[en] To study the impact on water quality of changes in land-use, climate, or other factors, the residence times of water within the watershed must be understood. One manner of addressing this issue is through the use of tritium data. It has been over 50 years since thermonuclear weapons began producing tritium in large quantities, and over 40 years since the peak production of tritium by weapons testing in 1962. Since then, the tritium concentrations in the atmosphere have decreased and leading to a perception that tritium is no longer useful for determining physical parameters of surface waters. However, surface waters are frequently composed of a mixture of recent precipitation and base flow, the latter of which can have residence times on the order of weeks to decades, or higher. The fraction of groundwater in surface flow and its age impacts the tritium concentrations of the surface flow. If the residence time of the waters composing base flow is decades or longer, it is frequently possible to estimate the age of the active groundwater component in the river basin. Data from the Rocky Mountain area of Colorado is analyzed during three timespans: 1. The four years immediately after the bomb tests; 2. the decades after the tests (1970-2000); 3. 2000 to the present, The early time period is found to be the best time to estimate the fraction of base flow in a system. Using the ratio of tritium concentration in precipitation/tritium concentration (C_p/C_r) in surface water, it is possible to estimate the residence times of water within watersheds both on historical and recent data. (author)