Module 2: Surface Organic Matter Simulation

Created 23/02/2023 - Last updated 05/03/2023

IMPORTANT NOTE: It is highly recommended that you upgrade your APSIM Next Gen version to at least version 2023.2.7164.0 or later.

The Effect of Residue Cover on Soil Water Storage during Fallow

Tracking the Decline of Cover as Residues Decompose.

APSIM simulates the influence of crop residues on the efficiency with which water is captured and retained during fallows. Residue cover declines as residues decompose. Residue decomposition is simulated in APSIM in response to weather, as well as the chemical composition of the residues. By doing this simulation you will reinforce skills learned in previous exercises and learn to do some basic editing of default values to “customise” your simulations.

This module assumes you have completed the previous module: “Module One: Fallow Simulation”. It will introduce you to the Surface Organic Matter module and demonstrate how surface residue decomposes over time.

1. For this module we will use the simulation we created in Exercise 1 (Module1) as a base. There is also a completed example in the training toolbox if you would prefer to use that. Open the file Module1.apsim.

2. Save the file as Module2.apsim in your Apsim Training folder. Remember to use Save As not Save or you will overwrite the old file.

3. Delete the Sand Fallow simulation. We’ll use Clay Fallow as the starting point. Also delete the graph.

4. Make a copy of Clay Fallow by dragging it to the simulations node in the tree and rename this new simulation to Clay Residue.

   • Your node tree should look like this:

5. Expand the new simulation then expand the paddock node. Click the SurfaceOrganicMatter module and change the initial surface residue to 3000 kg/ha.

6. Run both of the simulations, either by clicking on the simulations node at the top and then clicking Run, or selecting them individually and running them.

7. Create a graph of Date vs surface organic matter cover (SurfaceOrganicMatter.Cover) and Weather.Rain (right hand axis) for the Clay Residue simulation.

   • To do this:
     • right-click Clay Residue node.
     • change graph name to Organic Matter Cover
     • right-click Organic Matter Cover node,
       • double-click Add model...,
       • double-click Series
     • click Series node
       • change X to Clock.Today
       • change Y to SurfaceOrganicMatter.Cover
       • select Report as data source
       • change the type to Scatter
       • change line type to Solid
       • change marker type to None
       • change colour to yellow
       • rename this Series node as SOM.
     • Your node tree should look like this:

8. Create another series for rain under the Graph node.

   • To do this:
     • right-click Organic Matter Cover node
       • double-click Add model...
       • double-click Series
       • rename this series to Rain
     • click Rain node
       • select Report for Data Source
       • change X to Clock.Today
       • change Y to Weather.Rain
         • check on right? checkbox
       • change Type to Scatter
       • change Line type to Solid
       • change Marker type to None
       • change Colour to blue.
     • Your node tree should look like this:

The effect of cover decline on runoff and evaporation

We will compare the effect that ground cover has on runoff.

1. Graph both simulations, with Date vs runoff (cumulative) and Rain (right axis).
   • To do this:
     • right-click the Simulations node
     • click add model...
     • double-click Graph
2. Rename the graph to Runoff.

3. Let’s build the graph

   • To do this:
     • right-click Runoff graph
     • click Add model...
     • double-click Series
     • rename series to Cumulative Runoff
     • Set your series variables to below:
     • NOTE: If you only get one graph it means that one simulation has not been run yet.

4. Let’s add another Series for Rain

   • To do this:
     • right-click Runoff
     • click Add model
     • double-click Series
     • rename this series to Rain
     • set the series variables to below:

The effect of residue type on speed of decomposition

The APSIM residue model will decompose residues at differing rates according to the C:N ratio of the material. To demonstrate this we will reproduce the previous simulation but apply legume residues in the place of the wheat residues.

1. Create another copy of the Clay Residue simulation. Rename it to Clay Chickpea Residue. Remove the graph component.
2. Change the SurfaceOrganicMatter residue parameters to 3000 kg/ha of chickpea residue.
   • Also change the initial residue pool to ‘chickpea’.
3. Change the C:N ratio to 25.
4. Run the simulation.
5. Create a graph with all three residue simulations with residue as a function of time. Call the graph Cover.
   • To do this:
     • right-click the simulations node
     • click Add model...
     • double-click Graph
     • rename Graph to Cover
     • right-click Cover graph
     • click Add model...
     • double-click Series
6. Change the variables to match the image below:
7. If you’d like the lengend to appear on the right of the graph like the image:
   • Click one of the legend items
   • a menu will appear at the bottom of the graph and you can change the drop down menu’s value to top right or any other position to your liking.

Congratulations on completing module two!

Note: If you found any incorrect/outdated information in this tutorial, please let us know on GitHub by submitting an issue.