux and stock issues, and they should specify the location of water consumption and the (potentially distant) location of impact or water stress. This report uses terminology derived from the International Recommendations for Figure 3.1 Figure 3.1 Simplified water budget at a river basin scale Simplified water budget at the river basin scale Actual Evapotranspiration (ET a) Precipitation Actual External Inflow (Qi) (SW+GW) Imports / Exports / Returned Flow SW Inflow Agriculture Streamflow Total Actual Outflow (SW+GW) (Qo = Qo,s + Qo,n) Qo,n into neighbouring Public water supply Reservoir Storage Industry Qo,s into the sea GW Inflow SW Outflow River Basin - RB Other Water Abstractions GW Storage GW level Major Aquifer System - MAS GW Outflow 35
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Water is an essential resource for
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