Storm-runoff generation in the Permanente Creek drainage basin, west central California — An example of flood-wave effects on runoff composition

Variations in the isotopic and chemical composition of storm runoff in the 10.6-km 2 Permanente Creek basin, Santa Clara County, California, indicate that changes in water composition lag behind changes in streamflow. This lag occurs even though field observations and rainfall-runoff modeling indicate that much of the storm runoff must be composed of “new” water running off impervious surfaces. The apparent incompatibility posed by the presence of “old” water and the direct and indirect evidence that surface runoff contributes substantially to storm runoff can be explained if initial rises in streamflow result from effects of flood waves. Flood waves composed of old channel water reach downstream locations ahead of the new water derived from impervious areas. By this mechanism, streamflow can rise rapidly in response to surface runoff and still be composed of large amounts of old water. Data collected in Permanente Creek indicate that flood waves can occur even in small basins, at least when those basins contain impervious surfaces.