The United States Geological Survey (USGS) developed the Expert System for Calibration of HSPF (HSPEXP) in 1994. HSPEXP was designed to assist HSPF users in calibrating the model by calculating several calibration statistics, displaying graphs, and then advising the user to change the parameters based on several sets of rules. HSPEXP has been used extensively since its release and has been cited more than 1,000 times; however, HSPEXP support was discontinued and the model did not perform on most modern computers. Several organizations developed their own software to calculate calibration statistics, but these software programs were neither freely available nor were they open-source, limiting their usability. Using the existing codebase of BASINS, which is developed on VB.net, AQUA TERRA Consultants (a division of RESPEC) developed HSPEXP+ and released it freely in February 2015. This open-source software replicates most of the functionality of its predecessor HSPEXP and provides multiple additional reports and advanced graphing functionality that can assist a user in obtaining meaningful insights about the model on an overall basis and on an operation-by operation-basis. For example, in a single HSPEXP+ run during a hydrology calibration, a user can run the model, generate area reports to verify the model connections, various hydrology calibration statistics, obtain advice for parameter improvement, generate calibration graphs, and water balance reports on an annual and overall basis for the entire watershed as well as for each operation. The user can also generate similar nutrient balance reports for total nitrogen (TN), total phosphorus (TP), and sediment. A load-allocation report also accompanies these nutrient balance reports. The load-allocation reports provide the total and percent contribution of TN, TP, and sediment load at each reach after taking into account the gains and losses in each upstream reach.
This paper presents the case study of the HSPF model calibration of the Snake River Watershed in Minnesota. The Snake River Watershed is approximately 1,000 square miles located in the eastern part of the state. The Minnesota Pollution Control Agency (MPCA) contracted AQUA TERRA Consultants (a division of RESPEC) to develop HSPF models for several Hydrologic Unit Codes-8 (HUC-8) watersheds (including Snake River) to support their Total Maximum Daily Load (TMDL) development process. HSPEXP+ was used to assist in the hydrologic calibration at one location and water quality calibration at over ten locations in the watershed. Continuous, long-term USGS discharge data are available at one location for the calibration period, snow depth data are available at stations in and around the watershed, and grab samples of water quality data are available at different locations in the watershed. HSPEXP+ was used to generate snow-simulation graphs, hydrology calibration statistics and graphs, water balance reports, and advice on parameter adjustment to produce improved calibrations. Following the successful hydrology calibration, HSPEXP+ was used to generate sediment and nutrient balance reports to assist in sediment apportionment and nutrient loading rate calibration. HSPEXP+ was further used to generate water quality graphs at locations where observed data are available. In addition, HSPEXP+ provided the load-allocation report from the calibrated model, which suggested that pasture areas are the leading source of sediment, and cropland is the leading source of TN and TP in the Snake River Watershed.