  Water Supply Systems are essential infrastructures for the socio-economic
life of urban cities. To improve their reliability, water utilities undertake
several short- and long-term operational tasks based on technical and economic
constraints. These activities are motivated by many factors, including
increasing leakage rates due to infrastructure aging, increased consumer
demands and need for sustainable use of water and energy. European and national
regulatory bodies have promoted investment programs for allowing water
utilities to reach common standards of reliability and quality of service among
countries. Targets of management and operational achievements are usually
measured using specific performance indicators. The Italian Regulatory
Authority for Energy, Networks and Environment (ARERA) recently introduced the
Regulation of the technical performances of water utilities. Performances on
leakage management and investment plans of the utilities are thus based on two
indicators named M1a (linear leakage index) and M1b (percentage leakage index).
This paper analyzes in details the inconsistencies of the percentage leakage
index (M1b), mainly due to its mathematical formulation and the ambiguity of
defining water consumption as part of the total system inflow. The discussion
is supported by a real case study, where both indicators have been calculated
to assess their impact on management decisions and investment plans. The
inconsistencies of the percentage leakage index are further demonstrated for
various layouts of water supply systems.
