The effect of residue cover on soil water storage

Tracking the decline of cover as residues decompose

APSIM simulates the influence of crop residues on the efficiency with which soil water is captured and retained during fallows. But 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 and soil. 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 simulation will demonstrate how surface residue decomposes over time. You should use the previous simulation as a starting point for this simulation. You need to add an initial amount of surface residues.

  1. Reopen the previous file Fallow water balance.apsim. (Hint: c:\Apsim_workshop\fallow water balance.apsim)
  2. Save the file as Residue.apsim (Reminder: don’t forget to use the Save as button, NOT the Save, or you will save these changes to Fallow water balance.apsim)
  3. Remove the Silt2 Fallow simulation. We’re going to use Silt Fallow as our starting point for this exercise. Also remove the graph components. Select the graph component and press “delete” or right click on the component and select Delete.
  4. Make a copy of the Silt Fallow simulation by dragging to the top node in tree (Simulations).
  5. Rename second simulation to Silt rice residue
  6. Select the Surface Organic Matter node for editing:
    Set Organic Matter pool name: rice_stubble.
    Set Organic Matter type: rice.
    Set Initial surface residue: 2000 kg/ha.
  7. Run the simulation
  8. Create a graph of day vs surfaceom_cover and rain(right hand axis) for the Silt rice Residue simulation. Drag an XY graph from the “Graph toolbox” -> Graph -> “Graphs (XY)” onto the output file, and rename it to rice_cover. Remember to set “Point type” to None. To find out how to modify a graph see How To Modify a Graph Component

It can be seen that periods of high decomposition rate match with higher rainfall and low decomposition with dry periods.

The effect of cover decline on runoff

In this activity, a comparison will be made between two simulations: Silt Fallow and Silt Rice Residue.

  1. Drag the rice_cover graph that we’ve just created onto the simulations node at the top of the tree to create a copy.
  2. Graph Date vs runoff(cumulative)
  3. Rename this graph residues_runoff. Notice that data from both simulations (Silt Fallow and Silt rice residue) appears.
  4. Drag grahic node Plot to residues_runoff node to create Plot1
  5. Graph rain (this time select Type = Bar )

The effect of residue type on speed of decomposition

The APSIM Surface Organic Matter model will decompose residues at differing rates according to the C:N ratio of the material, amongst other parameters. To demonstrate this we will reproduce the previous simulation but apply legume residues in the place of the rice straw residues.

  1. Create another copy of the Silt rice Residue simulation and call it Silt cowpea residue. Delete the rice_cover graph
  2. Select Surface Organic Matter for editing:
    Set Organic Matter pool name: cowpea_stubble.
    Set Organic Matter type: cowpea.
    Set Initial surface residue: 2000 kg/ha.
    set C:N ratio of initial residue: 20.
    (Remember you may want to change the Surface Organic Matter pool name to something like Cowpea as well)
  3. Run this new simulation. (If you just select this simulation in the tree and click the run button it will only run this simulation instead of all of them.
  4. Create a new graph for Silt rice residue and Silt cowpea residue simulations with residue cover as a function of time (eg date). Add rain to the right hand axis.
  5. Rename the graph to “residue_type_cover”.