This paper presents a distribution-free statistical test aimed at detecting gradual changes in the hydrological behavior of watersheds. The proposed test uses a rainfall-runoff model to identify watershed behavior over successive time periods and a resampling approach to quantify the significance of trends. The method can be applied with any model deemed suitable for the studied watershed. To assess test efficiency, we used three different case studies: An afforested agricultural watershed, a burnt-over forested watershed, and a watershed covered by old-growth forest. All three watersheds had a long period of rainfall and runoff records (60, 35, and 40 years, respectively), on which stationarity could be tested. The test was shown to adequately detect gradual changes, and it can therefore be useful to identify hydrological trends, wherever rainfall and streamflow time series are available.