The Australian Water Balance Model (AWBM) was developed in the early 1990s by Walter Boughton and is now one of the most widely used rainfall-runoff models in Australia. Many GoldSim models incorporate AWBM for estimating runoff around mine sites and for other projects that simulate rainfall runoff in a watershed.
This version of AWBM is intended to be used as a module within other GoldSim models but also works as a standalone model. The inputs to the model are precipitation and potential evapotranspiration time series, along with model parameters that describe the catchment. The output is a runoff rate on a per unit area basis, which can be multiplied by the total catchment area for volumetric runoff.
User Guide: Integrating the AWBM Model into Your Larger Water Balance Model
This guide provides step-by-step instructions for integrating the AWBM module into your water balance model or any other dynamic system requiring simulated runoff inflows.
Step 1: Import AWBM
The first step is to obtain the AWBM model and incorporate it into your existing model. Download the AWBM GoldSim model file using the links at the bottom of this article.
Once downloaded, open the model and copy it (Ctrl-A to select all model components) then paste it into your model. This model will import a new Array Label Set: “SurfaceStores” with default index of (1,2,3).
The AWBM model will now be integrated into your model, but it still needs to be configured with site-specific inputs.
Step 2: Configure the AWBM Model Inputs
The default time series elements and parameters in the AWBM model are placeholders. You must replace them with site-specific data or output from another part of your model:
- Precipitation (Prec_TS): Provided as a time series representing daily rainfall in mm/day
- Potential Evapotranspiration (Evap_TS): Provided as a time series representing daily potential evapotranspiration in mm/day
Constant Input Parameters:
- Delta Time (DT): This input should be equal to the scheduled time step (see "Basic Step") defined in the Simulation Settings
- Initial Conditions:
- SSi=Initial capacity of the surface stores (as a fraction of average capacity)
- Si=Initial depth in the surface runoff store
- Bi=Initial depth in the baseflow store
- Baseflow Index (BFI): Controls partitioning between baseflow and surface runoff. Required: 0.0 <= BFI <= 1.0
- Surface runoff recession constant (Ks): Controls how quickly surface runoff drains. Required: 0.0 <= Ks <= 1.0
- Baseflow runoff recession constant (Kb): Controls how quickly baseflow runoff drains. Required: 0.0 <= Kb <= 1.0, should be greater than Ks
- Depth Capacity of Surface Stores (C[*]): Depth capacities of the three surface store components. Note these items do not represent individual catchments within a watershed but rather the depth capacity for different partial areas of a single catchment.
- Catchment Area (Catchment_Area): Set this to the size of your contributing catchment.
Step 4: Recommended Calibration Strategy
AWBM must be calibrated to match historical runoff data at your site. Calibration typically involves adjusting:
- Compare simulated runoff to observed runoff and adjust parameters iteratively
- Baseflow Index (BFI): Start with 0.6 - 0.7 and adjust based on streamflow behavior.
- Surface Runoff Recession Constant (Ks): Start with a value in the range of 0.6 < Ks < 0.9. Adjust to match observed short-term falling limb of the hydrograph.
- Baseflow Runoff Recession Constant (Kb): Start with a value in the range of: 0.9 < Kb < 1.0. Adjust to match observed long-term falling limb of the hydrograph.
- Storage Capacity (C): Increase if runoff is too flashy. From the AWBM documentation (Boughton, 2003): "By selecting a number of high-quality data sets, i.e. with very high correlation between calculated and actual monthly values of runoff, it was found that the average value of surface storage capacity Avg = C1*A1+C2*A2+C3*A3 was far more important to calibration than the individual set of capacities and partial areas." Because of this, it is recommended that you consider using a single scaling factor, evenly applied to all components of C for calibration.
You can also find tools in the GoldSim model library to assist with automatically calibrating your model using GoldSim's built-in optimization feature. This is demonstrated in some examples in our model library:
- Nash-Sutcliffe Model Efficiency Coefficient – GoldSim Help Center
- Root Mean Square Error (RMSE) – GoldSim Help Center
- Model Calibration Using Optimization – GoldSim Help Center
Once you have configured and calibrated AWBM: Run your GoldSim model with AWBM integrated. Verify that simulated runoff aligns with expected flows. Refine parameter values as needed to improve accuracy. AWBM will now simulate runoff inflows as part of your water balance model.
Notes:
- Unscheduled time steps have been avoided in AWBM by carefully structuring the model equations. Refer to the calculates in this location of the model: \Catchment_Runoff\AWBM\Surface_Stores
- Time step settings (e.g., daily step size) should align with your overall simulation settings.
- AWBM has been validated against the Australian eWater RRL Toolkit model, ensuring reliable performance (download the AWBM Verification below to see results).
If you have any questions or need further guidance, feel free to reach out!
Comments
9 comments
AWBM version 1.002 added:
Version Report
1. Fixed typo in Surface Stores
2. Re-organized some elements in the graphics pane
3. Rename "Runoff_Rate" and "Total_Runoff" for clarity
4. Revise some element descriptions for better clarity
Model version 1.007 released on Dec 20, 2018
Version Report
1. Changed the Precipitation and ET_Potential time series to represent "instantaneous value" as a rate in mm/d for easier incorporation into other models.
Model version 1.009 released on May 3, 2019
Version Report
1. A user interface was added so that Player users can edit and run AWBM
2. Added a warning if the model time step is different from K_Step constant
3. Updated the schematic
4. Added reporting periods
5. Changed simulation type to deterministic
Model version 1.011 released on August 1, 2019
Version Report
1. The model parameter "k-step" was exposed on the dashboard so player users can adjust the model's time step and make the same change to this factor, which will impact the recession coefficients for surface and baseflow.
Model version 1.013 released on September 27, 2019
Version Report
Model version 1.017 released on Oct 29, 2021
Version Report
Model version 1.019 released on Feb 2, 2022
Version Report
Version 1.022
Change Report:
Now, in GoldSim 15, you can print a version report. Below, is a print of versions since the last reported, with redundant information removed.
GoldSim Version Report
GoldSim Version: 15.0.0257
Current Date: 02/10/25 10:28:45
Model File: C:...\AWBM.gsm
Included Versions: 7 of 29
Version 1.028: Update documentation for runoff components
Created by: Jason
Created on: 02/10/25 09:12:54
Description: Updated documentation for both Baseflow and Surface Runoff equations to include a 0.05 mm cutoff. This threshold prevents numerical instability from near-zero withdrawal rates due to the asymptotic behavior inherent in the volume-dependent calculations.
Version 1.027: Improved Surface Stores functions
Created by: Jason
Created on: 02/10/25 09:00:05
Description: Revised Time_to_Next calculation to filter out very small differences between DT and TimestepLength. If( abs(DT - TimestepLength) <= 1 minute, DT is used instead of the raw difference. This prevents near-zero denominators in the overflow equation, avoiding runaway spikes during unscheduled events.
Version 1.026: Upgraded to GoldSim 15
Created by: Jason
Created on: 01/27/25 15:48:56
Description: <none>
Version 1.025: Allow for changing the basic time step of the model
Created by: Jason
Created on: 05/21/24 14:44:53
Description: Added new parameter "DT" for defining the basic time step of the time series data and simulation settings. This replaces the use of "1 day" used before and allows the model to work using any time step.
Version 1.024: Update display units
Created by: Jason
Created on: 05/16/24 09:53:47
Description: Updated display units to be metric
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