Adaption and Calibration of HSPF to Simulate Periphyton in Support of the Truckee River Water Quality TMDL

The Hydrological Simulation Program – FORTRAN (HSPF) model is a widely used watershed/water quality model supported by the U.S. Geological Survey (USGS) and U.S. Environmental Protection Agency (EPA). It is part of EPA's BASINS software system. The Cities of Reno and Sparks, Nevada chose HSPF as their long-term river management tool to evaluate water quality implications of alternative management scenarios because of its acceptance by regulatory agencies, historical use on the Truckee River, open source code, and large community of users.Water quantity and quality are critical issues for the Truckee River. Management and regulatory factors that influence the river include the established Total Maximum Daily Loads (TMDLs) for total phosphorus, total nitrogen, and total dissolved solids that protect designated uses in the river; the finalization of the Truckee River Operating Agreement (TROA) that will regulate reservoir releases, diversions and flow in the river; and the development of a regional wastewater treatment plan. These factors require the use of water quality models to evaluate the effects of various treatment and watershed management options on river water quality with regards to the established TMDLs.A new integrated model was developed as the long-term management tool for river water quality. This model was created by enhancing HSPF with periphyton routines from an existing water quality model for the system (DSSAMt – developed by Rapid Creek Research). The purpose of this paper is to document the changes made to HSPF, the process of verifying the new routines, and the challenges encountered during calibration. Issues that will be discussed include:Description of the new periphyton routines added to HSPF;Verification of the new routines;Discussion of the calibration strategy and goals; andComparison of the model results to historical modeling and observed data.The enhancements made to HSPF include additional growth limitation terms, loss terms, and increasing the number of benthic algal types that can be simulated. The additional growth terms include a temperature limitation, standard Michaelis-Menton nutrient limitation terms, a stream velocity limitation term on nutrient availability, a light limitation using the Steele equation, and a density limitation. Loss terms include both basal and photorespiration, a grazing and disturbance loss, and a scour loss. The enhancements were made to improve HSPF's ability to simulate periphyton, and better represent the current state-of-the-science in this area.Verification of the new periphyton routines were conducted by comparing HSPF output to simple spreadsheet models, and historical modeling results from the previous management model (DSSAMt). Since many of the periphyton routines added to HSPF were taken from the DSSAMt model, similar results were expected. The fit between the two models was good. The results of this comparison confirm that HSPF simulates nutrients, dissolved oxygen, and algae in a similar manner as DSSAMt, and indicates that the routines were implemented correctly.Comparison of the HSPF model to observed data was conducted using a data set that contained a complete set of both water quality data and periphyton data. The goals of model calibration were to achieve an adequate “goodness of fit” to observed data, while keeping model coefficients within a reasonable range. Results of the calibration showed that HSPF errors were less than or equal to the DSSAMt model available as of February 2002 (version 30b).