Step 2: Layered G&A Method for the Long-Term Post-Wildfire Hydrology Model

Last Modified: 2025-08-18 10:20:05.384

This tutorial walks you through configuring and calibrating an HEC-HMS model for long-term post-wildfire hydrology simulations.

Software Version

HEC-HMS version 4.13 was used to develop this tutorial.

Change the loss method from G&A to Layered G&A

The standard Green and Ampt (G&A) loss method is designed for single‐event simulations and should be replaced in a continuous model. Instead, use the Layered G&A method for long‐term post‐wildfire hydrology. It supports continuous runs and handles both pre‐ and post‐fire conditions, as outlined in the guidance table.

We have re-calibrated the HEC-HMS post-wildfire hydrology model for the Arroyo Seco watershed to the 18–19 January 2010 event using the Layered Green-Ampt infiltration method and then simulated it through 14 February 2019. The tables below compare the input parameters for the standard Green-Ampt versus the Layered Green-Ampt approaches. As you’ll see, we need additional data for Layer 2 in the layered method. Note that this calibration represents one of several reasonable outcomes rather than the optimal result.

In Step 2, we will

• compare the two model runs to quantify the improvement gained by the Layered G&A method, and

• identify opportunities to further refine the model for long-term post-wildfire hydrology.

Calibrated Input parameters for G&A method

Calibrated Input parameters for Layered G&A method

Compare G&A Method and Layered G&A Methods for Post-Wildfire hydrology Model

Select the Basin Models folder to expand the Watershed Explorer.
Select LGA_CUH_LR Basin Model and review all its methods and input parameters.
Run the LGA_CUH_LR_Event simulation, calibrated to the January 18–19, 2010 event using the Layered G&A infiltration loss method.
Examine the simulation results, then answer the questions below.

Question 1:
What are the results of the calibration?
- Computed peak discharge (value, date, and time)
- Total runoff volume
- Nash–Sutcliffe Efficiency
Based on these metrics, does the calibrated Layered G&A model outperform the standard G&A method, and is it robust enough to support a nine-year continuous simulation?
compared to USGS Arroyo Seco gage observations, both the layered and standard Green-Ampt models exhibit a strong match.
- Computed peak discharge for Layered G&A: 3,978 cfs on 18 Jan 2010 at 22:30
  Computed peak discharge for Standard G&A: 4,221 cfs on 18 Jan 2010 at 22:15
  Observed peak discharge: 4,230 cfs on 18 Jan 2010 at 22:30
- Total runoff volume: 1.39 in (computed for Layered G&A) vs. 1.24 in (computed for Standard G&A) vs. 1.04 in (observed)
- Nash–Sutcliffe efficiency: 0.970 (Layered G&A) vs. 0.953 (Layered G&A)
These metrics indicate that the standard G&A model marginally outperforms the alternative in event‐based simulations, but both produce similarly high‐quality calibrations that are adequate for establishing initial conditions in a nine‐year continuous run. Divergences between the two models, however, become pronounced over the long term.
New Results for the Layered G&A Method

Previous Results for the Standard G&A Method
Run the LGA_CUH_LR simulation (17Jan2020 - 14Feb1019) using the Layered G&A infiltration loss method.
Examine the simulation results, then answer the questions below.

Question 2:
What are the results for 9-year simulation?
- Computed peak discharge (value, date, and time)
- Total runoff volume
- Nash–Sutcliffe Efficiency
Based on these metrics, does the calibrated Layered G&A model outperform the standard G&A method, and is it robust enough to support a nine-year continuous simulation?
When comparing both results with USGS Arroyo Seco gage observations, a significant mismatch is evident, with the model increasingly overestimating flows in later years:
- Computed peak discharge for Layered G&A: 4,293 cfs on 28 Feb 2014 at 17:00
Computed peak discharge for Standard G&A: 5,538 cfs on 20 Jul 2015 at 01:45
Observed peak discharge: 4,620 cfs on 06 Feb 2010 at 13:45
- Total runoff volume: 101.09 in (Layered G&A) vs. 64.56 in (Standard G&A) vs. 42.45 in (observed)
- Nash–Sutcliffe efficiency: -0.966 (Layered G&A) vs. -1.880 in (Standard G&A)
These metrics show that both models currently fall short of delivering a nine-year continuous simulation suitable for emergency evacuation and maintenance planning. Although the Layered G&A results modestly outperformed the Standard G&A, further enhancements are needed to produce reliable long-term post-wildfire hydrology estimates.
New Results for the Layered G&A Method

Previous Results for the Standard G&A Method
Question 3:
What strategies can we employ to improve the model and ensure reliable long-term post-wildfire hydrology estimates?
Applying the Layered G&A method yields modest improvements in the nine-year continuous simulation. However, it remains insufficient for post-wildfire hydrology, since the method was developed solely for continuous simulation.
Next, incorporate post‐fire recovery processes—such as time-varying soil loss and canopy dynamics—to capture long‐term landscape changes. Based on this review, the following are recommended as minimum requirements for the long‐term post‐wildfire hydrology model:
- Include the Pak & Lee dynamic surface method (Applying the Pak and Lee Dynamic Surface Method for Post-wildfire Hydrologic Modeling) for the post‐wildfire hydrology model. Wildfires often bake the soil surface, creating a water-repellent crust (hydrophobic layer) that severely limits infiltration and surface storage, thereby increasing runoff. As rainfall gradually erodes this layer, infiltration rates recover over years, a process constant‐rate methods cannot capture. The Pak & Lee method dynamically updates surface properties, making it ideal for simulating both immediate post‐fire runoff spikes and longer‐term soil recovery.
- Incorporate the Dynamic Canopy Method to adjust canopy interception and evapotranspiration in long-term continuous simulations. This approach accommodates time-varying crop coefficients—as seen during post-fire vegetation recovery—ensuring a more realistic representation of changing canopy conditions.
Current results from the Layered G&A model indicate it remains inadequate for a nine-year continuous simulation to support post-wildfire emergency evacuation and maintenance planning. Further refinements are needed to produce reliable long-term hydrology estimates.