Task 3: Calibrate the Model to the Hopland Gage

In this task, you will use the HEC-HMS Combined Parameter Editor to calibrate the model at the Talmage Gage and Hopland Gage locations. The following figure shows the location of these two gages.

1. Select the HEC-HMS node in the Forecasts window, then click the Combined Parameter Editor action button.
2. Open HEC-HMS results at the Talmage Gage and the Hopland Gage junctions. Notice in the figure below, plots are open as well as summary tables. 
   
3. At the bottom of the Combined Parameter Editor is an drop down that lets you choose computation points in the model. Choose the Hopland computation point. When you press the compute forecast button on the bottom left of the Combined Parameter Editor, HEC-HMS will only run the model to the selected location. Go to the Deficit Constant Loss tab and click the Upper zone. Notice the in the element override table, the Ukiah Loc, Talmage Loc, and Hopland Loc subbasins are in the Upper zone. 
   
4. The first thing that stands out in the flow hydrographs is that the model under-predicts the runoff response for the first two events. This means the initial deficit is too high. Based on adjustment made when calibrating the model for Lake Mendocino and Lake Sonoma, the initial deficit should be in the 0 - 0.2 inch range. Change the initial deficit to 0.2 inches in the zonal override table and click compute. Results should be improved. 
5. Typically, zones are created so that there is only one location with observed flow. In our case, the are two locations with observed flow. Adjustments to Ukiah Loc and Talmage Loc subbasin parameters can improve model results at the Talmage Gage, and adjustments to parameters in the Hopland Loc subbasin can improve model results at the Hopland Gage. You should calibrate your model by working from upstream to downstream. As shown in the figure below, the element override table can be used to make adjustments at the individual element level. As shown, once you check the Override Box, you can edit parameter overrides for each subbasin. Change the Constant Loss Rates for the Ukiah Loc, Talmage Loc, and Hopland Loc subbasins using values shown below. 
   
   Go to the Linear Reservoir Baseflow tab. Use the element override table and change the GW 1 and GW 2 Fraction parameters for subbasins in the Upper zone to those shown below.
6. Rerun the simulation. Your results should be similar to those shown below. 
   

At this point, the model has been calibrated above both reservoirs and downstream of Lake Mendocino to the Hopland Gage. The effort put into calibrating the model to the lookback period is a quick process if you are familiar with the watershed and parameter impacts. As mentioned earlier, only the initial deficit, constant loss rate, and GW 1 and GW 2 fraction parameters will need adjustment between forecasts. Other model parameter should be consistent between forecasts, for many cases. 

HEC-HMS has tools that can be used to set initial reservoir storage and baseflow using observed flow gages. The Combined Parameter Editor can be used to quickly adjust parameter either on a zonal basis or for individual elements. You can access HEC-HMS plots and summary tables directly from the CAVI to visualize model performance. Additional results, spatial results, can be accessed directly from HEC-HMS. These tools, plus the capability to compute the model to specific locations in the watershed, contribute quick and efficient model calibration.