Trend Analysis of Non - Revenue Water using Mann-kendall Test and Sen's Slope Estimator: A Case of Sumbawanga Urban Water Supply and Sanitation Authority

Non-Revenue Water (NRW) remains a critical challenge for urban water utilities in sub-Saharan Africa, where system losses frequently exceed the International Water Association (IWA) benchmark of 20%. This study investigates monthly NRW trends at the Sumbawanga Urban Water Supply and Sanitation Authority (SUWASA) in Rukwa Region, southwestern Tanzania, over six years (2019–2024) using two robust non-parametric statistical techniques: The Mann-Kendall (MK) trend test and Sen's Slope Estimator (SSE). Monthly NRW data expressed as a percentage of system input volume were sourced from EWURA annual performance reports and SUWASA operational records. The Mann-Kendall test was applied to both monthly and annual time series to detect the direction of monotonic trends, while Sen's slope quantified the magnitude of change in percentage points per year. The analysis reveals a consistent downward tendency in NRW across all twelve months and the annual series, with an overall Sen's slope of −0.8675%/year and an annual Mann-Kendall statistic of Z = −1.127 (p = 0.260, τ = −0.467). Although no individual monthly trend attained statistical significance at α = 0.05 — a limitation attributable to the short observation window of n = 6 — the universal negative directionality of the Z-statistics and slope estimates constitutes meaningful evidence of gradual improvement. Annual mean NRW declined from 35.5% in 2019 to 27.53% in 2024. December exhibited the steepest decline (β = −1.667%/year, Z = −1.812, p = 0.070), while June and July showed no discernible trend. November recorded recurrent seasonal peaks (45.4% in 2021 and 2023), identifying a targeted intervention priority. These findings demonstrate the utility of non-parametric trend analysis for evidence-based NRW management in data-scarce, small-to-medium utility contexts and provide a quantitative baseline to guide infrastructure rehabilitation, active leakage control, and regulatory oversight at SUWASA