SENSITIVITY ANALYSIS AND CALIBRATION OF A HYDROLOGICAL MODEL FOR THE TMDL ANALYSIS IN THE CALLEGUAS CREEK WATERSHED

A hydrologic model was developed using Hydrologic Simulation Program – Fortran (HSPF) for the Calleguas Creek Watershed (CCW) in Ventura County, Southern California, for flood control analysis. The hydrologic model was originally manually calibrated using HSPEXP. To provide decision support in total maximum daily load (TMDL) development, the model was expanded to simulate suspended sediment and particle associated and dissolved fractions of select metals and selenium all of which were also manually calibrated. The HSPF water quality model was used it the development of the Metals and Selenium TMDL for the CCW. The sediment yield and concentrations were over-predicted by the model resulting in over-prediction of total metals and selenium.

A Pathogen TMDL is required for the CCW and it is desired to modify the available HSPF model of the watershed to simulate fate and transport of bacteria. Bacteria are difficult to measure and model and they can be attached to suspended sediment; thus, a detailed simulation of the sediment is required. For a Pathogen TMDL, it is commonly accepted that quantification of the uncertainty in the predictions is more important than the predicted value. In the present work, we decided to re-analyze the calibration of the hydrological model, and then add calibration of sediment simulations and bacteria. The goal of the new model is to better match low flow periods of record, sediment transport, and indicator bacteria.

The HSPF model was recalibrated using the automatic inversion code UCODE_2005. We evaluated composite and dimensionless sensitivities, and parameter correlations for most of the parameters involved in the representation of the hydrologic processes. Focusing on the parameters with high sensitivity and low correlations we started the HSPF calibration. To improve the quality of the calibration results, we used several targets: daily flows, monthly volumes, and exceedance times. Weighting of observations were assigned based on relative errors while trying to avoid assigning large weights to the peaks which could easily dominate the calibration. Results of the sensitivity analysis and of the hydrological model calibration will be presented and discussed.