Proceedings

Balancing Levels of Service with Estimates of Source Yield

Piper, B., Atkins Water Resources Management, UK

(free)

The term “level of service” (LoS) is widely used in water resource planning. In common with many
measures of risk, the term can be poorly understood and its interpretation appears to be far from
consistent.

In the documents placed in the public domain as part of the current water resource planning round in
England, each water company has stated its target level of service. This measure is expressed as the
number of years on average (or frequency) that measures such as hosepipe or sprinkler bans to control
demand would need to be implemented to reduce the risk of interruptions to supply. The introduction of
extreme measures such as rota cuts and/or standpipes is no longer considered to be acceptable for the
planning and management of water supply systems in the 21st Century. In the draft Water Resource
Management Plans (WRMP) the range of levels of service for the introduction of hosepipe bans ranges
from 1 in 10 years to 1 in 50 years.

Estimates of the yield of surface water resource systems are often expressed in terms of frequency. It is
much more difficult to express the yield of groundwater sources in terms of frequency.
Although they are expressed in similar ways, it may not always be appropriate to make a direct link in the
supply demand balance between demand-side and supply-side levels of service. For the supply demand
balance reported in the WRMP Tables the test is whether supplies under drought conditions are sufficient
to meet unrestricted demands in “dry” year conditions. However surface water yield estimates explicitly
take account of reductions in demand as a result of restrictions.

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