Runoff processes and small watershed

Full text: Small watersheds are a fundamental landscape unit for quantifying inputs and outputs of water, sediment and nutrients. Small watersheds have been used historically for defining runoff processes and flood response to storm precipitation. Early conceptualizations of runoff production during the International Hydrological Decade in the 1960s focused on the importance and movement of event water as overland flow to the stream channel. Use of mass balance mixing models using stable isotope tracers in the 1970s and 1980s directly challenged early ideas of where water goes when it rains, residence time of catchment waters and flow paths of subsurface runoff towards the stream. These data showed that the majority of water in the stream during a precipitation event was water that existed in the watershed prior to the event. While credible physical mechanisms of old water mobilization have only been defined in the past decade, stable isotope tracer approaches are now mature enough to offer new potential for informing new model structures of how small watersheds work. Isotope tracer data in small watersheds and mass balance separations also represent new ways of validating and calibrating watershed models. This presentation will chronicle the use of isotope tracers in small watersheds and provide examples of how these data can be used in models of runoff processes and for providing valuable input for water resource management at larger basin scales. (author)