Travel times and nonlinearity of flood runoff from tracer measurements on a small watershed

Flood runoff has been traced from seven injection points to the outlet of a 96‐acre watershed. Each radioactivity time record at the outlet provides a hydrograph of outflow of the labeled drop of water. The results yield direct information on the flood process of a type and accuracy not possible from conventional analysis of rainfall and runoff records. The relationships of travel time and average velocity with discharge are examined together with the variations of these relationships over the watershed. Average travel times are shown to be similar to the watershed lag. The nature of the nonlinearity of the flood runoff process is demonstrated. Although the process is grossly nonlinear at low flows, linearity is approximated at high flows. Spatial variations of travel time and hydrograph shape are shown to make contributions to nonlinearity that are additional to the lumped dependence on flood magnitude that is normally considered.