Expert-Informed Engineering Requirements for Distributed Soil-Moisture Sensing in Plastic Water-Pipe Leak Monitoring

Leak detection in underground drinking-water networks is especially difficult in plastic pipes because leak-generated acoustic and vibration signals attenuate strongly. This study investigates the concept of distributed soil-moisture monitoring as a potential complementary early-warning approach and derives stakeholder requirements for future system development. An expert questionnaire was distributed by email to 90 water-supply utilities in the Swiss cantons of Thurgau, Zurich, and Aargau. Fifty-five submissions were received (61.1% relative to invitations), of which 52 included consent for research use and were analysed (57.8% of invitations). Item-specific denominators, Wilson 95% confidence intervals, and exploratory exact and non-parametric subgroup tests were used. Current practice was dominated by acoustic methods (48/52) and baseline-consumption monitoring (41/52). Among valid quantitative answers, 29/47 respondents expected detection at or below 20 L/h (61.7%, 95% CI 47.4–74.2%), 44/46 preferred at least daily sampling (95.7%, 85.5–98.8%), 41/45 selected a price at or below EUR 20/m (91.1%, 79.3–96.5%), 36/46 specified a false-alarm tolerance of no more than one per year (78.3%, 64.4–87.7%), and 39/44 specified a missed-leak tolerance of no more than 5% (88.6%, 76.0–95.0%). These cumulative proportions include more demanding response categories and do not establish acceptance of performance exactly at each boundary. Experience-group comparisons yielded no statistically significant differences after Holm correction. The contribution is a traceable, uncertainty-aware requirement specification linked to engineering relationships and verification planning for power, spacing, latency, cost, false alarms, and missed detections. Limited precision, regional recruitment, and unfamiliarity with the monitoring approach constrain interpretation. The requirements are provisional design inputs; technical feasibility and final acceptance criteria require laboratory, field, and stakeholder validation.