The Dutch drinking water benchmark has been carried out every three years since 1997. In 2011, the benchmark was incorporated into the Drinking Water Act. As a result the benchmark switched from a voluntarily benchmark executed by the Association of Dutch water companies to a mandatory benchmark executed by the Human Environment and Transport Inspectorate. In addition, the drinking water companies have to hand in an improvement plan 6 months after the benchmark publication. Based on interviews with benchmark coordinators of drinking water companies the influence of the inclusion of the benchmark into the Drinking Water Act on the improvement driven part of the benchmark is studied. For this purpose, the following four issues are discussed: changes experienced as a consequence of the shifting from a voluntarily to a mandatory benchmark, influence of mandatory character of the benchmark on the drivers, identified by de Goede et al. (2016), the role of the improvement plans and a new instrument to stimulate improvement.

The differences between the voluntarily and mandatory benchmark are identified: the benchmark switched from being only an improvement driven instrument to an instrument for accountability as well and the drinking water organizations lost control over the development of the benchmark. The influence of the mandatory character on the drivers for performance improvement have been determined: only the driver 'enhanced transparency' is (positively) influenced. In order for the improvement plans to be able to have a positive effect on the stimulation of improvement, a feedback system should be implemented and the publications of the benchmark and improvement plans have to be faster. The benchmark is still thought to be useful, although an adaptive benchmark could stimulate improvement again.