Numerical model of a tracer test on the Santa Clara River, Ventura County,California

To better understand the flow processes, solute-transport processes, and gr ound-water/surface-water interactions on the Santa Clara River in Ventura C ounty, California, a 24-hour fluorescent-dye tracer study was performed und er steady-state flow conditions on a 45-km reach of the river. The study re ach includes perennial (uppermost and lowermost) subreaches and ephemeral s ubreaches of the lower Piru Creek and the middle Santa Clara River. The tra cer-test data were used to calibrate a one-dimensional flow model (DAFLOW) and a solute-transport model (BLTM). The dye-arrival times at each sample l ocation were simulated by calibrating the velocity parameters in DAFLOW. Th e simulations of dye transport indicated that (1) ground-water recharge exp lains the loss of mass in the ephemeral middle subreaches, and (2) groundwa ter recharge does not explain the loss of mass in the perennial uppermost a nd lowermost subreaches. The observed tracer curves in the perennial subrea ches were indicative of sorptive dye losses, transient storage, and (or) ph otodecay - these phenomena were simulated using a linear decay term. Howeve r, analysis of the linear decay terms indicated that photodecay was not a d ominant source of dye loss.