Water use efficiency has been highlighted as a key water indicator in the set of Sustainable Development Goals (SDGs). The method adopted by the SDG is evaluated and compared with specific sector approaches for evaluating the efficiency of freshwater use. The scale of the challenge for the SDG indicator is reviewed and the the range of evaluation methods that can be applied in the major water using sectors is illustrated. A case is made for looking in more detail at operational water accounting procedures in order to evaluate the scope for making the water use efficiency gains anticipated in SDG goal 6. The importance of measuring or estimating evaporative consumption of fresh- water and the quantity and quality of return flows is stressed noting that the measurement of these variables in the water balance are often the most difficult to report accurately at scale.
In practice some notable gains in water use efficiency have been made, particularly in the generation of thermal energy, and incremental gains in manufacturing processes and leakage control in municipal water supply systems are evident. The agriculture sector is more problematic. While the adoption of technology, including precision irrigation, has boosted the productivity of agriculture, there is little or no evidence of irrigation water- use efficiency measures ‘freeing up’ water for other uses or being returned to the environment as recharge or drainage. This is particularly the case in water scarce countries where it is observed that irrigated agriculture tends to ‘internalise’ efficiency gains through intensification and expansion of irrigated areas. Determining who will benefit from the adoption of water-use efficiency measures can be done with more precision where spatial planning tools can be deployed, but implementation will need explicit allocation policies to direct efficiency gains to desired beneficiaries.
It is expected that the technical scope for water-use efficiency to be improved locally and taken to scale will continue to improve in all economic sectors, but operational water accounting will be needed to validate any claimed efficiency gains. A review of various national initiatives suggests that the economic and political cost of improved technology and governance of water allocation need careful appraisal prior to any public investment. This includes water quality in particular when water efficiency measures may exacerbate concentration levels and attenuate dilution processes.
Systems of water use are multi-purpose and multi-functional so that the often-complex cascade of use, consumption and re-use inevitably leads to a set of environmental trade-offs that have to be appraised and negotiated. Slowing the growth of water withdrawals or making desired re-allocations of conserved water will need water policy instruments and management decisions to be place together with strong support for the adoption of ‘joined-up’ water technology.