The San Antonio River Authority (SARA), Lockwood, Andrews & Newnam, Inc. (LAN), and RESPEC have conducted a modeling effort of the Upper San Antonio River (USAR) Watershed using Hydrological Simulation Program – FORTRAN (HSPF). The effort included simulating two floodwater tunnel systems: the San Antonio River Tunnel (SART) and San Pedro Creek Tunnel (SPCT). The SART and SPCT are about three- and one-mile long, respectively. The tunnels act as inverted siphons to divert floodwater from the main channel to the tunnels. Typical recirculation flow is about 30 cfs and low-flow going downstream is 10-15 cfs for SART. A modeling approach of the SART and SPCT system was developed and tested using conditional Special Actions, a sophisticated feature of HSPF added for version 11. The high flow condition into the tunnels was modeled as a second exit. For the recirculation flow, a series of GENER Operations was added and the Special Actions developed to pump water from the downstream end into the tunnel if the flow drops below a certain threshold. When flows fall below a given threshold, a given volume of water is specified to pump from a reach downstream of the tunnel system into a reach at the upstream end of the tunnel system. This condition is evaluated at each time step of the model simulation period. Approach for water quality simulation of the tunnels was also developed and tested. Under high flow condition, water exiting tunnel reaches through the normal exit carries with it the typical simulated water quality. This presentation will cover the development, testing, and application of the modeling approach of the SART and SPCT system using conditional Special Actions for simulation of both flows and water quality.